Why guard cells occur in pairs

Tanaka, Y. Ethylene inhibits abscisic acid-induced stomatal closure in Arabidopsis. Tang, R. Torres, M. Six Arabidopsis thaliana homologues of the human respiratory burst oxidase gp91phox.

Plant J 14 , Vahisalu, T. SLAC1 is required for plant guard cell S-type anion channel function in stomatal signalling. Wang, X. Differential responses of abaxial and adaxial guard cells of broad bean to abscisic acid and calcium.

G protein regulation of ion channels and abscisic acid signaling in Arabidopsis guard cells. Ward, J. Roles of ion channels in initiation of signal transduction in higher plants. The Plant Cell 7 , Webb, A. The role of calcium in ABA-induced gene expression and stomatal movements.

Plant J 26 , Carbon dioxide induces increases in guard cell cytosolic free calcium. Plant Journal 9 , Xie, X. The identification of genes involved in the stomatal response to reduced atmospheric relative humidity. Curr Biol 16 , Xiong, L. Modulation of abscisic acid signal transduction and biosynthesis by an Sm-like protein in Arabidopsis.

Cell 1 , Yamaguchi-Shinozaki, K. Transcriptional regulatory networks in cellular responses and tolerance to dehydration and cold stresses. Annu Rev Plant Biol 57 , Yang, Y. Isolation of a strong Arabidopsis guard cell promoter and its potential as a research tool.

Plant Methods 4 , 6. Yoshida, R. Plant Cell Physiol. Yoshida, T. Young, J. Zhang, W. Osmo-sensitive and stretch-activated calcium-permeable channels in Vicia faba guard cells are regulated by actin dynamics. Phospholipase D alpha 1-derived phosphatidic acid interacts with ABI1 phosphatase 2C and regulates abscisic acid signaling. Zhang, X. Hydrogen peroxide is involved in abscisic acid-induced stomatal closure in Vicia faba. Zhang, Y. Zhao, R. Zheng, Z.

Zhu, J. Salt and drought stress signal transduction in plants. Plant Biol. Zhu, S. This interactive clickable resource includes the following features: Figure 1. Guard cells can respond to physiological stimuli even after they are isolated, allowing cell biological and time-resolved analyses of stomatal opening and closure in response to various stimuli.

D Corresponding authors, email: julian biomail. PubMed Allen, G. PubMed Anderson, J. PubMed Ashcroft, F. PubMed Assmann, S. PubMed Baum, G. PubMed Bihler, H. PubMed Blatt M. PubMed Blatt, M.

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Plants release oxygen during the day in the presence of natural light through the process of photosynthesis. While at night, the plants uptake oxygen and release carbon dioxide, which is called respiration.

Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Physics What is guard cell and its function?

Ben Davis December 14, High stiffness is concentrated at the polar ends of guard cell pairs based on published AFM data Carter et al.

Differential wall thickness is illustrated in cross-sectional views of stomata in both grasses A and dicots B. Growing plant cells are encased by a three-dimensional cell wall called the primary wall, wherein cellulose is embedded in a matrix containing hemicelluloses, pectins, and structural proteins Somerville et al.

The biosynthesis, modification, degradation, and reorganization of wall polymers and their interactions make the primary wall quite complex and dynamic Voiniciuc et al. The composition of the primary wall is diverse across plant species.

For instance, there are two major types of primary walls in flowering plants, based on the relative amounts and types of matrix polymers. For a given plant cell, wall composition undergoes spatiotemporal changes during cell development and differentiation, with older polymers such as middle lamellar pectins being deposited earlier and thus being farther from the plasma membrane, and nascent materials being laid down later and thus being closer to the cell surface Keegstra, Cell growth in the short term, such as over a few minutes, can involve large-scale reorientations of wall components Anderson et al.

Cellulose is synthesized at the cell surface by plasma membrane-localized cellulose synthase complexes CSCs Paredez et al. Cellulose is the most ordered wall polymer and is often oriented transversely to the growth axis of a cell, providing tensile strength to the wall Green, Hemicelluloses e.

Xyloglucan can intertwine with cellulose, forming junctions that serve as mechanical hotspots for wall loosening Park and Cosgrove, a , b. Xyloglucan in extended conformations can also bind to the hydrophobic faces of cellulose Zheng et al.

HG is the simplest and most abundant pectin domain. Highly methyl-esterified HG is exocytosed to the wall where it is then de-methyl-esterified by pectin methylesterases PMEs Wolf et al. In model species such as Arabidopsis, genes encoding these pectin-modifying and -degrading enzymes all exist in large families McCarthy et al.

Figure 2. Homogalacturonan HG is synthesized in the Golgi, and is de-methyl-esterified and degraded in the apoplast. Highly methyl-esterified HG is then exocytosed to the apoplast, where it is de-methyl-esterified by pectin methyl-esterases PMEs.

We have gained our knowledge of the primary wall predominantly from studies in tissue types that undergo irreversible expansion, such as roots Anderson et al.

Below, we will present an update on recent research and suggest future directions to advance our understanding of the molecular details of how guard cell walls are built to allow for fast and reversible stomatal movements. The polysaccharide components of guard cell walls have been identified mostly by imaging experiments, including polarized light microscopy for cellulose, and immunolabeling coupled with fluorescence microscopy or transmission electron microscopy in thin sections for hemicelluloses and pectins Palevitz and Hepler, ; Majewska-Sawka et al.

These techniques, although they are not quantitative, can reveal differences in wall composition between guard cells and neighboring cells. For example, LM15 antibody-labeled xyloglucan is more enriched in guard cells than in neighboring epidermal cells in Arabidopsis Amsbury et al.

Polysaccharide components of the guard cell wall that are conserved across species include cellulose, HG, and RG-I. Pectic arabinan, in particular, has been demonstrated to maintain the flexibility of guard cell walls in various species, since exogenous treatment with arabinanase in epidermal strips prevents stomatal opening or closure in species such as Commelina communis and Vicia faba Jones et al.

Cell wall structural proteins have also been found to be present in the guard cell wall by functional characterizations or immunolabeling approaches. Plants lacking FOCL1 have larger stomata and are impaired in controlling stomatal aperture and transpiration rate Hunt et al.

Despite the visualization of representative wall components, a global, quantitative analysis of guard cell wall composition is still missing. This is largely due to technical difficulties in isolating and enriching enough guard cell wall materials for quantitative compositional assays.

Knowing the relative amount of each wall component will aid the comparison of wall constitution between guard cells and other cell types, between dicots and monocots, and between wild type and mutant plants. Guard cell walls are synthesized and deposited in the apoplast during stomatal development Movie S1. As a result, their thickness gradually and differentially increases at different regions as stomata mature, with outer and inner periclinal walls eventually being thicker than ventral and dorsal walls Zhao and Sack, ; Merced and Renzaglia, ; Figure 1.

However, the molecular details of how these differentially thickened walls are synthesized and how their synthesis is spatially controlled are not clearly understood, raising several questions, e. Transcriptomic datasets of stomatal lineage cells Hachez et al. Cellulose is actively synthesized in young guard cells, and is likely to contribute to the build-up of wall strength to withstand the high turgor pressure inside a guard cell.

In Arabidopsis, although genes encoding primary wall-associated cellulose synthases CESAs are not highly expressed in stomatal lineage cells Adrian et al. Future studies of other glycosyltransferase families such as the cellulose synthase-like C CSLC family and the GAUT family, which are required for the synthesis of xyloglucan and pectins, respectively Cocuron et al.

Some components of the guard cell wall are conserved across terrestrial plants, but their distribution patterns can be distinct in different species. For example, in ferns, dicots, and some monocots where stomata are kidney-shaped, cellulose exhibits an overall radial arrangement Palevitz and Hepler, ; Fujita and Wasteneys, ; Rui and Anderson, ; Shtein et al.

Compared to cellulose, there are fewer studies on the spatial organization of matrix polysaccharides in guard cells, although stretches of de-methyl-esterified HG have been reported to be diffusely distributed in the periclinal wall, but enriched at the polar ends of guard cell pairs in Arabidopsis Carter et al. Visualization of the organization of the guard cell wall is challenging, partly due to the particularly thick cuticles that prevent penetration of many probes Voiniciuc et al. To address these issues and to learn how individual components of guard cell walls are distributed in 3D, one direction is to develop and apply a library of small fluorescent dyes Anderson and Carroll, that can penetrate the cuticle and bind to specific wall components in guard cells.

Dyes that are compatible with super-resolution imaging techniques such as structured illumination microscopy SIM Gustafsson, and stochastic optical reconstruction microscopy STORM Huang et al. Results from these dye-based imaging experiments should be interpreted with the caution that the dye might alter the function of the wall component to which it binds. One hypothesis we have raised is that stomatal movement is accompanied by the dynamic reorganization of wall components in guard cells Figure 3 ; Movie S1.

One piece of data that supports this hypothesis is that cellulose microfibrils in intact guard cells of Arabidopsis exhibit a relatively even distribution when stomata are open, but become more bundled and evidently fibrillar when stomata are closed Figure 3 ; Movie S1 Rui and Anderson, In addition, stomatal apertures during stomatal movements are larger in cesa3 je 5 mutants but smaller in xxt1 xxt2 mutants compared to wild type controls Rui and Anderson, These observations suggest that the construction of a wall that facilitates cellulose reorganization and proper control of stomatal aperture depends on sufficient levels of cellulose and xyloglucan Movie S1 Rui and Anderson, In addition to cellulose reorganization, we also proposed that during stomatal movements, pectins might undergo remodeling from being un-crosslinked in the open state to crosslinked in the closed state Figure 3 ; Movie S1 Rui et al.

To further test the above hypotheses, high-resolution imaging of multiple wall constituents in living guard cells during the movements of individual stomata will be needed to reveal and quantify any spatiotemporal changes in nanoscale wall organization.

Figure 3. Mature guard cells might use wall materials that have been deposited to meet their functional requirement for repeated elastic deformation. Note that 3D renderings were made via computerized segmentation of 3D microscopic images of actual guard cells. During stomatal opening, cellulose becomes diffusely distributed, and pectic homogalacturonan networks are uncoupled. See also Movie S1. A transposon insertional mutant of PME6 exhibits a narrower range of stomatal conductance in response to changes in CO 2 level or light intensity, as demonstrated by two independent studies Negi et al.

Although PME6 has not yet been biochemically confirmed to act as a PME, methyl-esterified pectin epitopes are more abundant in pme6 guard cell walls than in wild type controls, suggesting a link between cell wall composition and stomatal function in guard cells Amsbury et al. Alternatively, cell wall-modifying genes that are more highly expressed in guard cells than in guard mother cells are candidates for functional characterizations in stomatal maturation Adrian et al.

Genes encoding wall-modifying proteins can also be identified by screening using assays to test stomatal physiology. In a thermotolerance screen using a collection of Arabidopsis PME mutants, a pme34 mutant was found to be less tolerant to heat stress than wild type controls Huang et al. These data suggest that disrupting pectin modification by PME34 might impair water evaporation through stomata during heat stress.

Because de-methyl-esterified HG produced by PMEs can have contrasting mechanical effects on the wall depending on how the de-methyl-esterification occurs Hocq et al. Because PMEs enzymatically remove methyl groups from the pectin backbone in the apoplast whereas putative PMTs are localized in the Golgi Mouille et al.

Instead, the de-methyl-esterification patterns of HG generated by PME activity and their mechanical effects on the guard cell wall are more likely established during guard cell morphogenesis Movie S1. Such effects persist in the walls of mature guard cells to allow for repetitive stomatal movements. Other cell wall-modifying proteins act non-enzymatically, and include expansins, which can cause pH-dependent wall loosening and extension Cosgrove, These experimental data open up the possibility that expansin-mediated wall loosening might be another dynamic process that acts independently or synergistically with the reorganization of wall components during stomatal movement.

Plant cell walls can be degraded by exogenous or endogenous glycoside hydrolases GHs. Exogenous treatment with arabinanase in epidermal strips prevents stomatal opening or closure in species such as C.

The final step of stomatal development requires partial separation of the wall between sister guard cells Bergmann and Sack, , which based on analogous cell separation events in other plant tissues Liljegren, likely involves pectin degradation in the middle lamella. However, virtually no data have been reported to support this hypothesis, although pectic strands haven been shown to be present in newly formed stomata Carr et al.

In cotyledons, GFP-tagged PGX3 is enriched at sites of stomatal pore initiation and PGX3 expression is associated with pore dimensions, suggesting that pectin degradation by PGX3 contributes to the controlled cell separation between sister guard cells during stomatal pore formation Rui et al.

It remains to be tested whether additional mechanisms other than pectin degradation exist to separate sister guard cells at pore initiation sites, and how pectin degradation is spatially restricted to facilitate pore formation while retaining strong connections between the ends of sister guard cells at their poles Carter et al.

In adult true leaves of Arabidopsis, PGX3 regulates stomatal dynamics by fine-tuning the abundance of de-methyl-esterified HG and pectin molecular size, providing a molecular explanation for how pectins maintain the flexibility of guard cell walls during stomatal movement Movie S1 Rui et al. In addition to PGs, it would also be worthwhile to extend functional characterizations to genes encoding other wall-degrading enzymes, such as pectate lyases and glucanases.

Atomic force microscopy AFM has been used to visualize the pattern and movement of cellulose microfibrils on the nanoscale in onion epidermis Zhang et al. Unfortunately, cellulose microfibrils in the guard cell wall cannot be directly probed by AFM due to the presence of cuticles in aerial tissues.

However, stiffness distribution on the cellular scale in guard cells and neighboring pavement cells can still be revealed by AFM Sampathkumar et al. Recently, Carter et al. There has been growing interest in assessing responses to mechanical stress with cellular resolution in live tissues such as Arabidopsis cotyledons Bringmann and Bergmann, ; Robinson et al.

Bringmann and Bergmann applied stretch forces to whole cotyledons using elastic strips and observed that the distribution of polarity markers for stomatal stem cells follows the direction of tissue-wide tensile stress Bringmann and Bergmann, This finding should be interpreted with the caveats that polarity markers are not imaged simultaneously when stretch is applied and that the amount of force applied to the cotyledon is undefined.

A future avenue to understanding the stress-strain relationship of the guard cell wall is to apply ACME in leaves or epidermal peels, with the caution that mechanical stress is applied to whole tissue rather than to individual stomata.

In addition, the setup of an ACME for this purpose will require expertise and perhaps some customization. Stomatal opening and closure are driven by turgor changes in guard cells, but direct measurement of guard cell turgor is technically challenging mainly due to the small size of guard cells and their vacuoles Franks et al. To our knowledge, pressure values of guard cells have been determined by pressure probe in only a few species e.

They would normally only close in the dark when no carbon dioxide is needed for photosynthesis. Guard cells are adapted to their function by allowing gas exchange and controlling water loss within the leaf. The size of the stomatal opening is used by the plant to control the rate of transpiration and therefore limit the levels of water loss from the leaf.

This helps to stop the plant from wilting. Transport in plants and the structure of specialised plant cells Plants do not have a heart, blood or a circulation system, but they do need a transport system to move food, water and minerals around. Root hair cells Plants absorb water from the soil by osmosis.