Cavitation Resistance in Seedless Vascular Plants: The Structure and Function of Interconduit Pit Membranes

Plant water transport occurs through interconnected xylem conduits that are separated by partially digested regions in the cell wall known as pit membranes. These structures have a dual function. Their porous construction facilitates water movement between conduits while limiting the spread of air that may enter the conduits and render them dysfunctional during a drought. Pit membranes have been well studied in woody plants, but very little is known about their function in more ancient lineages such as seedless vascular plants. Here, we examine the relationships between conduit air seeding, pit hydraulic resistance, and pit anatomy in 10 species of ferns (pteridophytes) and two lycophytes. Air seeding pressures ranged from 0.8 ± 0.15 MPa (mean ± sd) in the hydric fern Athyrium filix-femina to 4.9 ± 0.94 MPa in Psilotum nudum, an epiphytic species. Notably, a positive correlation was found between conduit pit area and vulnerability to air seeding, suggesting that the rare-pit hypothesis explains air seeding in early-diverging lineages much as it does in many angiosperms. Pit area resistance was variable but averaged 54.6 MPa s m⁻¹ across all surveyed pteridophytes. End walls contributed 52% to the overall transport resistance, similar to the 56% in angiosperm vessels and 64% in conifer tracheids. Taken together, our data imply that, irrespective of phylogenetic placement, selection acted on transport efficiency in seedless vascular plants and woody plants in equal measure by compensating for shorter conduits in tracheid-bearing plants with more permeable pit membranes.Water transport in plants occurs under tension, which renders the xylem susceptible to air entry. This air seeding may lead to the rupture of water columns (cavitation) such that the air expands within conduits to create air-vapor embolisms that block further transport. (Zimmermann and Tyree, 2002). Excessive embolism such as that which occurs during a drought may jeopardize leaf hydration and lead to stomatal closure, overheating, wilting, and possibly death of the plant (Hubbard et al., 2001; Choat et al., 2012; Schymanski et al., 2013). Consequently, strong selection pressure resulted in compartmentalized and redundant plant vascular networks that are adapted to a species habitat water availability by way of life history strategy (i.e. phenology) or resistance to air seeding (Tyree et al., 1994; Mencuccini et al., 2010; Brodersen et al., 2012). The spread of drought-induced embolism is limited primarily by pit membranes, which are permeable, mesh-like regions in the primary cell wall that connect two adjacent conduits. The construction of the pit membrane is such that water easily moves across the membrane between conduits, but because of the small membrane pore size and the presence of a surface coating on the membrane (Pesacreta et al., 2005; Lee et al., 2012), the spread of air and gas bubbles is restricted up to a certain pressure threshold known as the air-seeding pressure (ASP). When xylem sap tension exceeds the air-seeding threshold, air can be aspirated from an air-filled conduit into a functional water-filled conduit through perhaps a large, preexisting pore or one that is created by tension-induced membrane stress (Rockwell et al., 2014). Air seeding leads to cavitation and embolism formation, with emboli potentially propagating throughout the xylem network (Tyree and Sperry, 1988; Brodersen et al., 2013). So, on the one hand, pit membranes are critical to controlling the spread of air throughout the vascular network, while on the other hand, they must facilitate the efficient flow of water between conduits (Choat et al., 2008; Domec et al., 2008; Pittermann et al., 2010; Schulte, 2012). Much is known about such hydraulic tradeoffs in the pit membranes of woody plants, but comparatively little data exist on seedless vascular plants such as ferns and lycophytes. Given that seedless vascular plants may bridge the evolutionary transition from bryophytes to woody plants, the lack of functional data on pit membrane structure in early-derived tracheophytes is a major gap in our understanding of the evolution of plant water transport.In woody plants, pit membranes fall into one of two categories: the torus-margo type found in most gymnosperms and the homogenous pit membrane characteristic of angiosperms (Choat et al., 2008; Choat and Pittermann, 2009). In conifers, water moves from one tracheid to another through the margo region of the membrane, with the torus sealing the pit aperture should one conduit become embolized. Air seeding occurs when water potential in the functional conduit drops low enough to dislodge the torus from its sealing position, letting air pass through the pit aperture into the water-filled tracheid (Domec et al., 2006; Delzon et al., 2010; Pittermann et al., 2010; Schulte, 2012; but see Jansen et al., 2012). Across north-temperate conifer species, larger pit apertures correlate with lower pit resistance to water flow (r_pit; MPa s m⁻¹), but it is the ratio of torus-aperture overlap that sets a species cavitation resistance (Pittermann et al., 2006, 2010; Domec et al., 2008; Hacke and Jansen, 2009). A similar though mechanistically different tradeoff exists in angiosperm pit membranes. Here, air seeding reflects a probabilistic relationship between membrane porosity and the total area of pit membranes present in the vessel walls. Specifically, the likelihood of air aspirating into a functional conduit is determined by the combination of xylem water potential and the diameter of the largest pore and/or the weakest zone in the cellulose matrix in the vessel’s array of pit membranes (Wheeler et al., 2005; Hacke et al., 2006; Christman et al., 2009; Rockwell et al., 2014). As it has come to be known, the rare-pit hypothesis suggests that the infrequent, large-diameter leaky pore giving rise to that rare pit reflects some combination of pit membrane traits such as variation in conduit membrane area (large or small), membrane properties (tight or porous), and hydrogel membrane chemistry (Hargrave et al., 1994; Choat et al., 2003; Wheeler et al., 2005; Hacke et al., 2006; Christman et al., 2009; Lee et al., 2012; Plavcová et al., 2013; Rockwell et al., 2014). The maximum pore size is critical because, per the Young-Laplace law, the larger the radius of curvature, the lower the air-water pressure difference under which the contained meniscus will fail (Jarbeau et al., 1995; Choat et al., 2003; Jansen et al., 2009). Consequently, angiosperms adapted to drier habitats may exhibit thicker, denser, smaller, and less abundant pit membranes than plants occupying regions with higher water availability (Wheeler et al., 2005; Hacke et al., 2007; Jansen et al., 2009; Lens et al., 2011; Scholz et al., 2013). However, despite these qualitative observations, there is no evidence that increased cavitation resistance arrives at the cost of higher r_pit. Indeed, the bulk of the data suggest that prevailing pit membrane porosity is decoupled from the presence of the single largest pore that allows air seeding to occur (Choat et al., 2003; Wheeler et al., 2005 Hacke et al., 2006, 2007).As water moves from one conduit to another, pit membranes offer considerable hydraulic resistance throughout the xylem network. On average, r_pit contributes 64% and 56% to transport resistance in conifers and angiosperms, respectively (Wheeler et al., 2005; Pittermann et al., 2006; Sperry et al., 2006). In conifers, the average r_pit is estimated at 6 ± 1 MPa s m⁻¹, almost 60 times lower than the 336 ± 81 MPa s m⁻¹ computed for angiosperms (Wheeler et al., 2005; Hacke et al., 2006; Sperry et al., 2006). Presumably, the high porosity of conifer pits compensates for the higher transport resistance offered by a vascular system composed of narrow, short, single-celled conduits (Pittermann et al., 2005; Sperry et al., 2006).Transport in seedless vascular plants presents an interesting conundrum because, with the exception of a handful of species, their primary xylem is composed of tracheids, the walls of which are occupied by homogenous pit membranes (Gibson et al., 1985; Carlquist and Schneider, 2001, 2007; but see Morrow and Dute, 1998, for torus-margo membranes in Botrychium spp.). At first pass, this combination of traits appears hydraulically maladaptive, but several studies have shown that ferns can exhibit transport capacities that are on par with more recently evolved plants (Wheeler et al., 2005; Watkins et al., 2010; Pittermann et al., 2011, 2013; Brodersen et al., 2012). Certainly, several taxa possess large-diameter, highly overlapping conduits, some even have vessels such as Pteridium aquilinum and many species have high conduit density, all of which could contribute to increased hydraulic efficiency (Wheeler et al., 2005; Pittermann et al., 2011, 2013). But how do the pit membranes of seedless vascular plants compare? Scanning electron micrographs of fern and lycopod xylem conduits suggest that they are thin, diaphanous, and susceptible to damage during specimen preparation (Carlquist and Schneider 2001, 2007). Consistent with such observations, two estimates of r_pit imply that r_pit in ferns may be significantly lower than in angiosperms; Wheeler et al. (2005) calculated r_pit in the fern Pteridium aquilinum at 31 MPa s m⁻¹, while Schulte et al. (1987) estimated r_pit at 1.99 MPa s m⁻¹ in the basal fern Psilotum nudum. The closest structural analogy to seedless vascular plant tracheids can be found in the secondary xylem of the early-derived vesselless angiosperms, in which tracheids possess homogenous pit membranes with r_pit values that at 16 MPa s m⁻¹ are marginally higher than those of conifers (Hacke et al., 2007). Given that xylem in seedless vascular plants is functionally similar to that in vesselless angiosperms, we expected convergent r_pit values in these two groups despite their phylogenetic distance. We tested this hypothesis, as well as the intrinsic cavitation resistance of conduits in seedless vascular plants, by scrutinizing the pit membranes of ferns and fern allies using the anatomical and experimental approaches applied previously to woody taxa. In particular, we focused on the relationship between pit membrane traits and cavitation resistance at the level of the individual conduit.     

Evidence for Hydraulic Vulnerability Segmentation and Lack of Xylem Refilling under Tension

The vascular system of grapevine (Vitis spp.) has been reported as being highly vulnerable, even though grapevine regularly experiences seasonal drought. Consequently, stomata would remain open below water potentials that would generate a high loss of stem hydraulic conductivity via xylem embolism. This situation would necessitate daily cycles of embolism repair to restore hydraulic function. However, a more parsimonious explanation is that some hydraulic techniques are prone to artifacts in species with long vessels, leading to the overestimation of vulnerability. The aim of this study was to provide an unbiased assessment of (1) the vulnerability to drought-induced embolism in perennial and annual organs and (2) the ability to refill embolized vessels in two Vitis species X-ray micro-computed tomography observations of intact plants indicated that both Vitis vinifera and Vitis riparia were relatively vulnerable, with the pressure inducing 50% loss of stem hydraulic conductivity = −1.7 and −1.3 MPa, respectively. In V. vinifera, both the stem and petiole had similar sigmoidal vulnerability curves but differed in pressure inducing 50% loss of hydraulic conductivity (−1.7 and −1 MPa for stem and petiole, respectively). Refilling was not observed as long as bulk xylem pressure remained negative (e.g. at the apical part of the plants; −0.11 ± 0.02 MPa) and change in percentage loss of conductivity was 0.02% ± 0.01%. However, positive xylem pressure was observed at the basal part of the plant (0.04 ± 0.01 MPa), leading to a recovery of conductance (change in percentage loss of conductivity = −0.24% ± 0.12%). Our findings provide evidence that grapevine is unable to repair embolized xylem vessels under negative pressure, but its hydraulic vulnerability segmentation provides significant protection of the perennial stem.The plant hydraulic system is located at the interface between soil water and the atmosphere. Evaporative demand from the atmosphere generates a tension within a continuous xylem water column, pulling water from the soil, through roots, stems, petioles, and leaves (Dixon, 1896). Under drought conditions, the overall resistance to water flow through the soil-plant continuum increases. Increased resistance to water flow results from changes in the resistance at multiple specific locations along the flow pathway: in the soil, at the soil-root interface, and in the roots, the main plant axis (i.e. stems and branches), the petioles, and the leaves. Two primary mechanisms controlling the resistance are stomatal closure (leaf-to-air water flow) and the loss of xylem hydraulic conductivity (soil-to-leaf water flow; Cochard et al., 2002). Stomatal closure is closely related to decreasing plant water status (Brodribb and Holbrook, 2003) and is often considered to be a protective mechanism against the loss of xylem hydraulic conductivity (Tyree and Sperry, 1988; Jones and Sutherland, 1991). Loss of xylem hydraulic conductivity occurs when the water potential of xylem sap reaches levels negative enough to disrupt the metastability of the water column, potentially resulting in embolism.Generally, high resistance to embolism is observed in species distributed in dry environments, whereas highly vulnerable species are distributed in wet environments (Maherali et al., 2004; Choat et al., 2012). Although grapevine (Vitis spp.) is widely cultivated, including in regions where it is frequently exposed to water deficit during the growing season (Lovisolo et al., 2010), recent studies have produced contrasting estimates of its resistance to embolism. Grapevine has been described as either vulnerable (Zufferey et al., 2011; Jacobsen and Pratt, 2012) or relatively resistant (Choat et al., 2010; Brodersen et al., 2013). In Vitis spp., and Vitis vinifera especially, stomatal closure is typically observed for midday leaf water potentials less than −1.5 MPa (Schultz, 2003). Thus, according to some studies, significant losses in xylem hydraulic conductivity should be observed before stomatal closure (Ψ₅₀ > −1 MPa; Jacobsen and Pratt, 2012; Jacobsen et al., 2015), implying that embolism would be commonplace.The risk of hydraulic dysfunction is mitigated along the hydraulic pathway by hydraulic segmentation (i.e. more distal organs such as leaves and petioles will be at greater risk to embolism than more basal organs such as the trunk; Tyree and Zimmermann 2002; Choat et al., 2005). This could promote hydraulic safety in larger, perennial organs, which represent a greater investment of resources for the plant. Hydraulic segmentation may occur in two ways. During transpiration, the xylem pressure will always be more negative in more distal parts of the pathway (leaves and petioles). All else being equal, this translates to a greater probability of embolism in distal organs. However, organs also may differ in their vulnerability to embolism, compensating or exacerbating the effects of differences in xylem pressure along the pathway. If leaves or petioles were more vulnerable to embolism than branches and the trunk, then they would be far more likely to suffer embolism during periods of water stress. This would allow petioles, leaves (Nolf et al., 2015), or even young branches (Rood et al., 2000) to become embolized without significant impacts on the trunk and larger branches. In grapevine, petioles have been described as extremely sensitive to cavitation (Ψ₅₀ of approximately −1 MPa; Zufferey et al., 2011). However, the hydraulic methods employed in those previous studies have been shown to be prone to artifacts (Wheeler et al., 2013; Torres-Ruiz et al., 2015), necessitating the use of a noninvasive assessment of drought-induced embolism.High-resolution computed tomography (HRCT) produces three-dimensional images of xylem tissue in situ, allowing for a noninvasive assessment of embolism resistance. This technique has provided robust results in various plant species with contrasting xylem anatomy (Charra-Vaskou et al., 2012, 2016; Dalla-Salda et al., 2014; Torres-Ruiz et al., 2014; Cochard et al., 2015; Knipfer et al., 2015; Bouche et al., 2016). Synchrotron-based tomography facilities allow the visualization of intact plants, offering a noninvasive, in vivo estimation of the loss of hydraulic conductivity within the xylem (Choat et al., 2016). Moreover, the quality of the x-ray beam in the synchrotron facilities provides high resolution and signal-to-noise ratio, making image analysis simple and accurate.If grapevine were as vulnerable to xylem embolism as suggested in some studies, refilling of embolized vessels would be expected to occur on a frequent (daily) basis in order to maintain hydraulic continuity (Sperry et al., 1994; Cochard et al., 2001; Hacke and Sperry, 2003; Charrier et al., 2013). Various refilling mechanisms have been proposed to date, including positive root/stem pressure and refilling while the xylem is under negative pressure via water droplet growth (Salleo et al., 1996; Brodersen et al., 2010; Knipfer et al., 2016). Positive pressure in the xylem sap can be related to mineral nutrition and soil temperature in autumn or spring (Ewers et al., 2001) and to soluble carbohydrate transport into the vessel lumen during winter (Améglio et al., 2001; Charrier et al., 2013). Refilling under negative pressure is based on the hypothesis that embolized vessels are isolated from surrounding functional vessels, permitting positive pressures to develop and the embolism to dissolve (Salleo et al., 1996; Tyree et al., 1999). This process has been related to the chemistry of conduit walls (Holbrook and Zwieniecki, 1999), the geometry of interconduit bordered pits (Zwieniecki and Holbrook, 2000), and phloem unloading (Nardini et al., 2011). While refilling via positive pressure has been described frequently (Sperry et al., 1987, 1994; Hacke and Sauter 1996; Cochard et al., 2001; Améglio et al., 2004; Cobb et al., 2007), refilling under negative pressure remains controversial (Cochard et al., 2013, 2015). In grapevine particularly, imaging techniques have provided evidence of refilling in embolized vessels (Brodersen et al., 2010), but uncertainties remain regarding the xylem water potential measurement at the position of the scan.The goal of this study was to provide a noninvasive assessment of (1) the vulnerability to drought-induced embolism in two widespread grapevine species in perennial (V. vinifera and Vitis riparia) and annual (V. vinifera) organs and (2) the ability to refill embolized vessels under positive or negative pressure (V. vinifera). This approach would indicate whether embolism formation and repair are likely to occur on a daily basis and/or if hydraulic segmentation could protect perennial organs from drought stress. Stems and petioles from intact V. vinifera ‘Cabernet Sauvignon’ and V. riparia plants were scanned using Synchrotron-based HRCT, characterizing their vulnerability to embolism and quantifying their ability to refill at different positions along the plant axis (base and apex) in relation to bulk xylem pressure. These data were integrated with other noninvasive techniques assessing leaf hydraulics and transpiration.     

Noninvasive Measurement of Vulnerability to Drought-Induced Embolism by X-Ray Microtomography

Hydraulic failure induced by xylem embolism is one of the primary mechanisms of plant dieback during drought. However, many of the methods used to evaluate the vulnerability of different species to drought-induced embolism are indirect and invasive, increasing the possibility that measurement artifacts may occur. Here, we utilize x-ray computed microtomography (microCT) to directly visualize embolism formation in the xylem of living, intact plants with contrasting wood anatomy (Quercus robur, Populus tremula × Populus alba, and Pinus pinaster). These observations were compared with widely used centrifuge techniques that require destructive sampling. MicroCT imaging provided detailed spatial information regarding the dimensions and functional status of xylem conduits during dehydration. Vulnerability curves based on microCT observations of intact plants closely matched curves based on the centrifuge technique for species with short vessels (P. tremula × P. alba) or tracheids (P. pinaster). For ring porous Q. robur, the centrifuge technique significantly overestimated vulnerability to embolism, indicating that caution should be used when applying this technique to species with long vessels. These findings confirm that microCT can be used to assess the vulnerability to embolism on intact plants by direct visualization.Theory describing the physiological mechanism that allows plants to extract water from the soil and transport it many tens of meters in height has often been the subject of intense debate (Tyree, 2003). Plants have evolved a water-transport system that relies on water sustaining a tensile force; as a result, xylem sap is at negative absolute pressures (Dixon and Joly, 1895; Melcher et al., 1998; Wei et al., 1999). However, this transport mechanism comes with its own set of problems. Most notably, water under tension is prone to cavitation, which results in the formation of gas bubbles (emboli) that block xylem conduits. Embolism reduces the capacity of the xylem tissue to deliver water to the canopy, where it is required to maintain adequate levels of cellular hydration (Tyree and Sperry, 1989). The probability of embolism occurring in the xylem increases during drought, with increasing tension in the xylem sap. During prolonged and severe droughts, xylem embolism can reach lethal levels, causing branch dieback and, ultimately, plant death (Davis et al., 2002; Brodribb and Cochard, 2009; Hoffmann et al., 2011; Choat, 2013; Urli et al., 2013). Water stress-induced embolism is now recognized as one of the principal causes of plant mortality in response to extreme drought events (Anderegg, 2015). In the face of increasingly severe droughts expected with rising global temperatures, hydraulic failure due to embolism has the potential to cause widespread dieback of trees across all major forest biomes (Choat et al., 2012).The majority of techniques used to estimate cavitation resistance are indirect and/or invasive, increasing the possibility of artifacts occurring during measurement (Cochard et al., 2013). Artifacts relating to invasive techniques are particularly relevant in this case, since xylem sap under tension is in a metastable state and may easily vaporize as a result of disturbance. Noninvasive imaging techniques offer the potential to make direct observations of xylem function in intact plants at high resolution and in real time. Noninvasive techniques include magnetic resonance imaging (MRI; Holbrook et al., 2001; Kaufmann et al., 2009; Choat et al., 2010) and, more recently, x-ray computed microtomography (microCT; Brodersen et al., 2010; Charra-Vaskou et al., 2012; McElrone et al., 2012). MicroCT provides superior spatial resolution to MRI, with resolutions below 2 μm attainable for a plant stem of 4 to 5 mm in diameter. This allows for detailed analysis of embolism formation and repair in the xylem, including spatial patterns of embolism spread between conduits (Brodersen et al., 2013; Dalla-Salda et al., 2014).However, noninvasive imaging techniques have seldom been used to validate indirect or invasive techniques used to estimate cavitation resistance. At this stage, only a handful of studies have utilized imaging technology to measure cavitation resistance in trees (Torres-Ruiz et al., 2014; Cochard et al., 2015), and these studies employed a destructive mode of the technique in which small branches were cut off the plant before scanning took place. Thus far, noninvasive imaging on intact plants has only been used to measure cavitation resistance in two species, grapevine (Vitis vinifera; Choat et al., 2010; Brodersen et al., 2013) and Sequoia sempervirens (Choat et al., 2015). Further measurement of cavitation resistance using noninvasive imaging on intact plants across a range of species, therefore, is a high priority.These comparisons are particularly important because of the current debate surrounding the invasive techniques (Cochard et al., 2013). Specifically, evidence from a variety of experiments suggests that centrifuge and air injection techniques underestimate cavitation resistance in species with long xylem vessels (Choat et al., 2010; Cochard et al., 2010; Ennajeh et al., 2011; Martin-StPaul et al., 2014; Torres-Ruiz et al., 2014; Wang et al., 2014). This artifact occurs when samples placed into centrifuge rotors or air injection collars have a large proportion of vessels that are cut open at both ends of the segment. A number of studies have disputed this open-vessel hypothesis and suggested that some versions of the centrifuge and air injection techniques provide reliable estimates of cavitation resistance (Jacobsen and Pratt, 2012; Sperry et al., 2012; Tobin et al., 2013). Because there will always be uncertainties associated with indirect measurements, noninvasive imaging using intact plants provides the best option for resolving these methodological issues.In this study, synchrotron-based microCT was utilized to investigate the formation of drought-induced embolism in the xylem of intact, potted plants. Three species were selected to provide a range of contrasting xylem structures: Quercus robur (ring porous), Populus tremula × Populus alba (diffuse porous), and Pinus pinaster (tracheid bearing). Visualizations of xylem embolism in the stems of these species during a sequence of natural dehydration were used to construct embolism vulnerability curves. We hypothesized that (1) vulnerability curves based on microCT observations would match vulnerability curves based on the centrifuge technique for species with short vessels (P. tremula × P. alba) or tracheid-based xylem (P. pinaster) and (2) the centrifuge technique would overestimate vulnerability to embolism in the long-vesseled species (Q. robur) due to the open-vessel artifact.     

In Vivo Visualizations of Drought-Induced Embolism Spread in Vitis vinifera

Long-distance water transport through plant xylem is vulnerable to hydraulic dysfunction during periods of increased tension on the xylem sap, often coinciding with drought. While the effects of local and systemic embolism on plant water transport and physiology are well documented, the spatial patterns of embolism formation and spread are not well understood. Using a recently developed nondestructive diagnostic imaging tool, high-resolution x-ray computed tomography, we documented the dynamics of drought-induced embolism in grapevine (Vitis vinifera) plants in vivo, producing the first three-dimensional, high-resolution, time-lapse observations of embolism spread. Embolisms formed first in the vessels surrounding the pith at stem water potentials of approximately –1.2 megapascals in drought experiments. As stem water potential decreased, embolisms spread radially toward the epidermis within sectored vessel groupings via intervessel connections and conductive xylem relays, and infrequently (16 of 629 total connections) through lateral connections into adjacent vessel sectors. Theoretical loss of conductivity calculated from the high-resolution x-ray computed tomography images showed good agreement with previously published nuclear magnetic resonance imaging and hydraulic conductivity experiments also using grapevine. Overall, these data support a growing body of evidence that xylem organization is critically important to the isolation of drought-induced embolism spread and confirm that air seeding through the pit membranes is the principle mechanism of embolism spread.Water is transported through the xylem under tension and in a metastable state, making it inherently vulnerable to cavitation, the rapid phase change of liquid water to vapor (Dixon and Joly, 1895; Hayward, 1971; Tyree and Sperry, 1989). The resulting gas embolisms block water transport in the affected xylem vessel. It is widely accepted that embolisms spread between adjacent conduits when the pressure differential between gas-filled and water-filled conduits reaches a critical point where water vapor is aspirated through the pit membrane from the neighboring conduit (Tyree and Sperry, 1989; Tyree and Zimmermann, 2002). The resulting spread of embolisms through the xylem effectively reduces the hydraulic conductivity of the network, impairing the capacity to replace transpired water. The consequences of embolism formation can be dramatic, and it is now considered to be one of the major physiological factors driving reductions in forest primary productivity and drought-induced mortality in woody plants (Anderegg et al., 2012; Choat et al., 2012).Embolism spread between conduits is necessarily dependent on the number and orientation of the interconduit connections, but little is known about the organization of those connections or the spatial dynamics of embolism spread in vivo (Tyree and Zimmermann, 2002; Brodersen et al., 2010). This knowledge gap is largely due to the lack of a nondestructive visualization tool with sufficient resolution to study the propagation and spread of embolism. Previous efforts to visualize embolism in vivo utilized either cryo-scanning electron microscopy (cryo-SEM) or NMR imaging. Cryo-SEM yields fine resolution of frozen plant tissue, revealing the functional status of xylem conduits (i.e. water- or air-filled) at the time of freezing (Canny, 1997; Melcher et al., 2003; Cobb et al., 2007; Mayr et al., 2007; Johnson et al., 2012). Both transverse (Hukin et al., 2005; Sun et al., 2007; Johnson et al., 2012) and longitudinal (Utsumi et al., 1999) cryo-SEM sections have been prepared, but only provide a snapshot of a single point in time and in a single, two-dimensional plane. Similarly, NMR imaging was used in several studies as a nondestructive visualization tool to study the functional status of the xylem in vivo (Holbrook et al., 2001; Clearwater and Clark, 2003). However, the resulting images are typically of insufficient resolution to determine anything other than whether xylem conduits were filled with water or air. Three-dimensional (3D) imaging with NMR is challenging and is not frequently employed (Kuroda et al., 2006). Despite the availability of NMR, studies using this technology are largely focused to the spread of embolism over long periods of time (e.g. weeks [Umebayashi et al., 2011] or months [Pérez-Donoso et al., 2007]) rather than the short-term dynamics of embolism spread over the course of a few hours.Recently, high-resolution x-ray computed tomography (HRCT), a nondestructive diagnostic imaging tool, has been successfully used to study plant tissue in vivo (Brodersen et al., 2010, 2011). Synchrotron-based HRCT is based on the same principles as medical computed tomography systems but yields data with a spatial resolution of less than 5 µm and a temporal resolution of less than 30 min. Brodersen et al. (2011) expanded on this technology to study and map the 3D organization of grapevine (Vitis vinifera) stems and found that the functional status of the xylem could be determined in vivo. Brodersen et al. (2010) visualized the dynamics of embolism repair (i.e. the metabolically active refilling of embolized xylem conduits) in live plants using HRCT, including the growth of water droplets emerging from xylem parenchyma surrounding embolized vessels that eventually led to the dissolution of trapped gas inside the vessels. While we now have a better understanding of embolism repair and the physiological consequences of embolism spread are well documented (Tyree and Zimmermann, 2002; McDowell et al., 2008; Cochard et al., 2009; Zwieniecki and Holbrook, 2009; Choat et al., 2012), the spatial dynamics and biophysics of embolism formation and spread in vivo have yet to be fully explored. Clearly, the spatial organization of xylem conduits plays a critical role in embolism repair and is likely even more influential in embolism spread, as direct connections between conduits are the most likely pathway through the network. Building on these findings and new techniques, we aimed to take advantage of HRCT imaging to provide the first high-resolution visualization of the spread of drought-induced embolism.     

Focus on Water: Down-Regulation of Plasma Intrinsic Protein1 Aquaporin in Poplar Trees Is Detrimental to Recovery from Embolism

During their lifecycles, trees encounter multiple events of water stress that often result in embolism formation and temporal decreases in xylem transport capacity. The restoration of xylem transport capacity requires changes in cell metabolic activity and gene expression. Specifically, in poplar (Populus spp.), the formation of xylem embolisms leads to a clear up-regulation of plasma membrane protein1 (PIP1) aquaporin genes. To determine their role in poplar response to water stress, transgenic Populus tremula × Populus alba plants characterized by the strong down-regulation of multiple isoforms belonging to the PIP1 subfamily were used. Transgenic lines showed that they are more vulnerable to embolism, with 50% percent loss of conductance occurring 0.3 MPa earlier than in wild-type plants, and that they also have a reduced capacity to restore xylem conductance during recovery. Transgenic plants also show symptoms of a reduced capacity to control percent loss of conductance through stomatal conductance in response to drought, because they have a much narrower vulnerability safety margin. Finally, a delay in stomatal conductance recovery during the period of stress relief was observed. The presented results suggest that PIP1 genes are involved in the maintenance of xylem transport system capacity, in the promotion of recovery from stress, and in contribution to a plant’s control of stomatal conductance under water stress.Long-distance water transport in vascular plants occurs in a conduit network of nonliving cells connecting roots to leaves (Sperry, 2003). Often under drought conditions, the water column within the lumen of xylem vessels or tracheids can be subjected to tensions that result in cavitation and the subsequent formation of embolisms (Holbrook and Zwieniecki, 2008). This hydraulic failure within the xylem network can cause tissue damage, loss of plant productivity, and ultimately, plant death (Tyree and Sperry, 1989; Sperry et al., 1998; Zwieniecki and Holbrook, 2009). Plants have evolved several strategies to prevent and/or mitigate the effects of hydraulic failure caused by embolism and restore xylem transport capacity after embolism occurs (Stiller and Sperry, 2002; Nardini et al., 2011; Secchi and Zwieniecki, 2012). These strategies include passive, often long-term responses, like the growth of new vessels/tracheids or dieback followed by the growth of new shoots (shrubs), or active, often fast responses that result in the restoration of hydraulic conductivity by (1) creating positive pressure through root or stem pressure in the complete transport system (xylem level; Cochard et al., 1994; Ewers et al., 1997; Yang et al., 2012) or (2) enabling positive pressures in specific, embolized conduits, despite negative pressure in the surrounding xylem (conduit level; Salleo et al., 2004; Nardini et al., 2011; Brodersen and McElrone, 2013).Although embolism formation is a purely physical process related to the degree of tension in the water column and a wood’s physicochemical properties (Brennen, 1995; Tyree and Zimmermann, 2002), embolism removal requires that empty vessels fill with water against existing energy gradients as the bulk of water in the xylem remains under tension caused by transpiration. Thus, recovery from embolism cannot happen spontaneously and necessitates some physiological activities that promote water flow into embolized vessels (Holbrook and Zwieniecki, 1999; Thomas Tyree et al., 1999; Salleo et al., 2004; Zwieniecki and Holbrook, 2009; Secchi et al., 2011). Visual evidence from cryo-scanning electron microscopy studies, magnetic resonance imaging observations, and computed tomography scans showed that water (xylem sap) can return to empty vessels, suggesting that plants do have the ability to restore functionality in the xylem (Holbrook et al., 2001; Clearwater and Goldstein, 2005; Scheenen et al., 2007). Brodersen et al. (2010) showed that water droplets preferentially form on the vessel walls adjacent to parenchyma cells and that these droplets grow until the lumen completely refills. In addition, scientific support for the existence of embolism/refilling cycles in intact stems of Acer rubrum are provided using magnetic resonance imaging (Zwieniecki et al., 2013). Droplet formation on the walls of empty vessels that are in contact with parenchyma cells support predictions that these living cells supply both water and energy to drive the restoration of xylem hydraulic function.Processes related to water transport across the cellular membrane involve plasma intrinsic protein (PIP; aquaporins) moderators, and thus, the role of PIPs must be considered when contemplating how plants recover from embolism formation. Plant aquaporins show a great diversity and are classified into five major homologous groups that reflect specific subcellular localizations (Prado and Maurel, 2013). Among different aquaporin gene families (26-like intrinsic proteins, tonoplast intrinsic proteins, X unrecognized intrinsic proteins, small basic intrinsic proteins, and PIPs; Danielson and Johanson, 2008), the PIPs represent the largest number of members and can be further divided into two subfamilies, PIP1 and PIP2. There is a large body of evidence that aquaporins from the PIP2 subfamily contribute to water transport. The generation of data has been multidisciplinary and involved the use of chemical blockers, the down-regulation and up-regulation of genes in plants, and the expression of these proteins in oocytes (Hukin et al., 2002; Postaire et al., 2010; Shatil-Cohen et al., 2011). Expression levels of several PIP and TIP members change after the dynamic of increasing water stress and recovery in many woody plants, including walnut (Juglans regia), poplar (Populus trichocarpa.), and grapevine Vitis vinifera; (Sakr et al., 2003; Secchi et al., 2011; Perrone et al., 2012a, 2012b; Laur and Hacke, 2013; Pou et al., 2013). Furthermore, an increase in the expression of PIP2.1 and PIP2.2 genes was observed in vessel-associated parenchyma cells in walnuts at the same time that recovery from embolism was taking place (Sakr et al., 2003). The role of genes from the PIP1 subfamily in tree responses to water stress is less well-understood. PIP1s were shown to have little to no water channel activity when expressed in oocytes on their own. However, coexpression of PIP1.1 proteins with an isoform from the PIP2 subfamily led to higher membrane permeability than that observed with the expression of a single PIP2 protein (Fetter et al., 2004; Secchi and Zwieniecki, 2010). With respect to their role in mediating water stress, it was shown that the expression level of several PIP1 genes in poplar changed significantly during the onset of stress, during recovery, during the formation of embolisms after water stress, and under no stress conditions but with induced embolism, whereas the expression of PIP2 genes remained mostly unresponsive (Secchi and Zwieniecki, 2010; Secchi et al., 2011; Secchi and Zwieniecki, 2011).Despite significant effort invested in elucidating the contribution of aquaporins to the regulation of xylem hydraulic capacity throughout the progression of drought and recovery from water stress, evidence of their active role in vivo is only partially confirmed. Genetic approaches provide a reliable and effective strategy for determining the physiological function of aquaporin genes in plant water relations. However, most studies thus far have been conducted on herbaceous plants (Kaldenhoff et al., 1998; Postaire et al., 2010). For example, Arabidopsis (Arabidopsis thaliana) plants expressing PIP antisense genes exhibit an impaired ability to recover from water stress (Martre et al., 2002), and knockout mutants exhibit reduced leaf hydraulic conductivity (Da Ines et al., 2010). The Nicotiana tabacum aquaporin1 (NtAQP1) down-regulated tobacco plants show reduced root hydraulic conductivity and lower water stress resistance (Siefritz et al., 2002). RNA technology, although not often used for woody plants, has been adapted for grapevine (Perrone et al., 2012a, 2012b) and Eucalyptus spp. trees (Tsuchihira et al., 2010); in both cases, analysis focused on overexpressing specific isoforms of aquaporin genes. The PIP2;4 root-specific aquaporin enhanced water transport in transformed Vitis spp. plants under well-watered conditions but not under water stress (Perrone et al., 2012a, 2012b), whereas Eucalyptus spp. hybrid clones overexpressing two Raphanus sativus genes (RsPIP1;1 and RsPIP2;1) did not display any increase in drought tolerance (Tsuchihira et al., 2010). To date, no research on the recovery from embolism formation in woody plants with impaired aquaporin expression has been conducted.In this study, we used poplar transgenic plants characterized by a strong down-regulation of PIP1 genes to test the role of this aquaporin subfamily in the plant response to water stress and subsequent recovery from stress. Although transformed poplars did not show morphologically different phenotypes compared with wild-type plants, they were found to be more sensitive to imposed water stress, resulting in increased vulnerability to embolism formation and the loss of stomatal conductance. We also noted a reduced capacity of transformed plants to restore xylem water transport.     

Mechanical Failure of Fine Root Cortical Cells Initiates Plant Hydraulic Decline during Drought

Root systems perform the crucial task of absorbing water from the soil to meet the demands of a transpiring canopy. Roots are thought to operate like electrical fuses, which break when carrying an excessive load under conditions of drought stress. Yet the exact site and sequence of this dysfunction in roots remain elusive. Using in vivo x-ray computed microtomography, we found that drought-induced mechanical failure (i.e. lacunae formation) in fine root cortical cells is the initial and primary driver of reduced fine root hydraulic conductivity (Lp_r) under mild to moderate drought stress. Cortical lacunae started forming under mild drought stress (−0.6 MPa Ψ_stem), coincided with a dramatic reduction in Lp_r, and preceded root shrinkage or significant xylem embolism. Only under increased drought stress was embolism formation observed in the root xylem, and it appeared first in the fine roots (50% loss of hydraulic conductivity [P₅₀] reached at −1.8 MPa) and then in older, coarse roots (P₅₀ = −3.5 MPa). These results suggest that cortical cells in fine roots function like hydraulic fuses that decouple plants from drying soil, thus preserving the hydraulic integrity of the plant’s vascular system under early stages of drought stress. Cortical lacunae formation led to permanent structural damage of the root cortex and nonrecoverable Lp_r, pointing to a role in fine root mortality and turnover under drought stress.Root systems of woody plants consist of both coarse and nonwoody fine roots. Fine roots can constitute as little as 1% of the total root surface area (Kramer and Bullock, 1966), yet are critically important for biogeochemical cycling in terrestrial ecosystems as they constitute the primary exchange surface between plants and soil (Jackson et al., 1997). They are responsible for the vast majority of water absorption in woody root systems (Gambetta et al., 2013; Kramer and Boyer, 1995; Kramer and Bullock, 1966) and mediate backward flow of water from a plant to the soil via a process called hydraulic redistribution, which can alter regional climate (Richards and Caldwell, 1987; Lee et al., 2005). Fine roots also modify the soil through carbon exudation and stimulation of microbial activity (McCormack et al., 2015), and their production and annual turnover represent 33% of global net primary productivity (Jackson et al., 1997; McCormack et al., 2015). Elucidating details of fine root function and responses to stress can thus improve our understanding of how these plant organs can influence ecosystem carbon, nutrient, and water cycles.Fine roots are traditionally defined as all roots <2-mm diameter, but recent work has emphasized the need to delineate this diameter class into distinct functional groups. By separating fine roots into a shorter-lived absorptive pool and a longer-lived transport pool, McCormack et al. (2015) showed that fine root functionality can alter estimates of global net primary productivity by 30%. This work highlights our still-limited understanding of fine root functionality, the mechanisms underlying their lifespan and turnover, and how those traits respond to abiotic stress (Lukac, 2012; Tierney and Fahey, 2002; Guo et al., 2008). Fine root mortality during drought has been linked to increased root respiration and inhibited photosynthate transport to roots (e.g. Marshall, 1986) but could also be attributed to hydraulic dysfunction (Jackson et al., 2000). Portions of root systems are thought to operate analogously to a hydraulic fuse in an electrical circuit and designed to fail hydraulically when carrying excessive current under drought stress (Zimmermann, 1983; Jackson et al., 2000). However, the exact location of these hydraulic fuses in the root system has yet to be identified. Axial water transport in the xylem is considered a weak link, as roots of numerous species have been shown to be more susceptible to drought-induced xylem embolism compared to other organs within the same plant (i.e. trunks, stems, tap roots; Alder et al., 1996; Hacke and Sauter, 1996; McElrone et al., 2004; Pratt et al., 2015; Johnson et al., 2016). Moreover, Sperry and Ikeda (1997) found that smaller roots were the most vulnerable plant organ to xylem embolism, which would localize failure to inexpensive, distal, and easily replaceable portions of a root system. Such a design is considered effective, because it is widely assumed that the hydraulic capacity of smaller distal roots is readily repaired upon rewatering via xylem embolism removal (Domec et al., 2006; Jackson et al., 2000). While much work has demonstrated that xylem embolism reduces hydraulic capacity under severe drought stress (Brodribb et al., 2016a, 2016b; Choat et al., 2012), its contribution under mild to moderate stress is less clear (Choat et al., 2016; Cochard and Delzon, 2013; Cochard et al., 2013; McElrone et al., 2012; Wheeler et al., 2013; Choat et al., 2010; Torres-Ruiz et al., 2015). Work is still needed to resolve the location and sequence of root hydraulic dysfunction under drought and what tissues are involved in each stage of this process, especially under mild stress where fine root hydraulic conductivity (Lp_r) is known to decrease dramatically (Aroca et al., 2012).Before entering the xylem for long distance transport, water absorbed by roots must traverse a series of cell layers that include the epidermis, cortex, and endodermis (Steudle and Peterson 1998). Hydraulic resistance is much greater along this radial pathway compared to the axial transport pathway in the xylem (e.g. Frensch and Hsiao, 1993; Frensch and Steudle, 1989). The resistance differential between radial and axial pathways persists or increases in magnitude as fine Lp_r decreases under mild to moderate drought stress. While decreased fine root permeability under drought has been attributed to root shrinkage (Passioura, 1988; Nobel and Cui, 1992), changes in membrane permeability via aquaporins (Maurel et al., 2015; Aroca et al., 2012; North, 2004), development of suberized apoplastic barriers over longer periods of drought (Barrios-Masias et al., 2015; North and Nobel, 1991), or mechanical damage in cortical cells (i.e. lacunae formation; North and Nobel, 1991), the integration of these responses particularly under mild stress is still lacking. Elucidating the physiological mechanism that drives this response could help to resolve long-standing questions about fine root functionality, lifespan, and turnover.Here, we originally aimed to study whether fine roots function as the primary hydraulic fuse that disconnects a plant from drying soil. We studied the sequence of events during soil drying from saturated to severe drought conditions in coarse and fine roots of grapevines, which are considered a model species and have long been characterized as highly susceptible to drought-induced embolism. While performing these experiments, we discovered that the fine root cortex was radically changing under mild drought stress that preceded any embolism formation. We then performed hydraulic measurements and fluorescence light microscopy to investigate how fine Lp_r is affected by the formation of cortical lacunae that should significantly increase radial hydraulic resistance to flow.     

Tyloses and Phenolic Deposits in Xylem Vessels Impede Water Transport in Low-Lignin Transgenic Poplars: A Study by Cryo-Fluorescence Microscopy

Of 14 transgenic poplar genotypes (Populus tremula × Populus alba) with antisense 4-coumarate:coenzyme A ligase that were grown in the field for 2 years, five that had substantial lignin reductions also had greatly reduced xylem-specific conductivity compared with that of control trees and those transgenic events with small reductions in lignin. For the two events with the lowest xylem lignin contents (greater than 40% reduction), we used light microscopy methods and acid fuchsin dye ascent studies to clarify what caused their reduced transport efficiency. A novel protocol involving dye stabilization and cryo-fluorescence microscopy enabled us to visualize the dye at the cellular level and to identify water-conducting pathways in the xylem. Cryo-fixed branch segments were planed in the frozen state on a sliding cryo-microtome and observed with an epifluorescence microscope equipped with a cryo-stage. We could then distinguish clearly between phenolic-occluded vessels, conductive (stain-filled) vessels, and nonconductive (water- or gas-filled) vessels. Low-lignin trees contained areas of nonconductive, brown xylem with patches of collapsed cells and patches of noncollapsed cells filled with phenolics. In contrast, phenolics and nonconductive vessels were rarely observed in normal colored wood of the low-lignin events. The results of cryo-fluorescence light microscopy were supported by observations with a confocal microscope after freeze drying of cryo-planed samples. Moreover, after extraction of the phenolics, confocal microscopy revealed that many of the vessels in the nonconductive xylem were blocked with tyloses. We conclude that reduced transport efficiency of the transgenic low-lignin xylem was largely caused by blockages from tyloses and phenolic deposits within vessels rather than by xylem collapse.Secondary xylem in woody plants is a complex vascular tissue that functions in mechanical support, conduction, storage, and protection (Carlquist, 2001; Tyree and Zimmermann, 2002). The xylem must provide a sufficient and safe water supply throughout the entire pathway from roots to leaves for transpiration and photosynthesis. It is well established that enhanced water conductivity of xylem can increase total plant carbon gain (Domec and Gartner, 2003; Santiago et al., 2004; Brodribb and Holbrook, 2005a). According to the Hagen-Poiseuille equation, xylem conductivity should scale with vessel lumen diameter to the fourth power (Tyree and Zimmermann, 2002). Indeed, xylem conductivity largely depends on anatomical features, including conduit diameters and frequencies (Salleo et al., 1985; McCulloh and Sperry, 2005). However, there are hydraulic limits to maximum vessel diameters, because xylem conduits have to withstand the strong negative pressures of the transpiration stream that could cause cell collapse or embolisms within vessels that are structurally inadequate to withstand these forces (Tyree and Sperry, 1989; Lo Gullo et al., 1995; Hacke et al., 2000). To some extent, stomatal regulation of transpiration limits the negative pressures that the xylem experiences (Tardieu and Davies, 1993; Cochard et al., 2002; Meinzer, 2002; Brodribb and Holbrook, 2004; Buckley, 2005; Franks et al., 2007; Woodruff et al., 2007). Nevertheless, plants rely on an array of structural reinforcements of xylem to ensure the safety of water transport. The size of xylem elements, vessel redundancy, intervessel pit and membrane geometries, and the thickness, microstructure, and chemical composition of cell walls are among the features that regulate tradeoffs between efficiency and safety of xylem water transport (Baas and Schweingruber, 1987; Hacke et al., 2001; Domec et al., 2006; Ewers et al., 2007; Choat et al., 2008).The xylem cell wall is made up of cellulose bundles that are hydrogen bonded with hemicelluloses, which are in turn embedded within a lignin matrix (Mansfield, 2009; Salmén and Burgert, 2009). Besides providing this matrix for the cell wall itself, lignin is thought to contribute to many of the mechanical and physical characteristics of wood as well as conferring passive resistance to the spread of pathogens within a plant (Niklas, 1992; Boyce et al., 2004; Davin et al., 2008). Lignin typically represents 20% to 30% of the dry mass of wood and therefore is among the most abundant stores of carbon in the biosphere (Zobel and van Buijtenen, 1989). The complex molecular structure and biosynthetic pathway of various types of lignins have been studied extensively (Boerjan et al., 2003; Ralph et al., 2004, 2007; Higuchi, 2006; Boudet, 2007; Davin et al., 2008). The monomeric composition of lignin varies between different cell types of the same species depending on the functional specialization of the cell (Yoshinaga et al., 1992; Watanabe et al., 2004; Xu et al., 2006). The composition and amount of lignin in wild plants varies in response to climatic conditions (Donaldson, 2002) or gravitational and mechanical demands (Pruyn et al., 2000; Kern et al., 2005; Rüggeberg et al., 2008). It is clear that plants are capable of regulating the lignification pattern in differentiating cells, which provides them with flexibility for responding to environmental stresses (Donaldson, 2002; Koehler and Telewski, 2006; Ralph et al., 2007; for review, see Vanholme et al., 2008).Whereas some level of lignin is a requisite for all vascular plants, it is often an unwanted product in the pulp and paper industry because it increases the costs of paper production and associated water treatments necessary for environmental protection (Chen et al., 2001; Baucher et al., 2003; Peter et al., 2007). Reducing the lignin content of the raw biomass material may allow more efficient hydrolysis of polysaccharides in biomass and thus facilitate the production of biofuel (Chen and Dixon, 2007). With the ultimate goal of development of wood for more efficient processing, much research has been aimed at the production of genetically modified trees with altered lignin biosynthesis (Boerjan et al., 2003; Boudet et al., 2003; Li et al., 2003; Halpin, 2004; Ralph et al., 2004, 2008; Chiang, 2006; Coleman et al., 2008a, 2008b; Vanholme et al., 2008; Wagner et al., 2009). It is now technically possible to achieve more than 50% reductions of lignin content in xylem of poplar (Populus spp.; Leplé et al., 2007; Coleman et al., 2008a, 2008b), but the consequences of such reduction on plant function have received relatively little attention (Koehler and Telewski, 2006). In-depth studies on the xylem structure and functional performance of transgenic plants with low lignin are limited, despite the need to assess their long-term sustainability for large-scale production (Anterola and Lewis, 2002; Hancock et al., 2007; Coleman et al., 2008b, Voelker, 2009; Horvath et al., 2010).Genetically modified plants are suitable models for studying fundamental questions of the physiological role of lignin because of the possibility of controlling lignification without the confounding effects encountered when comparing across plant tissues or stages of development (Koehler and Telewski, 2006; Leplé et al., 2007; Coleman et al., 2008b). Research on Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum) has shown that down-regulation of lignin biosynthesis can have diverse effects on plant metabolism and structure, including changes in the lignin amount and composition (p-hydroxyphenyl/guaiacyl/syringyl units ratio) as well as the collapse of xylem vessel elements (Lee et al., 1997; Sewalt et al., 1997; Piquemal et al., 1998; Chabannes et al., 2001; Jones et al., 2001; Franke et al., 2002; Dauwe et al., 2007). Among temperate hardwoods, poplar has been established as a model tree for genetic manipulations because of its ecological and economic importance, fast growth, ease of vegetative propagation, and its widespread use in traditional breeding programs (Bradshaw et al., 2001; Brunner et al., 2004). The question of how manipulation of lignin can affect the anatomy and physiological function of xylem in poplar has been addressed in part by several research groups (Anterola and Lewis, 2002; Boerjan et al., 2003; Leplé et al., 2007; Coleman et al., 2008b). Some studies that involved large lignin reductions reported no significant alterations in the xylem anatomy (Hu et al., 1999; Li et al., 2003). However, in many other experiments, reduced total lignin content was associated with significant growth retardation, alterations in the lignin monomer composition, irregularities in the xylem structure (Anterola and Lewis, 2002; Leplé et al., 2007; Coleman et al., 2008b), and the patchy occurrence of collapsed xylem cells (Coleman et al., 2008b; Voelker, 2009). Furthermore, severely down-regulated lignin biosynthesis has resulted in greatly reduced xylem water-transport efficiency (Coleman et al., 2008b; Lachenbruch et al., 2009; Voelker, 2009). It is generally assumed that the reduced water transport ability of xylem with very low lignin contents is caused by collapsed conduits and/or increased embolism due to the entry of air bubbles into the water-conducting cells (Coleman et al., 2008b; Wagner et al., 2009), but detailed anatomical investigations of the causes of impaired xylem conductivity of low-lignin trees are lacking. Analysis of the anatomical basis for the properties of xylem conduits in plants with genetically manipulated amounts and composition of lignin can provide a deeper understanding of the physiological role of lignin as well as the lower limit of down-regulation of lignin biosynthesis at which trees can still survive within natural environments.One of the approaches for the suppression of lignin biosynthesis is down-regulation of 4-coumarate:coenzyme A ligase (4CL), an enzyme that functions in phenylpropanoid metabolism by producing the monolignol precursor p-coumaroyl-CoA (Kajita et al.,1997; Allina et al., 1998; Hu et al., 1998; Harding et al., 2002; Jia et al., 2004; Costa et al., 2005; Friedmann et al., 2007; Wagner et al., 2009). In a 2-year-long field trial on the physiological performance of poplar (Populus tremula × Populus alba) transgenic clones, out of 14 genotypes with altered lignin biosynthesis (down-regulated 4CL), five showed dramatically reduced wood-specific conductivity (k_s) compared with that of control trees (Voelker, 2009). Those mutants with the severely reduced k_s were also characterized by having the lowest wood lignin contents (up to an approximately 40% reduction) in the study. Trees with transgenic events characterized by the formation of abnormally brown wood exhibited regular branch dieback at the end of the growing season, despite having been regularly watered (Voelker, 2009). Our objective was to identify the structural features responsible for reduced transport efficiency in the xylem of transgenic poplars with extremely low lignin contents. We employed fluorescence and laser scanning confocal microscopy for anatomical analyses of xylem structure as well as dye-flow experiments followed by cryo-fluorescence microscopy to visualize the functioning water-conductive pathways in xylem at the cellular level. We report the frequent occurrence of tyloses and phenolic depositions in xylem vessels of strongly down-regulated trees that may be the cause of their reduced xylem conductivity.     

Calcineurin B-Like Protein-Interacting Protein Kinase CIPK21 Regulates Osmotic and Salt Stress Responses in Arabidopsis

The role of calcium-mediated signaling has been extensively studied in plant responses to abiotic stress signals. Calcineurin B-like proteins (CBLs) and CBL-interacting protein kinases (CIPKs) constitute a complex signaling network acting in diverse plant stress responses. Osmotic stress imposed by soil salinity and drought is a major abiotic stress that impedes plant growth and development and involves calcium-signaling processes. In this study, we report the functional analysis of CIPK21, an Arabidopsis (Arabidopsis thaliana) CBL-interacting protein kinase, ubiquitously expressed in plant tissues and up-regulated under multiple abiotic stress conditions. The growth of a loss-of-function mutant of CIPK21, cipk21, was hypersensitive to high salt and osmotic stress conditions. The calcium sensors CBL2 and CBL3 were found to physically interact with CIPK21 and target this kinase to the tonoplast. Moreover, preferential localization of CIPK21 to the tonoplast was detected under salt stress condition when coexpressed with CBL2 or CBL3. These findings suggest that CIPK21 mediates responses to salt stress condition in Arabidopsis, at least in part, by regulating ion and water homeostasis across the vacuolar membranes.Drought and salinity cause osmotic stress in plants and severely affect crop productivity throughout the world. Plants respond to osmotic stress by changing a number of cellular processes (Xiong et al., 1999; Xiong and Zhu, 2002; Bartels and Sunkar, 2005; Boudsocq and Lauriére, 2005). Some of these changes include activation of stress-responsive genes, regulation of membrane transport at both plasma membrane (PM) and vacuolar membrane (tonoplast) to maintain water and ionic homeostasis, and metabolic changes to produce compatible osmolytes such as Pro (Stewart and Lee, 1974; Krasensky and Jonak, 2012). It has been well established that a specific calcium (Ca²⁺) signature is generated in response to a particular environmental stimulus (Trewavas and Malhó, 1998; Scrase-Field and Knight, 2003; Luan, 2009; Kudla et al., 2010). The Ca²⁺ changes are primarily perceived by several Ca²⁺ sensors such as calmodulin (Reddy, 2001; Luan et al., 2002), Ca²⁺-dependent protein kinases (Harper and Harmon, 2005), calcineurin B-like proteins (CBLs; Luan et al., 2002; Batistič and Kudla, 2004; Pandey, 2008; Luan, 2009; Sanyal et al., 2015), and other Ca²⁺-binding proteins (Reddy, 2001; Shao et al., 2008) to initiate various cellular responses.Plant CBL-type Ca²⁺ sensors interact with and activate CBL-interacting protein kinases (CIPKs) that phosphorylate downstream components to transduce Ca²⁺ signals (Liu et al., 2000; Luan et al., 2002; Batistič and Kudla, 2004; Luan, 2009). In several plant species, multiple members have been identified in the CBL and CIPK family (Luan et al., 2002; Kolukisaoglu et al., 2004; Pandey, 2008; Batistič and Kudla, 2009; Weinl and Kudla, 2009; Pandey et al., 2014). Involvement of specific CBL-CIPK pair to decode a particular type of signal entails the alternative and selective complex formation leading to stimulus-response coupling (D’Angelo et al., 2006; Batistič et al., 2010).Several CBL and CIPK family members have been implicated in plant responses to drought, salinity, and osmotic stress based on genetic analysis of Arabidopsis (Arabidopsis thaliana) mutants (Zhu, 2002; Cheong et al., 2003, 2007; Kim et al., 2003; Pandey et al., 2004, 2008; D’Angelo et al., 2006; Qin et al., 2008; Tripathi et al., 2009; Held et al., 2011; Tang et al., 2012; Drerup et al., 2013; Eckert et al., 2014). A few CIPKs have also been functionally characterized by gain-of-function approach in crop plants such as rice (Oryza sativa), pea (Pisum sativum), and maize (Zea mays) and were found to be involved in osmotic stress responses (Mahajan et al., 2006; Xiang et al., 2007; Yang et al., 2008; Tripathi et al., 2009; Zhao et al., 2009; Cuéllar et al., 2010).In this report, we examined the role of the Arabidopsis CIPK21 gene in osmotic stress response by reverse genetic analysis. The loss-of-function mutant plants became hypersensitive to salt and mannitol stress conditions, suggesting that CIPK21 is involved in the regulation of osmotic stress response in Arabidopsis. These findings are further supported by an enhanced tonoplast targeting of the cytoplasmic CIPK21 through interaction with the vacuolar Ca²⁺ sensors CBL2 and CBL3 under salt stress condition.     

Focus on Ethylene: Ethylene and the Regulation of Physiological and Morphological Responses to Nutrient Deficiencies

To cope with nutrient deficiencies, plants develop both morphological and physiological responses. The regulation of these responses is not totally understood, but some hormones and signaling substances have been implicated. It was suggested several years ago that ethylene participates in the regulation of responses to iron and phosphorous deficiency. More recently, its role has been extended to other deficiencies, such as potassium, sulfur, and others. The role of ethylene in so many deficiencies suggests that, to confer specificity to the different responses, it should act through different transduction pathways and/or in conjunction with other signals. In this update, the data supporting a role for ethylene in the regulation of responses to different nutrient deficiencies will be reviewed. In addition, the results suggesting the action of ethylene through different transduction pathways and its interaction with other hormones and signaling substances will be discussed.When plants suffer from a mineral nutrient deficiency, they develop morphological and physiological responses (mainly in their roots) aimed to facilitate the uptake and mobilization of the limiting nutrient. After the nutrient has been acquired in enough quantity, these responses need to be switched off to avoid toxicity and conserve energy. In recent years, different plant hormones (e.g. ethylene, auxin, cytokinins, jasmonic acid, abscisic acid, brassinosteroids, GAs, and strigolactones) have been implicated in the regulation of these responses (Romera et al., 2007, 2011, 2015; Liu et al., 2009; Rubio et al., 2009; Kapulnik et al., 2011; Kiba et al., 2011; Iqbal et al., 2013; Zhang et al., 2014).Before the 1990s, there were several publications relating ethylene and nutrient deficiencies (cited in Lynch and Brown [1997] and Romera et al. [1999]) without establishing a direct implication of ethylene in the regulation of nutrient deficiency responses. In 1994, Romera and Alcántara (1994) published an article in Plant Physiology suggesting a role for ethylene in the regulation of Fe deficiency responses. In 1999, Borch et al. (1999) showed the participation of ethylene in the regulation of P deficiency responses. Since then, evidence has been accumulating in support of a role for ethylene in the regulation of both Fe (Romera et al., 1999, 2015; Waters and Blevins, 2000; Lucena et al., 2006; Waters et al., 2007; García et al., 2010, 2011, 2013, 2014; Yang et al., 2014) and P deficiency responses (Kim et al., 2008; Lei et al., 2011; Li et al., 2011; Nagarajan and Smith, 2012; Wang et al., 2012, 2014c). Both Fe and P may be poorly available in most soils, and plants develop similar responses under their deficiencies (Romera and Alcántara, 2004; Zhang et al., 2014). More recently, a role for ethylene has been extended to other deficiencies, such as K (Shin and Schachtman, 2004; Jung et al., 2009; Kim et al., 2012), S (Maruyama-Nakashita et al., 2006; Wawrzyńska et al., 2010; Moniuszko et al., 2013), and B (Martín-Rejano et al., 2011). Ethylene has also been implicated in both N deficiency and excess (Tian et al., 2009; Mohd-Radzman et al., 2013; Zheng et al., 2013), and its participation in Mg deficiency has been suggested (Hermans et al., 2010).In this update, we will review the information supporting a role for ethylene in the regulation of different nutrient deficiency responses. For information relating ethylene to other aspects of plant mineral nutrition, such as N₂ fixation and responses to excess of nitrate or essential heavy metals, the reader is referred to other reviews (for review, see Maksymiec, 2007; Mohd-Radzman et al., 2013; Steffens, 2014).     

Surveying Rubisco Diversity and Temperature Response to Improve Crop Photosynthetic Efficiency

The threat to global food security of stagnating yields and population growth makes increasing crop productivity a critical goal over the coming decades. One key target for improving crop productivity and yields is increasing the efficiency of photosynthesis. Central to photosynthesis is Rubisco, which is a critical but often rate-limiting component. Here, we present full Rubisco catalytic properties measured at three temperatures for 75 plants species representing both crops and undomesticated plants from diverse climates. Some newly characterized Rubiscos were naturally “better” compared to crop enzymes and have the potential to improve crop photosynthetic efficiency. The temperature response of the various catalytic parameters was largely consistent across the diverse range of species, though absolute values showed significant variation in Rubisco catalysis, even between closely related species. An analysis of residue differences among the species characterized identified a number of candidate amino acid substitutions that will aid in advancing engineering of improved Rubisco in crop systems. This study provides new insights on the range of Rubisco catalysis and temperature response present in nature, and provides new information to include in models from leaf to canopy and ecosystem scale.In a changing climate and under pressure from a population set to hit nine billion by 2050, global food security will require massive changes to the way food is produced, distributed, and consumed (Ort et al., 2015). To match rising demand, agricultural production must increase by 50 to 70% in the next 35 years, and yet the gains in crop yields initiated by the green revolution are slowing, and in some cases, stagnating (Long and Ort, 2010; Ray et al., 2012). Among a number of areas being pursued to increase crop productivity and food production, improving photosynthetic efficiency is a clear target, offering great promise (Parry et al., 2007; von Caemmerer et al., 2012; Price et al., 2013; Ort et al., 2015). As the gatekeeper of carbon entry into the biosphere and often acting as the rate-limiting step of photosynthesis, Rubisco, the most abundant enzyme on the planet (Ellis, 1979), is an obvious and important target for improving crop photosynthetic efficiency.Rubisco is considered to exhibit comparatively poor catalysis, in terms of catalytic rate, specificity, and CO₂ affinity (Tcherkez et al., 2006; Andersson, 2008), leading to the suggestion that even small increases in catalytic efficiency may result in substantial improvements to carbon assimilation across a growing season (Zhu et al., 2004; Parry et al., 2013; Galmés et al., 2014a; Carmo-Silva et al., 2015). If combined with complimentary changes such as optimizing other components of the Calvin Benson or photorespiratory cycles (Raines, 2011; Peterhansel et al., 2013; Simkin et al., 2015), optimized canopy architecture (Drewry et al., 2014), or introducing elements of a carbon concentrating mechanism (Furbank et al., 2009; Lin et al., 2014a; Hanson et al., 2016; Long et al., 2016), Rubisco improvement presents an opportunity to dramatically increase the photosynthetic efficiency of crop plants (McGrath and Long, 2014; Long et al., 2015; Betti et al., 2016). A combination of the available strategies is essential for devising tailored solutions to meet the varied requirements of different crops and the diverse conditions under which they are typically grown around the world.Efforts to engineer an improved Rubisco have not yet produced a “super Rubisco” (Parry et al., 2007; Ort et al., 2015). However, advances in engineering precise changes in model systems continue to provide important developments that are increasing our understanding of Rubisco catalysis (Spreitzer et al., 2005; Whitney et al., 2011a, 2011b; Morita et al., 2014; Wilson et al., 2016), regulation (Andralojc et al., 2012; Carmo-Silva and Salvucci, 2013; Bracher et al., 2015), and biogenesis (Saschenbrecker et al., 2007; Whitney and Sharwood, 2008; Lin et al., 2014b; Hauser et al., 2015; Whitney et al., 2015).A complementary approach is to understand and exploit Rubisco natural diversity. Previous characterization of Rubisco from a limited number of species has not only demonstrated significant differences in the underlying catalytic parameters, but also suggests that further undiscovered diversity exists in nature and that the properties of some of these enzymes could be beneficial if present in crop plants (Carmo-Silva et al., 2015). Recent studies clearly illustrate the variation possible among even closely related species (Galmés et al., 2005, 2014b, 2014c; Kubien et al., 2008; Andralojc et al., 2014; Prins et al., 2016).Until recently, there have been relatively few attempts to characterize the consistency, or lack thereof, of temperature effects on in vitro Rubisco catalysis (Sharwood and Whitney, 2014), and often studies only consider a subset of Rubisco catalytic properties. This type of characterization is particularly important for future engineering efforts, enabling specific temperature effects to be factored into any attempts to modify crops for a future climate. In addition, the ability to coanalyze catalytic properties and DNA or amino acid sequence provides the opportunity to correlate sequence and biochemistry to inform engineering studies (Christin et al., 2008; Kapralov et al., 2011; Rosnow et al., 2015). While the amount of gene sequence information available grows rapidly with improving technology, knowledge of the corresponding biochemical variation resulting has yet to be determined (Cousins et al., 2010; Carmo-Silva et al., 2015; Sharwood and Whitney, 2014; Nunes-Nesi et al., 2016).This study aimed to characterize the catalytic properties of Rubisco from diverse species, comprising a broad range of monocots and dicots from diverse environments. The temperature dependence of Rubisco catalysis was evaluated to tailor Rubisco engineering for crop improvement in specific environments. Catalytic diversity was analyzed alongside the sequence of the Rubisco large subunit gene, rbcL, to identify potential catalytic switches for improving photosynthesis and productivity. In vitro results were compared to the average temperature of the warmest quarter in the regions where each species grows to investigate the role of temperature in modulating Rubisco catalysis.