Spatiotemporal Regulation of Lateral Root Organogenesis in Arabidopsis by Cytokinin

The architecture of a plant’s root system, established postembryonically, results from both coordinated root growth and lateral root branching. The plant hormones auxin and cytokinin are central endogenous signaling molecules that regulate lateral root organogenesis positively and negatively, respectively. Tight control and mutual balance of their antagonistic activities are particularly important during the early phases of lateral root organogenesis to ensure continuous lateral root initiation (LRI) and proper development of lateral root primordia (LRP). Here, we show that the early phases of lateral root organogenesis, including priming and initiation, take place in root zones with a repressed cytokinin response. Accordingly, ectopic overproduction of cytokinin in the root basal meristem most efficiently inhibits LRI. Enhanced cytokinin responses in pericycle cells between existing LRP might restrict LRI near existing LRP and, when compromised, ectopic LRI occurs. Furthermore, our results demonstrate that young LRP are more sensitive to perturbations in the cytokinin activity than are developmentally more advanced primordia. We hypothesize that the effect of cytokinin on the development of primordia possibly depends on the robustness and stability of the auxin gradient.     

ERECTA Family Genes Regulate Auxin Transport in the Shoot Apical Meristem and Forming Leaf Primordia

Leaves are produced postembryonically at the flanks of the shoot apical meristem. Their initiation is induced by a positive feedback loop between auxin and its transporter PIN-FORMED1 (PIN1). The expression and polarity of PIN1 in the shoot apical meristem is thought to be regulated primarily by auxin concentration and flow. The formation of an auxin maximum in the L1 layer of the meristem is the first sign of leaf initiation and is promptly followed by auxin flow into the inner tissues, formation of the midvein, and appearance of the primordium bulge. The ERECTA family genes (ERfs) encode leucine-rich repeat receptor-like kinases, and in Arabidopsis (Arabidopsis thaliana), this gene family consists of ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERL2. Here, we show that ERfs regulate auxin transport during leaf initiation. The shoot apical meristem of the er erl1 erl2 triple mutant produces leaf primordia at a significantly reduced rate and with altered phyllotaxy. This phenotype is likely due to deficiencies in auxin transport in the shoot apex, as judged by altered expression of PIN1, the auxin reporter DR5rev::GFP, and the auxin-inducible genes MONOPTEROS, INDOLE-3-ACETIC ACID INDUCIBLE1 (IAA1), and IAA19. In er erl1 erl2, auxin presumably accumulates in the L1 layer of the meristem, unable to flow into the vasculature of a hypocotyl. Our data demonstrate that ERfs are essential for PIN1 expression in the forming midvein of future leaf primordia and in the vasculature of emerging leaves.Leaves are formed during postembryonic development by the shoot apical meristem (SAM), a dome-shaped organ with a stem cell reservoir at the top and with leaf initiation taking place slightly below in the peripheral zone. The initiation of leaf primordia depends on the establishment of auxin maxima at the site of initiation (Braybrook and Kuhlemeier, 2010). Auxin is polarly transported through the epidermal layer of the meristem to the incipient primordium initiation site (Heisler et al., 2005) and then moves inward, where it promotes the formation of a vascular strand (Scarpella et al., 2006; Bayer et al., 2009). The developing vascular tissue acts as an auxin sink, depleting auxin in the epidermal layer (Scarpella et al., 2006). PIN1, an auxin efflux protein, is a central player in the formation of auxin maxima and is involved in the transport of auxin in both the epidermis and the forming vascular strand during leaf initiation (Benková et al., 2003; Reinhardt et al., 2003). PIN1 is the earliest marker for midvein formation (Scarpella et al., 2006), which starts to form before a leaf primordium bulges out of the meristem. The mechanisms determining PIN1 expression and polar localization in the SAM are central to understanding leaf initiation. In the L1 layer of the SAM, PIN1 is polarly localized in the plasma membrane toward cells with higher auxin concentration (Jönsson et al., 2006; Smith et al., 2006). Formation of the vein is explained by the canalization hypothesis, in which high auxin flux reinforces PIN1 expression (Kramer, 2008). Of all plasma membrane-localized PIN family transporters, only PIN1 has been detected in the vegetative SAM and linked with the initiation of rosette leaves (Guenot et al., 2012). At the same time, rosette leaves are positioned nonrandomly in pin1 mutants, suggesting that additional PIN1-independent mechanisms also have a role in regulating leaf initiation (Guenot et al., 2012).Here, we investigate the role of ERECTA family receptor-like kinases during leaf initiation in Arabidopsis (Arabidopsis thaliana). Previously, ERECTA family genes (ERfs) have been shown to be involved in the regulation of epidermis development and of plant growth along the apical-basal/proximodistal axis in aboveground organs (Torii et al., 1996; Shpak et al., 2004, 2005). Triple erecta (er), erecta-like1 (erl1), and erl2 mutants (er erl1 erl2) form a rosette with small, round leaves that lack petiole elongation. During the reproductive stage, the main inflorescence stem exhibits striking elongation defects and reduced apical dominance. ER has been implicated in vascular development, with the er mutation causing radial expansion of xylem (Ragni et al., 2011) and premature differentiation of vascular bundles (Douglas and Riggs, 2005). Recently, the dwarfism of described mutants was attributed to the function of ERf genes in the phloem, where they perceive signals from the endodermis (Uchida et al., 2012a). In the epidermis, all three genes inhibit the initial decision of protodermal cells to become meristemoid mother cells (Shpak et al., 2005). In addition, ERL1 and to a lesser extent ERL2 are important for maintaining cell proliferative activity in stomata lineage cells and for preventing terminal differentiation of meristemoids into guard mother cells. The activity of ERf receptors in the epidermis is regulated by a different set of peptides than in the phloem. EPIDERMAL PATTERNING FACTOR1 (EPF1) and EPF2 are expressed in stomatal precursor cells. They inhibit the development of new stomata in the vicinity of a forming stoma (Hara et al., 2007, 2009; Hunt and Gray, 2009). EPF-LIKE9 (EPFL9)/stomagen is expressed in the mesophyll, and, in contrast, it promotes the development of stomata (Kondo et al., 2010; Sugano et al., 2010). EPFL4 and EPFL6/CHALLAH are expressed in the endodermis, and their perception by phloem-localized ERfs is critical for stem elongation (Uchida et al., 2012a).While ERfs are very strongly expressed in the vegetative SAM and in forming leaf primordia, only recently has it become clear that these genes are involved in the regulation of meristem size and leaf initiation (Uchida et al., 2012b, 2013). It was suggested that ERfs regulate stem cell homeostasis in the SAM via buffering its cytokinin responsiveness by an unknown mechanism (Uchida et al., 2013). Here, we further investigate the involvement of ERfs in the control of leaf initiation and phyllotaxy. Our data suggest that ERfs are essential for PIN1 expression in the vasculature of forming leaf primordia. Based on analysis of the DR5rev::GFP reporter, auxin may accumulate in the L1 layer of the SAM in the mutant but is not able to move into the vasculature, consistent with drastically reduced PIN1pro:PIN1-GFP expression there. These data suggest that the convergence of PIN1 expression in the inner tissues of the SAM during leaf initiation is a complex process involving intercellular communications enabled by ERfs. The importance of ERfs for efficient auxin transport is further supported by reduced phototropic response in the er erl1 erl2 mutant.     

Regulation of Histone Methylation and Reprogramming of Gene Expression in the Rice Inflorescence Meristem

Rice inflorescence meristem (IM) activity is essential for panicle development and grain production. How chromatin and epigenetic mechanisms regulate IM activity remains unclear. Genome-wide analysis revealed that in addition to genes involved in the vegetative to reproductive transition, many metabolic and protein synthetic genes were activated in IM compared with shoot apical meristem and that a change in the H3K27me3/H3K4me3 ratio was an important factor for the differential expression of many genes. Thousands of genes gained or lost H3K27me3 in IM, and downregulation of the H3K27 methyltransferase gene SET DOMAIN GROUP 711 (SDG711) or mutation of the H3K4 demethylase gene JMJ703 eliminated the increase of H3K27me3 in many genes. SDG711-mediated H3K27me3 repressed several important genes involved in IM activity and many genes that are silent in the IM but activated during floral organogenesis or other developmental stages. SDG711 overexpression augmented IM activity and increased panicle size; suppression of SDG711 by RNA interference had the opposite effect. Double knockdown/knockout of SDG711 and JMJ703 further reduced panicle size. These results suggest that SDG711 and JMJ703 have agonistic functions in reprogramming the H3K27me3/H3K4me3 ratio and modulating gene expression in the IM.     

The dicer-like1 Homolog fuzzy tassel Is Required for the Regulation of Meristem Determinacy in the Inflorescence and Vegetative Growth in Maize

Plant architecture is determined by meristems that initiate leaves during vegetative development and flowers during reproductive development. Maize (Zea mays) inflorescences are patterned by a series of branching events, culminating in floral meristems that produce sexual organs. The maize fuzzy tassel (fzt) mutant has striking inflorescence defects with indeterminate meristems, fasciation, and alterations in sex determination. fzt plants have dramatically reduced plant height and shorter, narrower leaves with leaf polarity and phase change defects. We positionally cloned fzt and discovered that it contains a mutation in a dicer-like1 homolog, a key enzyme required for microRNA (miRNA) biogenesis. miRNAs are small noncoding RNAs that reduce target mRNA levels and are key regulators of plant development and physiology. Small RNA sequencing analysis showed that most miRNAs are moderately reduced in fzt plants and a few miRNAs are dramatically reduced. Some aspects of the fzt phenotype can be explained by reduced levels of known miRNAs, including miRNAs that influence meristem determinacy, phase change, and leaf polarity. miRNAs responsible for other aspects of the fzt phenotype are unknown and likely to be those miRNAs most severely reduced in fzt mutants. The fzt mutation provides a tool to link specific miRNAs and targets to discrete phenotypes and developmental roles.     

Focus Issue on Roots: Low Crown Root Number Enhances Nitrogen Acquisition from Low-Nitrogen Soils in Maize

In developing nations, low soil nitrogen (N) availability is a primary limitation to crop production and food security, while in rich nations, intensive N fertilization is a primary economic, energy, and environmental cost to crop production. It has been proposed that genetic variation for root architectural and anatomical traits enhancing the exploitation of deep soil strata could be deployed to develop crops with greater N acquisition. Here, we provide evidence that maize (Zea mays) genotypes with few crown roots (crown root number [CN]) have greater N acquisition from low-N soils. Maize genotypes differed in their CN response to N limitation in greenhouse mesocosms and in the field. Low-CN genotypes had 45% greater rooting depth in low-N soils than high-CN genotypes. Deep injection of ¹⁵N-labeled nitrate showed that low-CN genotypes under low-N conditions acquired more N from deep soil strata than high-CN genotypes, resulting in greater photosynthesis and plant N content. Under low N, low-CN genotypes had greater biomass than high-CN genotypes at flowering (85% in the field study in the United States and 25% in South Africa). In the field in the United States, 1.8× variation in CN was associated with 1.8× variation in yield reduction by N limitation. Our results indicate that CN deserves consideration as a potential trait for genetic improvement of N acquisition from low-N soils.Maize (Zea mays) is one of the world’s most important crops and is a staple food in Latin America and Africa. Maize production requires a large amount of fertilizer, especially nitrogen (N). In the United States, N fertilizers represent the greatest economic and energy costs for maize production (Ribaudo et al., 2011). However, on-farm studies across the northcentral United States revealed that more than half of applied N is not taken up by maize plants and is vulnerable to losses from volatilization, denitrification, and leaching, which pollute air and water resources (Cassman et al., 2002). Conversely, in developing countries, suboptimal N availability is a primary limitation to crop yields and, therefore, food security (Azeez et al., 2006). Increasing yield in these areas is an urgent concern, since chemical fertilizers are not affordable (Worku et al., 2007). Cultivars with greater N acquisition from low-N soils could help alleviate food insecurity in poor nations as well as reduce environmental degradation from excessive fertilizer use in developed countries.The two major soil N forms available to plants are ammonium and nitrate. Nitrate is the main N form in most maize production environments (Miller and Cramer, 2004). Nitrate is highly mobile in soil, and the spatiotemporal availability of soil N is rather complex. In the simplest case, N fertilizers applied to the soil and/or N released from the mineralization of soil organic matter are rapidly converted to nitrate by soil microbes. After irrigation and precipitation events, nitrate moves with water to deeper soil strata. Leaching of nitrate from the root zone has been shown to be a significant cause of low recovery of N fertilizer in commercial agricultural systems (Raun and Johnson, 1999; Cassman et al., 2002). Differences in root depth influence the ability of plants to acquire N. Studies using ¹⁵N-labeled nitrate placed at different soil depths showed that only plants with deep rooting can acquire N sources from deep soil strata, which would otherwise have been lost through leaching (Kristensen and Thorup-Kristensen, 2004a, 2004b). Therefore, selection for root traits enhancing rapid deep soil exploration could be used as a strategy to improve crop N efficiency.The maize root system consists of embryonic and postembryonic components. The embryonic root system consists of two distinct root classes: a primary root and a variable number of seminal roots formed at the scutellar node. The postembryonic root system consists of roots that are formed at consecutive shoot nodes and lateral roots, which are initiated in the pericycle of all root classes. Shoot-borne or nodal roots that are formed belowground are called crown roots, whereas those that are formed aboveground are designated brace roots (Hochholdinger, 2009). While the primary root and seminal roots are essential for the establishment of seedlings after germination, nodal roots and particularly crown roots make up most of the maize root system and are primarily responsible for soil resource acquisition later in development (Hoppe et al., 1986).Lynch (2013) proposed an ideotype for superior N and water acquisition in maize called Steep, Cheap, and Deep (SCD), which integrates root architectural, anatomical, and physiological traits to increase rooting depth and, therefore, the capture of N in leaching environments. One such trait is crown root number (CN). CN is an aggregate trait consisting of the number of belowground nodal whorls and the number of roots per whorl. The crown root system dominates resource acquisition during vegetative growth after the first few weeks and remains important during reproductive development (Hochholdinger et al., 2004). CN in maize ranges from five to 50 under fertile conditions (Trachsel et al., 2011). At the low end of this range, crown roots may be too spatially dispersed to sufficiently explore the soil. There is also a risk of root loss to herbivores and pathogens. If roots are lost in low-N plants, there may be too few crown roots left to support the nutrient, water, and anchorage needs of the plant. At the high end, a large number of crown roots may compete with each other for water and nutrients as well as incur considerable metabolic costs for the plant (Fig. 1). The SCD ideotype proposes that there is an optimal CN for N capture in maize (Lynch, 2013). Under low-N conditions, resources for root growth and maintenance are limiting, and nitrate is a mobile resource that can be captured by a dispersed root system. The optimal CN should tend toward the low end of the phenotypic variation to make resources available for the development of longer, deeper roots rather than more crown roots. According to the SCD ideotype, in low-N soils, maize genotypes with fewer crown roots could explore soils at greater depth, resulting in greater N acquisition, growth, and yield than genotypes with many crown roots.Open in a separate windowFigure 1.Visualization of the maize root system of low- and high-CN genotypes at 40 d after germination. Crown roots are colored in blue, and seminal roots are in red. The CN is eight in the low-CN genotype and 46 in the high-CN genotype. (Image courtesy of Larry M. York.)The objective of this study was to test the hypotheses that (1) low-CN genotypes have greater rooting depth than high-CN genotypes in low-N soils; (2) low-CN genotypes are better at acquiring deep soil N than high-CN genotypes; and (3) low-CN genotypes have greater biomass and yield than high-CN genotypes in low-N conditions.     

Abscisic Acid Regulation of Root Hydraulic Conductivity and Aquaporin Gene Expression Is Crucial to the Plant Shoot Growth Enhancement Caused by Rhizosphere Humic Acids

The physiological and metabolic mechanisms behind the humic acid-mediated plant growth enhancement are discussed in detail. Experiments using cucumber (Cucumis sativus) plants show that the shoot growth enhancement caused by a structurally well-characterized humic acid with sedimentary origin is functionally associated with significant increases in abscisic acid (ABA) root concentration and root hydraulic conductivity. Complementary experiments involving a blocking agent of cell wall pores and water root transport (polyethylenglycol) show that increases in root hydraulic conductivity are essential in the shoot growth-promoting action of the model humic acid. Further experiments involving an inhibitor of ABA biosynthesis in root and shoot (fluridone) show that the humic acid-mediated enhancement of both root hydraulic conductivity and shoot growth depended on ABA signaling pathways. These experiments also show that a significant increase in the gene expression of the main root plasma membrane aquaporins is associated with the increase of root hydraulic conductivity caused by the model humic acid. Finally, experimental data suggest that all of these actions of model humic acid on root functionality, which are linked to its beneficial action on plant shoot growth, are likely related to the conformational structure of humic acid in solution and its interaction with the cell wall at the root surface.Numerous studies have illustrated the relevant role of dissolved organic matter (DOM) present in soil solution and aquatic reservoirs (lakes, rivers, etc.) in the biological and chemical evolution of both natural and anthropogenic ecosystems (Stevenson, 1994; Tipping, 2002; Chen et al., 2004; Trevisan et al., 2011; Berbara and García, 2014; Canellas and Olivares, 2014; Mora et al., 2014a, 2014b). In many studies, DOM fractionation is made by using the methodology proposed by the International Humic Substances Society. Fractions obtained are operationally named humic acid (HA), fulvic acid, humin, and nonhumic fraction, which includes more hydrophilic compounds (polycarboxylic acids, aminoacids, sugars, etc.; Swift, 1996). Many studies have reported that HAs obtained from either organic materials (soils, soil sediments, composted wastes, etc.) or water reservoirs (rivers, lakes, etc.), extracted with alkaline water solutions, or isolated by resin fixation, reverse osmosis, or ultrafiltration (Alberts and Takács, 2004) affected the development of diverse plant species (for instance, cucumber [Cucumis sativus], tomato [Solanum lycopersicum], maize [Zea mays], wheat [Triticum aestivum], Arabidopsis [Arabidopsis thaliana], and rapeseed [Brassica Napus]) through common signaling pathways, which involved key phytoregulators, such as indole acetic acid (IAA)-nitric oxide (NO; Zandonadi et al., 2010; Canellas et al., 2011; Trevisan et al., 2011; Mora et al., 2012, 2014a), ethylene, and abscisic acid (ABA) in roots (Mora et al., 2012, 2014a) as well as cytokinins in shoots (Mora et al., 2010, 2014b). Recently, Mora et al., 2014a showed that the HA ability to enhance both shoot growth and ABA root concentration in cucumber was regulated by IAA and NO root signaling pathways. However, despite all of this information, the nature of a possible primary, common action on plant roots of HAs with diverse origin and structure remains elusive.Recently, Asli and Neumann (2010) described a new mechanism by which high concentrations of HAs extracted from diverse organic sources decreased shoot plant growth. This mechanism involved the reduction of root hydraulic conductivity (Lp_r) resulting from the fouling of root cell wall pores because of the accumulation and aggregation of HA molecules at root surface. Although the concentration of HAs used by Asli and Neumann (2010) (1 g L⁻¹) is much higher than that related to HA plant growth promotion ability (50–250 mg L⁻¹; Rose et al., 2014), the results do raise the hypothesis that the primary, still unknown event emerging from the interaction of humic substances with root surface cells might involve an unspecific, physical action on root permeability and water uptake. This event might trigger a chain of secondary events in the root that, in turn, would affect specific hormone signaling pathways, which may regulate shoot and root growth. This HA action on plant development would be positive (increasing) or negative (decreasing) depending on HAs concentration in the rhizosphere.To explore the suitability of this hypothesis, we have tested the potential role of Lp_r in the main mechanism by which HAs promote shoot growth in cucumber. To this end, we used a well-characterized and modeled sedimentary humic acid (SHA) at a concentration (100 mg of SHA organic carbon [C] L⁻¹) that was associated with plant shoot growth promotion in previous studies (Mora, 2009; Mora et al., 2014a, 2014b). We also investigated the functional relationships between these effects of SHA on Lp_r and shoot growth as well as in some shoot water-related parameters (leaf stomatal conductance [G_s] and ABA) and those caused by SHA on IAA-NO and ABA root signaling pathways. Finally, taking into account that root plasma membrane aquaporins (plasma membrane intrinsic proteins [PIPs]) are involved in the ABA regulation of Lp_r in other plant systems, we also studied the role of PIPs in SHA effects on plant shoot growth.The results obtained here show that SHA enhances shoot growth in cucumber through ABA-dependent increases in both Lp_r and root PIPs (CsPIPs) gene up-regulation.     

Focus Issue on Roots: Root Cortical Aerenchyma Enhances Nitrogen Acquisition from Low-Nitrogen Soils in Maize

Suboptimal nitrogen (N) availability is a primary constraint for crop production in developing nations, while in rich nations, intensive N fertilization carries substantial environmental and economic costs. Therefore, understanding root phenes that enhance N acquisition is of considerable importance. Structural-functional modeling predicts that root cortical aerenchyma (RCA) could improve N acquisition in maize (Zea mays). We evaluated the utility of RCA for N acquisition by physiological comparison of maize recombinant inbred lines contrasting in RCA grown under suboptimal and adequate N availability in greenhouse mesocosms and in the field in the United States and South Africa. N stress increased RCA formation by 200% in mesocosms and by 90% to 100% in the field. RCA formation substantially reduced root respiration and root N content. Under low-N conditions, RCA formation increased rooting depth by 15% to 31%, increased leaf N content by 28% to 81%, increased leaf chlorophyll content by 22%, increased leaf CO₂ assimilation by 22%, increased vegetative biomass by 31% to 66%, and increased grain yield by 58%. Our results are consistent with the hypothesis that RCA improves plant growth under N-limiting conditions by decreasing root metabolic costs, thereby enhancing soil exploration and N acquisition in deep soil strata. Although potential fitness tradeoffs of RCA formation are poorly understood, increased RCA formation appears be a promising breeding target for enhancing crop N acquisition.Nitrogen (N) deficiency is one of the most limiting factors in maize (Zea mays) production worldwide (Ladha et al., 2005). In developing countries such as those in sub-Saharan Africa, less than 20 kg N ha⁻¹ is applied to fields of smallholder farmers due to high fertilizer cost (Azeez et al., 2006; Worku et al., 2007). In developed countries, intensive N fertilization is used to maintain satisfactory yield (Tilman et al., 2002). In the United States, N fertilizers are the greatest economic and energy cost for maize production (Ribaudo et al., 2011). However, less than half of the N applied to crops is actually acquired, and most of the remaining N becomes a source of environmental pollution (Raun and Johnson, 1999; Smil, 1999; Tilman et al., 2002). For example, N and phosphorus (P) effluents into marine systems from agriculture cause eutrophication and hypoxic zones (Diaz and Rosenberg, 2008; Robertson and Vitousek, 2009). Nitrate contamination in surface water and groundwater systems poses serious health risks, such as methemoglobinemia and N-nitroso-induced cancers (UNEP and WHRC, 2007). Emission of nitrous oxides from agricultural activities contributes to ozone damage and global warming (Kulkarni et al., 2008; Sutton et al., 2011). Furthermore, the production of N fertilizers requires considerable energy from fossil fuels, and since energy costs have risen in recent years, farmers face economic pressure from increasing N fertilizer costs, which are linked to higher food prices. It is estimated that a 1% increase in crop N efficiency could save more than $1 billion (U.S.) annually worldwide (Kant et al., 2011). Therefore, even a small improvement in N efficiency would have significant positive impacts on the environment and the economy.Soil N is heterogenous and dynamic. The bioavailability of soil N depends on the balance between the rates of mineralization, nitrification, and denitrification. These processes are determined by several factors, including soil composition, microbial activity, soil temperature, and soil water status (Miller and Cramer, 2004). The predominant form of soil N available to plants in most agricultural systems is nitrate, which is highly soluble in water and thus mobile in the soil (Barber, 1995; Marschner, 1995). Mineralization of organic matter and/or the application of N fertilizer at the beginning of the growing season followed by precipitation and irrigation create a pulse of nitrate that may exceed the N acquisition capacity of seedlings and leach below the root zone. Therefore, it has been proposed that increasing the speed of root exploration of deep soil strata could benefit N acquisition (Lynch, 2013). However, the structural investments and metabolic expenditures of root systems are substantial and can exceed half of daily photosynthesis (Lambers et al., 2002). Therefore, full consideration of the costs and benefits of root systems is crucial for identifying root traits to improve crop production, especially in water- and nutrient-deficient environments (Lynch, 2007). Taking rhizoeconomics and the spatiotemporal availability of soil N into account, Lynch (2013) proposed a root ideotype for enhanced N acquisition in maize called Steep, Cheap, and Deep, in which Steep refers to architectural phenes and Cheap refers to phenes that reduce the metabolic cost of soil exploration. One element of this ideotype is abundant root cortical aerenchyma (RCA).RCA consists of enlarged air spaces in the root cortex (Esau, 1977). RCA is known to form in response to hypoxia, and the role of RCA in improving oxygen transport to roots of many plant species under hypoxic conditions has been well researched (Vartapetian and Jackson, 1997; Jackson and Armstrong, 1999; Mano and Omori, 2007, 2013). Interestingly, RCA can also form in response to drought and edaphic stresses such as N, P, and sulfur deficiencies (Drew et al., 1989; Bouranis et al., 2003; Fan et al., 2003; Zhu et al., 2010a), which suggests that the benefit of RCA extends beyond facilitating oxygen transport. Several lines of evidence suggest that RCA enhances root metabolic efficiency under stress. Fan et al. (2003) found that RCA formation significantly reduced root segment respiration and P content of root tissue, which allowed greater shoot growth in soils with low P availability. Under drought, maize genotypes with high RCA formation had greater root length, deeper rooting, better leaf water status, and 8 times greater yield than closely related genotypes with low RCA (Zhu et al., 2010a). Effects of RCA on root respiration were more pronounced for large-diameter roots compared with small-diameter roots (Jaramillo et al., 2013). Results from the functional-structural plant model SimRoot showed that RCA formation could be an adaptive response to deficiency of N, P, and potassium by decreasing the metabolic cost of soil exploration. By reducing root respiration, RCA decreases the carbon cost of soil exploration, and by decreasing the N and P content of root tissue, RCA permits internal reallocation of nutrients to growing root tissue, which is particularly beneficial under conditions of low N and P availability (Postma and Lynch, 2011a). Under suboptimal P availability, RCA increased the growth of a simulated 40-d-old maize plant by 70% (Postma and Lynch, 2011b). In the case of N, RCA increased the growth of simulated maize plants up to 55% in low-N conditions, and plants benefit from RCA more in high-N-leaching environments than in low-N-leaching environments (Postma and Lynch, 2011a). In addition, the formation of RCA decreases critical soil nutrient levels, defined as the soil fertility below which growth is reduced, suggesting that cultivars with high RCA may require less fertilizer under nonstressed conditions. These in silico results suggest that RCA has potential utility for improving crop nutrient acquisition in both high- and low-input agroecosystems.The overall objective of this research was to assess the utility of RCA for N acquisition in maize under N-limiting conditions. Maize near-isophenic recombinant inbred lines (RILs) sharing a common genetic background (i.e. descending from the same parents) with common root phenotypes but contrasting in RCA formation were grown under N stress to test the hypothesis that RCA formation is associated with reduced root respiration, reduced tissue nutrient content, greater rooting depth, enhanced N acquisition, and therefore greater plant growth and yield under N limitation.     

Arabidopsis DELLA and JAZ Proteins Bind the WD-Repeat/bHLH/MYB Complex to Modulate Gibberellin and Jasmonate Signaling Synergy

Integration of diverse environmental and endogenous signals to coordinately regulate growth, development, and defense is essential for plants to survive in their natural habitat. The hormonal signals gibberellin (GA) and jasmonate (JA) antagonistically and synergistically regulate diverse aspects of plant growth, development, and defense. GA and JA synergistically induce initiation of trichomes, which assist seed dispersal and act as barriers to protect plants against insect attack, pathogen infection, excessive water loss, and UV irradiation. However, the molecular mechanism underlying such synergism between GA and JA signaling remains unclear. In this study, we revealed a mechanism for GA and JA signaling synergy and identified a signaling complex of the GA pathway in regulation of trichome initiation. Molecular, biochemical, and genetic evidence showed that the WD-repeat/bHLH/MYB complex acts as a direct target of DELLAs in the GA pathway and that both DELLAs and JAZs interacted with the WD-repeat/bHLH/MYB complex to mediate synergism between GA and JA signaling in regulating trichome development. GA and JA induce degradation of DELLAs and JASMONATE ZIM-domain proteins to coordinately activate the WD-repeat/bHLH/MYB complex and synergistically and mutually dependently induce trichome initiation. This study provides deep insights into the molecular mechanisms for integration of different hormonal signals to synergistically regulate plant development.