Title: Flow‐limiting valve for <scp>ABA</scp> signalling in stomatal guard cells
Abstract: New PhytologistVolume 200, Issue 4 p. 943-945 CommentaryFree Access Flow-limiting valve for ABA signalling in stomatal guard cells Toshinori Kinoshita, Corresponding Author Toshinori Kinoshita Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Nagoya, 464-8602 Japan(tel +81 52 789 4778; email [email protected]) Search for more papers by this author Toshinori Kinoshita, Corresponding Author Toshinori Kinoshita Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Nagoya, 464-8602 Japan(tel +81 52 789 4778; email [email protected]) Search for more papers by this author First published: 04 November 2013 https://doi.org/10.1111/nph.12579Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Stomatal pores, surrounded by paired guard cells and located in the epidermes of plants, regulate gas exchange in response to both environmental and endogenous signals. Stomatal opening facilitates both transpiration and CO2 entry to enable photosynthesis. When water is scarce, the plant hormone abscisic acid (ABA) promotes stomatal closure and inhibits stomatal opening, thereby reducing water loss (Kim et al., 2010). Recently, a family of START domain proteins, the pyrabactin resistance/pyrabactin resistance 1-like/regulatory component of ABA receptors (PYR/PYL/RCARs), has been convincingly identified as a group of cytosolic ABA receptors (Cutler et al., 2010). Protein kinases also play critical roles in ABA signalling; an ABA-activated protein kinase (AAPK) was identified in Vicia faba and shown to be a positive regulator in guard cell ABA signalling (Li et al., 2000), and subsequently, Open Stomata 1 (OST1), the Arabidopsis ortholog of AAPK, was identified (Mustilli et al., 2002; Yoshida et al., 2002). OST1 belongs to the SRK2/SnRK2 serine/threonine protein kinase family with 10 members. Recognition of ABA by PYR/PYL/RCARs activates members of the protein kinase SRK2/SnRK2 family, including OST1, via inactivation of negative regulators, the type-2C protein phosphatases (PP2Cs) that include ABI1 and ABI2. Today, the early acting ABA-signalling pathway, PYR/PYL/RCARs-PP2Cs-SRK2/SnRK2s, is considered to mediate most ABA-triggered plant responses, including the inhibition of seed germination and root growth, stomatal closure, and the expression of particular genes (Cutler et al., 2010). In the case of ABA-induced stomatal closure, OST1 (SnRK2.6 or SRK2E) has been suggested to act as a critical output factor early in the ABA-signalling pathway. However, the molecular mechanism by which OST1 mediates ABA-signalling in guard cells is incompletely understood. In this issue of New Phytologist, Acharya et al. (pp. 1049–1063). report that OST1 plays a central and limiting role in ABA-mediated stomatal regulation in Arabidopsis, based on a comprehensive set of experiments. 'OST1 is essential and limiting for ABA regulation of guard cell ion channels and NADPH oxidases, and rate-limiting for ABA-induction of stomatal closure.' Role of OST1 in the ABA inhibition of guard cell K+in currents During stomatal opening, the plasma membrane K+in channel (inward K+ channels) mediates K+ uptake in response to plasma membrane hyperpolarization, which is effected by the activity of plasma membrane H+-ATPase in guard cells. Several genes, such as KAT1, KAT2, AKT1, AKT2/3, and AtKC1, have been shown to act as inward K+ current facilitators. The K+in current in guard cells has been shown to be inhibited by ABA (Kim et al., 2010), and KAT1, one of the major K+in channels, has been observed to be phosphorylated by OST1 in vitro (Sato et al., 2009). However, the regulation of K+in currents by OST1 has yet to be investigated directly in guard cells. To clarify the regulation of the K+in channel by OST1, K+in channel activity in an ost1 mutant and OST1 overexpressor lines, and the interaction between OST1 and the K+in channel were examined. The results showed that ost1 mutants are insensitive to ABA inhibition of the K+in channel, whereas OST1 overexpressors are ABA-hypersensitive. In addition, OST1 was shown to interact with the K+in channel KAT1 in plant cells. These results indicate that OST1 is required for the ABA regulation of K+in channels in guard cells presumably via phosphorylation. It should be noted that OST1 overexpressors show normal K+in currents in the absence of ABA, indicating that the regulation of K+in currents by OST1 strictly depends on ABA. Although not reported here by Acharya et al., ABA also suppresses blue-light-dependent phosphorylation/activation of the plasma membrane H+-ATPase in guard cells via OST1 activation (Hayashi et al., 2011). Inhibition of both the important components for stomatal opening, the H+-ATPase and K+in channel, by ABA would be expected to inhibit stomatal opening (Fig. 1). Figure 1Open in figure viewerPowerPoint Schematic drawing of the regulation of abscisic acid (ABA)-signalling in guard cells by open stomata 1 (OST1). This model integrates data fromAcharya et al. (this issue of New Phytologist, pp. 1049–1063) and relevant literature. Arrowheads, activation; short perpendicular bars, inhibition. OST1 is required for ABA-induced activation of guard cells slows anion currents ABA-induced stomatal closure is driven by efflux of K+ from guard cells; the ions pass through voltage-dependent outward-rectifying K+ channels in the plasma membrane. Activation of these K+ channels requires depolarization of the membrane, achieved principally by activation of anion channels within the plasma membrane (Kim et al., 2010). A slow anion channel, SLAC1, is a critical player in ABA-mediated stomatal closure (Negi et al., 2008; Vahisalu et al., 2008). Acharya et al. found that OST1 is also a critical and limiting component for the regulation of ABA-mediated activation of SLAC1, and that OST1 physically interacts with SLAC1. An earlier study indicated that the co-expression of SLAC1 with OST1 in an oocyte system activated anion channel activity in the absence of ABA (Geiger et al., 2009). However, Acharya et al. demonstrate that in the native guard cells, overexpression of OST1 does not alter basal (non-ABA stimulated) anion currents, suggesting the requirement of ABA-activated OST1 in guard cells for the activation of slow anion currents. In addition, OST1 overexpressors showed larger anion currents in guard cells only in the presence of ABA. Taken together, these results reveal the importance of assays conducted directly in plant cells and tissues. It is worth noting that calcium-dependent protein kinases, such as CPK3, CPK6, CPK21, and CPK23, have been shown to be involved in ABA-induced activation of slow anion currents/SLAC1 activation (Mori et al., 2006; Geiger et al., 2010; Brandt et al., 2012). However, the accumulated results with respect to OST1 and CPKs suggest that OST1 plays a more prominent role than CPKs in planta. Further investigation is required to clarify the contribution/significance of CPKs in ABA-induced stomatal closure. OST1 in the ABA-induced activation of guard cell Ca2+ currents and reactive oxygen species (ROS) production Ca2+ is an important second messenger in guard cell ABA signalling (Kim et al., 2010). ABA activates Ca2+-permeable channels in the plasma membrane of guard cells. However, whether OST1 plays a role in the ABA activation of Ca2+ channel activity has yet to be investigated. Acharya et al. show that OST1 is also a critical component in the regulation of the ABA-mediated activation of Ca2+ currents, in that OST1 overexpressors exhibited enhanced Ca2+ currents in guard cells only in the presence of ABA. In addition, ABA induces reactive oxygen species (ROS) production in guard cells, and ROS are also important secondary messengers in ABA signalling. Two nicotinamide adenine dinucleotide phosphate (NADPH) oxidase catalytic subunits in Arabidopsis, AtrbohD and AtrbohF, are involved in ABA-induced ROS production, Ca2+ channel activation, and stomatal closure (Kwak et al., 2003). Acharya et al. provide evidence that OST1 not only interacts with the NADPH oxidase AtrbohF, but also with AtrbohD. The ost1 mutant showed ABA-induced ROS production; however, enhanced ROS generation was observed in the guard cells of OST1 overexpression lines. Taken together, these findings suggest that OST1 is a positive regulator of Atrboh and ROS production. Concluding perspectives and future research Acharya et al. clearly demonstrate that OST is a critical limiting component in the ABA regulation of stomatal apertures, ion channels, and NADPH oxidases in Arabidopsis guard cells (Fig. 1), and greatly contribute to a comprehensive understanding of OST1 functions, which has been sought since the discovery of AAPK and OST1 in ABA regulation of stomatal apertures. Moreover, Acharya et al. provide data showing that the overexpression of OST1 in the absence of ABA does not affect the stomatal phenotype or target activities. These findings suggest that normal stomatal responses in OST1 overexpressors would be expected in the absence of ABA, and that overexpression of OST1 and/or its orthologs could be a valuable candidate gene approach for the engineering of drought-resistant crops, by accelerating stomatal closure in response to water limitation. References Acharya BR, Jeon BW, Zhang W, Assmann SM. 2013. Open Stomata 1 (OST1) is limiting in abscisic acid responses of Arabidopsis guard cells. New Phytologist 200: 1049– 1063. Brandt B, Brodsky DE, Xue S, Negi J, Iba K, Kangasjarvi J, Ghassemian M, Stephan AB, Hu H, Schroeder JI. 2012. Reconstitution of abscisic acid activation of SLAC1 anion channel by CPK6 and OST1 kinases and branched ABI1 PP2C phosphatase action. Proceedings of the National Academy of Sciences, USA 109: 10593– 10598. Cutler SR, Rodriguez PL, Finkelstein RR, Abrams SR. 2010. Abscisic acid: emergence of a core signaling network. Annual Review of Plant Biology 61: 651– 679. 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Plant and Cell Physiology 43: 1473– 1483. Citing Literature Volume200, Issue4December 2013Pages 943-945 FiguresReferencesRelatedInformation