An emerging signalling complex critical for modulating the stomatal aperture in response to environmental stimuli.

 

The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action

by Sirichandra C., Wasilewska A.,Vlad F., Valon C., Leung J. (2009)

Institut des Sciences du Végetal, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France.

in J Exp Bot 2009, 60: 1439-1463. – doi:10.1093/jxb/ern340 pmid:19181866 – 

Abstract/FREE Full Text – PubMed – 

https://www.ncbi.nlm.nih.gov/pubmed/19181866?dopt=Abstract

 

Abstract

Stomatal guard cells are functionally specialized epidermal cells usually arranged in pairs surrounding a pore. Changes in ion fluxes, and more specifically osmolytes, within the guard cells drive opening/closing of the pore, allowing gas exchange while limiting water loss through evapo-transpiration.

Adjustments of the pore aperture to optimize these conflicting needs are thus centrally important for land plants to survive, especially with the rise in CO(2) associated with global warming and increasing water scarcity this century. The basic biophysical events in modulating membrane transport have been gradually delineated over two decades. Genetics and molecular approaches in recent years have complemented and extended these earlier studies to identify major regulatory nodes.

In Arabidopsis, the reference for guard cell genetics, stomatal opening driven by K(+) entry is mainly through KAT1 and KAT2, two voltage-gated K(+) inward-rectifying channels that are activated on hyperpolarization of the plasma membrane principally by the OST2 H(+)-ATPase (proton pump coupled to ATP hydrolysis).

By contrast, stomatal closing is caused by K(+) efflux mainly through GORK, the outward-rectifying channel activated by membrane depolarization. The depolarization is most likely initiated by SLAC1, an anion channel distantly related to the dicarboxylate/malic acid transport protein found in fungi and bacteria.

Beyond this established framework, there is also burgeoning evidence for the involvement of additional transporters, such as homologues to the multi-drug resistance proteins (or ABC transporters) as intimated by several pharmacological and reverse genetics studies.

General inhibitors of protein kinases and protein phosphatases have been shown to profoundly affect guard cell membrane transport properties. Indeed, the first regulatory enzymes underpinning these transport processes revealed genetically were several protein phosphatases of the 2C class and the OST1 kinase, a member of the SnRK2 family.

Taken together, these results are providing the first glimpses of an emerging signalling complex critical for modulating the stomatal aperture in response to environmental stimuli.

An emerging signalling complex critical for modulating the stomatal aperture

Photo credit: JXB

Fig. 1.

Genetic screen of Arabidopsis mutants sensitive to progressive drought. (a) Closing (ABA, drought) and opening (light) of the stomatal pore alter the transpirational flux (top) with amplitudes sufficiently large to be visualized by remote infrared thermography (bottom). In comparison to wild-type plants, mutants that are impaired in stomatal closure in conditions of water deficit are therefore cooler (plants noted by circles in the infrared images). Figures are modified from those in Merlot et al. (2002) and Roelfsema and Hedrich (2005). (b) Summary of transport events at the plasma membrane of the guard cell. In responding to drought or ABA, the anion channel (SLAC) is activated, which drives depolarization of the plasma membrane, while the activity of the proton pump ATPase (OST2), needed for plasma membrane hyperpolarization, is inhibited. Both of these membrane transporters are regulated by Ca²⁺, and, in the case of OST2, the inhibition is most likely mediated, at least in part, by the calcium-sensitive PSK5 kinase. Membrane depolarization also leads to activation of the K⁺-outward-rectifying channel (GORK) to expel K⁺ and inhibition of the K⁺-inward-rectifying channels (KAT1 and KAT2) for K⁺ influx, both of these events are coordinated in opposing fashion by a pH-sensitive pathway that is in part membrane limited.

The guard cell as a single-cell model towards understanding drought tolerance and abscisic acid action

by Sirichandra C., Wasilewska A., Vlad F., Valon C., Leung J. (2009)

in J. Exp. Bot. 2009, 60: 1439-1463.

(PubMed Abstract | Publisher Full Text)

Abstract

Stomatal guard cells are functionally specialized epidermal cells usually arranged in pairs surrounding a pore. Changes in ion fluxes, and more specifically osmolytes, within the guard cells drive opening/closing of the pore, allowing gas exchange while limiting water loss through evapo-transpiration.

Adjustments of the pore aperture to optimize these conflicting needs are thus centrally important for land plants to survive, especially with the rise in CO₂ associated with global warming and increasing water scarcity this century. The basic biophysical events in modulating membrane transport have been gradually delineated over two decades. Genetics and molecular approaches in recent years have complemented and extended these earlier studies to identify major regulatory nodes.

In Arabidopsis, the reference for guard cell genetics, stomatal opening driven by K⁺ entry is mainly through KAT1 and KAT2, two voltage-gated K⁺ inward-rectifying channels that are activated on hyperpolarization of the plasma membrane principally by the OST2 H⁺-ATPase (proton pump coupled to ATP hydrolysis). By contrast, stomatal closing is caused by K⁺ efflux mainly through GORK, the outward-rectifying channel activated by membrane depolarization.

The depolarization is most likely initiated by SLAC1, an anion channel distantly related to the dicarboxylate/malic acid transport protein found in fungi and bacteria. Beyond this established framework, there is also burgeoning evidence for the involvement of additional transporters, such as homologues to the multi-drug resistance proteins (or ABC transporters) as intimated by several pharmacological and reverse genetics studies.

General inhibitors of protein kinases and protein phosphatases have been shown to profoundly affect guard cell membrane transport properties. Indeed, the first regulatory enzymes underpinning these transport processes revealed genetically were several protein phosphatases of the 2C class and the OST1 kinase, a member of the SnRK2 family.

Taken together, these results are providing the first glimpses of an emerging signalling complex critical for modulating the stomatal aperture in response to environmental stimuli.

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