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

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.

Type 2C protein phosphatases directly regulate abscisic acid-activated protein kinases in Arabidopsis

“…In contrast, dominant mutations in PP2C, such as abi1-1, allow the protein to avoid RCAR/PYR-binding (6,7), resulting in the constitutive inactivation of SnRK2s. At present, the mechanisms underlying the internal phosphorylation of SnRK2 remain unclear, although autophosphorylation is one of possible regulations (2). Further analysis will be required to clarify this issue.…”

“…The phytohormone abscisic acid (ABA) plays a central role in such responses (1,2), and is also involved in many developmental processes (3) and defense systems (4). Thus, ABA functions as a key molecule that unifies and regulates biotic and abiotic stress responses and the developmental status of the plant.…”

MYB30 regulates root cell elongation under abscisic acid signaling

“…In guard cells and plant shoot parts, ROS levels have been reported to be increased by ABA [16]. To investigate whether ABA induces ROS levels in the root, we stained Col-0 and myb30 specimens with dihydroethidium (DHE) and BES-H 2 O 2 -Ac (O 2 − and H 2 O 2 indicators, respectively) after 24 h treatment with 5 µM ABA (Figure 3).…”

The Arabidopsis ZINC FINGER PROTEIN3 Interferes with Abscisic Acid and Light Signaling in Seed Germination and Plant Development

“…ABA regulates seed maturation, promotes seed dormancy and desiccation tolerance and the synthesis of seed storage proteins and lipids, and inhibits phase transitions from embryonic growth to germination and from vegetative to reproductive growth. In addition to its role in plant development, ABA controls responses to drought and salt stresses, including stomatal closure, activation of genes involved in osmotic adjustment, ion compartmentalization, regulation of shoot versus root growth, and modifications of root hydraulic conductivity (Verslues and Zhu, 2005;Sirichandra et al, 2009;Fujita et al, 2011;Finkelstein, 2013).…”