Xyloglucan in the Cell Walls of Suspension-Cultured Rice Cells

The xyloglucan present in the 24% KOH extract of the cell wallsof suspension-cultured rice cells was characterized by fragmentationanalysis with Trichoderma viride cellulase and Aspergillus oryzaeß-D-glucosidase. The xyloglucan is composed mainlyof the following oligosaccharide units: Results showed that the xyloglucan of suspension-cultured ricecells is more extensively branched than is that of rice seedlings.Another structural characteristic of the former xyloglucan isthe presence of D-galactosyl-D-xylosyl side chains that arenot found in the latter. (Received June 15, 1984; Accepted January 11, 1985)     

Stomatal Guard Cells Are Totipotent

It has been successfully demonstrated, using epidermis explants of sugar beet (Beta vulgaris L.), that stomatal guard cells retain full totipotent capacity. Despite having one of the highest degrees of morphological adaptation and a unique physiological specialization, it is possible to induce a re-expression of full (embryogenic) genetic potential in these cells in situ by reversing their highly differentiated nature to produce regenerated plants via a callus stage. The importance of these findings both to stomatal research and to our understanding of cytodifferentiation in plants is discussed.     

Control of Ion Fluxes in Stomatal Guard Cells

Opening and closing of the stomatal pore is associated with very large changes in K-salt accumulation in stomatal guard cells. This review discusses the ionic relations of guard cells in relation to the general pattern of transport processes in plant cells, in plasmalemma and tonoplast, involving primary active transport of protons, proton-linked secondary active transport, and a number of gated ion channels. The evidence available suggests that the initiation of stomatal opening is regulated through the uptake mechanisms, whereas initiation of stomatal closing is regulated by control of ion efflux at the plasmalemma, and of fluxes to and from the vacuole. In response to a closing signal there are large transient increases in efflux of both Cl^? (or Br^?) and Rb⁺ (K⁺) at the plasmalemma, with also a probable increase in anion flux from vacuole to cytoplasm and decrease in anion flux from cytoplasm to vacuole. A speculative hypothetical sequence of events is discussed, by which the primary response to a closing signal is an increase in Ca²⁺ influx at the plasmalemma, producing depolarisation and increase in cytoplasmic Ca²⁺. The consequent opening of Ca²⁺-sensitive Cl^? channels, and voltage-sensitive K⁺ channels (also Ca²⁺-sensitive?) in the plasmalemma, and of a Ca²⁺-sensitive nonspecific channel in the tonoplast, could produce the flux effects identified by the tracer work; this speculation is also consistent with the Ca²⁺-sensitivity of the response to closing signals and with evidence from patch clamping that such channels exist in at least some plant cells, though not yet all shown in guard cells.     

Control of Volume and Turgor in Stomatal Guard Cells

Water loss from plants is determined by the aperture of stomatal pores in the leaf epidermis, set by the level of vacuolar accumulation of potassium salt, and hence volume and turgor, of a pair of guard cells. Regulation of ion fluxes across the tonoplast, the key to regulation of stomatal aperture, can only be studied by tracer flux measurements. There are two transport systems in the tonoplast. The first is a Ca²⁺-activated channel, inhibited by phenylarsine oxide (PAO), responsible for the release of vacuolar K⁺(Rb⁺) in response to the “drought” hormone, abscisic acid (ABA). This channel is sensitive to pressure, down-regulated at low turgor and up-regulated at high turgor, providing a system for turgor regulation. ABA induces a transient stimulation of vacuolar ion efflux, during which the flux tracks the ion content (volume, turgor), suggesting ABA reduces the set-point of a control system. The second system, which is PAO-insensitive, is responsible for an ion flux from vacuole to cytoplasm associated with inward water flow following a hypo-osmotic transfer. It is suggested that this involves an aquaporin as sensor, and perhaps also as responder; deformation of the aquaporin may render it ion-permeable, or, alternatively, the deformed aquaporin may signal to an associated ion channel, activating it. Treatment with inhibitors of aquaporins, HgCl₂ or silver sulfadiazine, produces a large transient increase in ion release from the vacuole, also PAO-insensitive. It is suggested that this involves the same aquaporin, either rendered directly ion-permeable, or signalling to activate an associated ion channel.     

Roles of Cellulose and Xyloglucan in Determining the Mechanical Properties of Primary Plant Cell Walls

The primary cell walls of growing and fleshy plant tissue mostly share a common set of molecular components, cellulose, xyloglucan (XyG), and pectin, that are required for both inherent strength and the ability to respond to cell expansion during growth. To probe molecular mechanisms underlying material properties, cell walls and analog composites from Acetobacter xylinus have been measured under small deformation and uniaxial extension conditions as a function of molecular composition. Small deformation oscillatory rheology shows a common frequency response for homogenized native cell walls, their sequential extraction residues, and bacterial cellulose alone. This behavior is characteristic of structuring via entanglement of cellulosic rods and is more important than cross-linking with XyG in determining shear moduli. Compared with cellulose alone, composites with XyG have lower stiffness and greater extensibility in uniaxial tension, despite being highly cross-linked at the molecular level. It is proposed that this is due to domains of cross-linked cellulose behaving as mechanical elements, whereas cellulose alone behaves as a mat of individual fibrils. The implication from this work is that XyG/cellulose networks provide a balance of extensibility and strength required by primary cell walls, which is not achievable with cellulose alone.     

保卫细胞碳代谢与气孔运动

作为气孔运动渗透调节的代谢基础 ,气孔保卫细胞的碳代谢有特殊的调控机理。本文介绍了气孔保卫细胞中参与碳代谢的主要酶的特性及调控特点 ,特别是保卫细胞叶绿体中催化苹果酸形成的PEP羧化酶 ,其磷酸化和去磷酸化参与了保卫细胞信号传递。保卫细胞碳代谢调控在气孔运动调节中的作用 ,并讨论了保卫细胞碳代谢与能量代谢的关系     

Kinetics of Integration of Xyloglucan into the Walls of Suspension-Cultured Rose Cells

To study the kinetics of synthesis, wall-binding and degradationof xyloglucan, we incubated suspension-cultured rose cells for0–5–24 h in L-[1-³H]arabinose. >95% of the [³H]arabinosewas taken up within 2 h. UDP-Pentoses were maximally labelledwithin 0–5 h and had lost most of their ³H by 2 h afterthe addition of [³H]arabinose. Therefore, the 24 h experimentresembled a pulse-chase rgime. The [³H]xyloglucan formed wasfractionated into four cellular pools [detergent-extractable(interpreted as cytoplasmic), and guanidinium thiocyanate-,06 M NaOH- and 60 M NaOH-extractable (interpreted as progressivelymore firmly wall-bound)]; soluble extracellular xyloglucan wascollected as a fifth pool. All five pools of xyloglucan hadstarted accumulating ³H at their respective maximal rates by     

一氧化氮在拟南芥气孔发育中的功能研究

为了研究一氧化氮(nitric oxide,NO)在气孔发育中的功能,本实验利用NO的释放剂硝普钠(SNP)处理拟南芥野生型,结果显示,SNP处理后的子叶气孔指数(SI)和%(GMC+M)都较未处理组明显升高。继续检测拟南芥内源一氧化氮升高的突变体nox1和内源一氧化氮降低的突变体noa1的气孔参数,结果显示,nox1的气孔指数(SI)和%(GMC+M)相对野生型明显提高,noa1的气孔指数(SI)和%(GMC+M)低于野生型。荧光定量PCR结果进一步显示,NO抑制了气孔发育相关基因MUTE,SCRM,SCRM2的表达。综上结果表明,NO通过调控气孔发育相关基因的表达影响气孔发育。     

Guard Cell Chloroplasts Are Essential for Blue Light-Dependent Stomatal Opening in Arabidopsis

Blue light (BL) induces stomatal opening through the activation of H⁺-ATPases with subsequent ion accumulation in guard cells. In most plant species, red light (RL) enhances BL-dependent stomatal opening. This RL effect is attributable to the chloroplasts of guard cell, the only cells in the epidermis possessing this organelle. To clarify the role of chloroplasts in stomatal regulation, we investigated the effects of RL on BL-dependent stomatal opening in isolated epidermis, guard cell protoplasts, and intact leaves of Arabidopsis thaliana. In isolated epidermal tissues and intact leaves, weak BL superimposed on RL enhanced stomatal opening while BL alone was less effective. In guard cell protoplasts, RL enhanced BL-dependent H⁺-pumping and DCMU, a photosynthetic electron transport inhibitor, eliminated this effect. RL enhanced phosphorylation levels of the H⁺-ATPase in response to BL, but this RL effect was not suppressed by DCMU. Furthermore, DCMU inhibited both RL-induced and BL-dependent stomatal opening in intact leaves. The photosynthetic rate in leaves correlated positively with BL-dependent stomatal opening in the presence of DCMU. We conclude that guard cell chloroplasts provide ATP and/or reducing equivalents that fuel BL-dependent stomatal opening, and that they indirectly monitor photosynthetic CO₂ fixation in mesophyll chloroplasts by absorbing PAR in the epidermis.     

Potassium Loss from Stomatal Guard Cells at Low Water Potentials

The potassium content of guard cells and the resistance to viscousflow of air through the leaf were determined in sunflower (Helianthusannuus) subjected to low leaf water potentials under illuminatedconditions. In intact plants desiccated slowly by withholdingwater from the soil, large losses in guard cell K occurred asleaf water potentials decreased. Leaf viscous resistance increased,indicating stomatal closure. Similar results were obtained whendetached leaf segments were desiccated rapidly. Upon rehydrationof leaves, no stomatal opening was observed initially, despiteleaf water potentials at predesiccated levels. After severalhours, however, re-entry of K occurred and stomata became fullyopen. Turgid leaf segments floated on an ABA solution showedlosses of guard cell K and closure of stomata as rapidly andcompletely as those brought about by desiccation. It is concludedthat stomatal closure at low water potentials under illuminatedconditions is not controlled solely by water loss from the tissuebut involves the loss of osmoticum from the guard cells as well.This in turn decreases the turgor difference between the guardcells and the surrounding cells, and closing occurs.     

Localization of Xyloglucan in the Macromolecular Complex Composed of Xyloglucan and Cellulose in Pea Stems

A macromolecular complex composed of xyloglucan and cellulosewas isolated from elongating regions of stems of etiolated pea(Pisum sativum L. var Alaska) seedlings and binding of a xyloglucan-specificantibody was examined after treatment of the complex with endo-1,4-ß-glucanaseor 24% KOH. The antibody bound to the complex but the extentof binding was reduced after treatment of the complex with endo-1,4-ß-glucanaseand was hardly detectable after treatment with 24% KOH. Themolecular weight of the xyloglucan that remained (5%) in theß-glucanase-treated complexes was less than 9,200.Pea xyloglucan was allowed to bind to enzymeand alkali-treatedcomplexes to generaly reconstituted complexes. The amount ofthe antibody that bound to each type of reconstituted complexwas similar but was much lower than that bound to the nativecomplex. Immunogold labeling indicated that most of the antigenwas widely distributed between microfibrils in the native complex,whereas the antigen appeared to be confined to the microfibrilsin the reconstituted complexes. These findings suggest thata part of each xyloglucan molecule is strongly associated withcellulose microfibrils while the rest is free of the microfibrilsin the native complex. ¹This work was supported in part by a grant from the YamadaScience Foundation.     

植物保卫细胞离子通道在气孔运动中的作用

介绍保卫细胞质膜和液泡膜上的离子通道活性变化及其在气孔运动中的作用,同时对各种刺激引发气孔运动过程中的信使Ca2 、H2O2和pH等对离子通道的调节作用作了概述.     

Photocontrol of the Functional Coupling between Photosynthesis and Stomatal Conductance in the Intact Leaf : Blue Light and Par-Dependent Photosystems in Guard Cells

The photocontrol of the functional coupling between photosynthesis and stomatal conductance in the leaf was investigated in gas exchange experiments using monochromatic light provided by lasers. Net photosynthesis and stomatal conductance were measured in attached leaves of Malva parviflora L. as a function of photon irradiance at 457.9 and 640.0 nanometers.

Photosynthetic rates and quantum yields of photosynthesis were higher under red light than under blue, on an absorbed or incident basis.

Stomatal conductance was higher under blue than under red light at all intensities. Based on a calculated apparent photon efficiency of conductance, blue and red light had similar effects on conductance at intensities higher than 0.02 millimoles per square meter per second, but blue light was several-fold more efficient at very low photon irradiances. Red light had no effect on conductance at photon irradiances below 0.02 millimoles per square meter per second. These observations support the hypothesis that stomatal conductance is modulated by two photosystems: a blue light-dependent one, driving stomatal opening at low light intensities and a photosynthetically active radiation (PAR)-dependent one operating at higher irradiances.

When low intensity blue light was used to illuminate a leaf already irradiated with high intensity, 640 nanometers light, the leaf exhibited substantial increases in stomatal conductance. Net photosynthesis changed only slightly. Additional far-red light increased net photosynthesis without affecting stomatal conductance. These observations indicate that under conditions where the PAR-dependent system is driven by high intensity red light, the blue light-dependent system has an additive effect on stomatal conductance.

The wavelength dependence of photosynthesis and stomatal conductance demonstrates that these processes are not obligatorily coupled and can be controlled by light, independent of prevailing levels of intercellular CO₂. The blue light-dependent system in the guard cells may function as a specific light sensor while the PAR-dependent system supplies a CO₂-modulated energy source providing functional coupling between the guard cells and the photosynthesizing mesophyll.

     

Modeling the Calcium Signaling in Stomatal Guard Cells under the Action of Abscisic Acid

A theoretical model of calcium signaling is presented that simulates oscillations of cytoplasmic calcium concentration ([Ca²⁺]_cyt) in stomatal guard cells under the action of abscisic acid. The model is based on the kinetics of inositol 1,4,5-trisphosphate-sensitive calcium channels of endoplasmic reticulum and cyclic ADP-ribose-sensitive calcium channels of the tonoplast. The operation of two energy-dependent pumps—the Ca²⁺-ATPase of the endoplasmic reticulum and the Ca²⁺/H⁺ antiporter of the tonoplast—is also included in the model. It is shown that the removal of excessive Ca²⁺ from the cytoplasm by the tonoplast Ca²⁺/H⁺ antiporter is the main factor accounting for generation of [Ca²⁺]_cyt oscillations at a wide range of ABA concentrations (0.01–1 M). The long period of [Ca²⁺]_cyt oscillations in plant cells is explained by a slow release from inhibition of inositol 1,4,5-trisphosphate-gated calcium channels.     

Xyloglucan Deficiency Disrupts Microtubule Stability and Cellulose Biosynthesis in Arabidopsis,Altering Cell Growth and Morphogenesis

Xyloglucan constitutes most of the hemicellulose in eudicot primary cell walls and functions in cell wall structure and mechanics. Although Arabidopsis (Arabidopsis thaliana) xxt1 xxt2 mutants lacking detectable xyloglucan are viable, they display growth defects that are suggestive of alterations in wall integrity. To probe the mechanisms underlying these defects, we analyzed cellulose arrangement, microtubule patterning and dynamics, microtubule- and wall-integrity-related gene expression, and cellulose biosynthesis in xxt1 xxt2 plants. We found that cellulose is highly aligned in xxt1 xxt2 cell walls, that its three-dimensional distribution is altered, and that microtubule patterning and stability are aberrant in etiolated xxt1 xxt2 hypocotyls. We also found that the expression levels of microtubule-associated genes, such as MAP70-5 and CLASP, and receptor genes, such as HERK1 and WAK1, were changed in xxt1 xxt2 plants and that cellulose synthase motility is reduced in xxt1 xxt2 cells, corresponding with a reduction in cellulose content. Our results indicate that loss of xyloglucan affects both the stability of the microtubule cytoskeleton and the production and patterning of cellulose in primary cell walls. These findings establish, to our knowledge, new links between wall integrity, cytoskeletal dynamics, and wall synthesis in the regulation of plant morphogenesis.The primary walls of growing plant cells are largely constructed of cellulose and noncellulosic matrix polysaccharides that include hemicelluloses and pectins (Carpita and Gibeaut, 1993; Somerville et al., 2004; Cosgrove, 2005). Xyloglucan (XyG) is the most abundant hemicellulose in the primary walls of eudicots and is composed of a β-1,4-glucan backbone with side chains containing Xyl, Gal, and Fuc (Park and Cosgrove, 2015). XyG is synthesized in the Golgi apparatus before being secreted to the apoplast, and its biosynthesis requires several glycosyltransferases, including β-1,4-glucosyltransferase, α-1,6-xylosyltransferase, β-1,2-galactosyltransferase, and α-1,2-fucosyltransferase activities (Zabotina, 2012). Arabidopsis (Arabidopsis thaliana) XYLOGLUCAN XYLOSYLTRANSFERASE1 (XXT1) and XXT2 display xylosyltransferase activity in vitro (Faik et al., 2002; Cavalier and Keegstra, 2006), and strikingly, no XyG is detectable in the walls of xxt1 xxt2 double mutants (Cavalier et al., 2008; Park and Cosgrove, 2012a), suggesting that the activity of XXT1 and XXT2 are required for XyG synthesis, delivery, and/or stability.Much attention has been paid to the interactions between cellulose and XyG over the past 40 years. Currently, there are several hypotheses concerning the nature of these interactions (Park and Cosgrove, 2015). One possibility is that XyGs bind directly to cellulose microfibrils (CMFs). Recent data indicating that crystalline cellulose cores are surrounded with hemicelluloses support this hypothesis (Dick-Pérez et al., 2011). It is also possible that XyG acts as a spacer-molecule to prevent CMFs from aggregating in cell walls (Anderson et al., 2010) or as an adapter to link cellulose with other cell wall components, such as pectin (Cosgrove, 2005; Cavalier et al., 2008). XyG can be covalently linked to pectin (Thompson and Fry, 2000; Popper and Fry, 2005, 2008), and NMR data demonstrate that pectins and cellulose might interact to a greater extent than XyG and cellulose in native walls (Dick-Pérez et al., 2011). Alternative models exist for how XyG-cellulose interactions influence primary wall architecture and mechanics. One such model posits that XyG chains act as load-bearing tethers that bind to CMFs in primary cell walls to form a cellulose-XyG network (Carpita and Gibeaut, 1993; Pauly et al., 1999; Somerville et al., 2004; Cosgrove, 2005). However, results have been accumulating against this tethered network model, leading to an alternative model in which CMFs make direct contact, in some cases mediated by a monolayer of xyloglucan, at limited cell wall sites dubbed “biomechanical hotspots,” which are envisioned as the key sites of cell wall loosening during cell growth (Park and Cosgrove, 2012a; Wang et al., 2013; Park and Cosgrove, 2015). Further molecular, biochemical, and microscopy experiments are required to help distinguish which aspects of the load-bearing, spacer/plasticizer, and/or hotspot models most accurately describe the functions of XyG in primary walls.Cortical microtubules (MTs) direct CMF deposition by guiding cellulose synthase complexes in the plasma membrane (Baskin et al., 2004; Paredez et al., 2006; Emons et al., 2007; Sánchez-Rodriguez et al., 2012), and the patterned deposition of cellulose in the wall in turn can help determine plant cell anisotropic growth and morphogenesis (Baskin, 2005). Disruption of cortical MTs by oryzalin, a MT-depolymerizing drug, alters the alignment of CMFs, suggesting that MTs contribute to CMF organization (Baskin et al., 2004). CELLULOSE SYNTHASE (CESA) genes, including CESA1, CESA3, and CESA6, are required for normal CMF synthesis in primary cell walls (Kohorn et al., 2006; Desprez et al., 2007), and accessory proteins such as COBRA function in cellulose production (Lally et al., 2001). Live-cell imaging from double-labeled YFP-CESA6; CFP-ALPHA-1 TUBULIN (TUA1) Arabidopsis seedlings provides direct evidence that cortical MTs determine the trajectories of cellulose synthesis complexes (CSCs) and patterns of cellulose deposition (Paredez et al., 2006). Additionally, MT organization affects the rotation of cellulose synthase trajectories in the epidermal cells of Arabidopsis hypocotyls (Chan et al., 2010). Recently, additional evidence for direct guidance of CSCs by MTs has been provided by the identification of CSI1/POM2, which binds to both MTs and CESAs (Bringmann et al., 2012; Li et al., 2012). MICROTUBULE ORGANIZATION1 (MOR1) is essential for cortical MT organization (Whittington et al., 2001), but disruption of cortical MTs in the mor1 mutant does not greatly affect CMF organization (Sugimoto et al., 2003), and oryzalin treatment does not abolish CSC motility (Paredez et al., 2006).Conversely, the organization of cortical MTs can be affected by cellulose synthesis. Treatment with isoxaben, a cellulose synthesis inhibitor, results in disorganized cortical MTs in tobacco cells, suggesting that inhibition of cellulose synthesis affects MT organization (Fisher and Cyr, 1998), and treatment with 2,6-dichlorobenzonitrile, another cellulose synthesis inhibitor, alters MT organization in mor1 plants (Himmelspach et al., 2003). Cortical MT orientation in Arabidopsis roots is also altered in two cellulose synthesis-deficient mutants, CESA6^52-isx and kor1-3, suggesting that CSC activity can affect MT arrays (Paredez et al., 2008). Together, these results point to a bidirectional relationship between cellulose synthesis/patterning and MT organization.MTs influence plant organ morphology, but the detailed mechanisms by which they do so are incompletely understood. The dynamics and stability of cortical MTs are also affected by MT-associated proteins (MAPs). MAP18 is a MT destabilizing protein that depolymerizes MTs (Wang et al., 2007), MAP65-1 functions as a MT crosslinker, and MAP70-1 functions in MT assembly (Korolev et al., 2005; Lucas et al., 2011). MAP70-5 stabilizes existing MTs to maintain their length, and its overexpression induces right-handed helical growth (Korolev et al., 2007); likewise, MAP20 overexpression results in helical cell twisting (Rajangam et al., 2008). CLASP promotes microtubule stability, and its mutant is hypersensitive to microtubule-destabilizing drug oryzalin (Ambrose et al., 2007). KATANIN1 (KTN1) is a MT-severing protein that can sever MTs into short fragments and promote the formation of thick MT bundles that ultimately depolymerize (Stoppin-Mellet et al., 2006), and loss of KTN1 function results in reduced responses to mechanical stress (Uyttewaal et al., 2012). In general, cortical MT orientation responds to mechanical signals and can be altered by applying force directly to the shoot apical meristem (Hamant et al., 2008). The application of external mechanical pressure to Arabidopsis leaves also triggers MT bundling (Jacques et al., 2013). Kinesins, including KINESIN-13A (KIN-13A) and FRAGILE FIBER1 (FRA1), have been implicated in cell wall synthesis (Cheung and Wu, 2011; Fujikura et al., 2014). The identification of cell wall receptors and sensors is beginning to reveal how plant cell walls sense and respond to external signals (Humphrey et al., 2007; Ringli, 2010); some of them, such as FEI1, FEI2, THESEUS1 (THE1), FERONIA (FER), HERCULES RECEPTOR KINASE1 (HERK1), WALL ASSOCIATED KINASE1 (WAK1), WAK2, and WAK4, have been characterized (Lally et al., 2001; Decreux and Messiaen, 2005; Kohorn et al., 2006; Xu et al., 2008; Guo et al., 2009; Cheung and Wu, 2011). However, the relationships between wall integrity, cytoskeletal dynamics, and wall synthesis have not yet been fully elucidated.In this study, we analyzed CMF patterning, MT patterning and dynamics, and cellulose biosynthesis in the Arabidopsis xxt1 xxt2 double mutant that lacks detectable XyG and displays altered growth (Cavalier et al., 2008; Park and Cosgrove, 2012a). To investigate whether and how XyG deficiency affects the organization of CMFs and cortical MTs, we observed CMF patterning in xxt1 xxt2 mutants and Col (wild-type) controls using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and confocal microscopy (Hodick and Kutschera, 1992; Derbyshire et al., 2007; Anderson et al., 2010; Zhang et al., 2014). We also generated transgenic Col and xxt1 xxt2 lines expressing GFP-MAP4 (Marc et al., 1998) and GFP-CESA3 (Desprez et al., 2007), and analyzed MT arrays and cellulose synthesis using live-cell imaging. Our results show that the organization of CMFs is altered, that MTs in xxt1 xxt2 mutants are aberrantly organized and are more sensitive to external mechanical pressure and the MT-depolymerizing drug oryzalin, and that cellulose synthase motility and cellulose content are decreased in xxt1 xxt2 mutants. Furthermore, real-time quantitative RT-PCR measurements indicate that the enhanced sensitivity of cortical MTs to mechanical stress and oryzalin in xxt1 xxt2 plants might be due to altered expression of MT-stabilizing and wall receptor genes. Together, these data provide insights into the connections between the functions of XyG in wall assembly, the mechanical integrity of the cell wall, cytoskeleton-mediated cellular responses to deficiencies in wall biosynthesis, and cell and tissue morphogenesis.     

Arrangement of Cellulose Microfibrils in Walls of Elongating Parenchyma Cells

The arrangement of cellulose microfibrils in walls of elongating parenchyma cells of Avena coleoptiles, onion roots, and celery petioles was studied in polarizing and electron microscopes by examining whole cell walls and sections. Walls of these cells consist firstly of regions containing the primary pit fields and composed of microfibrils oriented predominantly transversely. The transverse microfibrils show a progressive disorientation from the inside to the outside of the wall which is consistent with the multinet model of wall growth. Between the pit-field regions and running the length of the cells are ribs composed of longitudinally oriented microfibrils. Two types of rib have been found at all stages of cell elongation. In some regions, the wall appears to consist entirely of longitudinal microfibrils so that the rib forms an integral part of the wall. At the edges of such ribs the microfibrils can be seen to change direction from longitudinal in the rib to transverse in the pit-field region. Often, however, the rib appears to consist of an extra separate layer of longitudinal microfibrils outside a continuous wall of transverse microfibrils. These ribs are quite distinct from secondary wall, which consists of longitudinal microfibrils deposited within the primary wall after elongation has ceased. It is evident that the arrangement of cellulose microfibrils in a primary wall can be complex and is probably an expression of specific cellular differentiation.     

Deficient Glutathione in Guard Cells Facilitates Abscisic Acid-Induced Stomatal Closure but Does Not Affect Light-Induced Stomatal Opening

We investigated the role of glutathione (GSH) in stomatal movements using a GSH deficient mutant, chlorinal-1 (ch1-1). Guard cells of ch1-1 mutants accumulated less GSH than wild types did. Light induced stomatal opening in ch1-1 and wild-type plants. Abscisic acid (ABA) induced stomatal closure in ch1-1 mutants more than wild types without enhanced reactive oxygen species (ROS) production. Therefore, GSH functioned downstream of ROS production in the ABA signaling cascade.     

Actin Filaments in Mature Guard Cells Are Radially Distributed and Involved in Stomatal Movement

Stomatal movements, which regulate gas exchange in plants, involve pronounced changes in the shape and volume of the guard cell. To test whether the changes are regulated by actin filaments, we visualized microfilaments in mature guard cells and examined the effects of actin antagonists on stomatal movements. Immunolocalization on fixed cells and microinjection of fluorescein isothiocyanate-phalloidin into living guard cells of Commelina communis L. showed that cortical microfilaments were radially distributed, fanning out from the stomatal pore site, resembling the known pattern of microtubules. Treatment of epidermal peels with phalloidin prior to stabilizing microfilaments with m-maleimidobenzoyl N-hydroxysuccimimide caused dense packing of radial microfilaments and an accumulation of actin around many organelles. Both stomatal closing induced by abscisic acid and opening under light were inhibited. Treatment of guard cells with cytochalasin D abolished the radial pattern of microfilaments; generated sparse, poorly oriented arrays; and caused partial opening of dark-closed stomata. These results suggest that microfilaments participate in stomatal aperture regulation.     

茉莉酸甲酯诱导保卫细胞气孔关闭的信号转导机制

气孔是由植物器官表面成对的保卫细胞围成的小孔,气孔运动控制气体交换,与植物逆境应答和生长发育等生物学过程密切相关,受多种因子调控,茉莉酸甲酯(MeJA)是其中之一。与ABA类似,MeJA也可诱导气孔关闭,但是其机理尚不清楚。该文综述了近年来MeJA调控气孔运动的信号转导机制进展,包括Ca2+、胞质pH、活性氧和NO等第二信使对气孔开闭的影响以及COI1、JAR1、RCN1和TGG1/2等信号组分之间的调控关系,并讨论了保卫细胞中MeJA与ABA信号途径的相互作用。