The Effect of SnRK26 on SLAC1 and KAT1

PP2Cs inhibit SnRK2.6 (open stomata 1, OST1) via dephosphorylation of Serine 175, which is in the kinase activation loop (Umezawa et al. 2009; Vlad et al. 2009) . SnRK2.6 regulates fast ABA responses resulting in stomata closure. SnRK2.6 directly interacts with slow anion channel-associated 1 (SLAC1), S-type anion channel on the plasma membrane of guard cells. SnRK2.6 phos-phorylates the N-terminal, cytoplasmic region of this channel. This phosphorylation is abolished by ABI1 (Geiger et al. 2009). PYR1, in the presence of ABA, is able to remove this inhibition (Fig. 19.1). Derepression of SLAC1 was also achieved with RCAR1(PYL9) (Geiger et al. 2010).

Fig. 19.1 Scheme of ABA derepression of SnRK in the cytoplasm and nucleus

Activation of this channel stimulates anion efflux which causes membrane depolarization. Geiger et al. (2010) reported that SLAC1 is regulated also by Ca-dependent protein kinase CPK23. SLAC1 regulation involves interaction with other Ca-dependent protein kinases, such as CPK3 and CPK6 (Mori et al. 2006).

SnRK2.6 phosphorylates also cation channel KAT1 (inward K+ channel, Sato et al. 2009). In this case, however, phosphorylation results in inhibition of the activity of channel involved in stomata opening.

The other members of this group are SnRK2.2 and SnRK2.3. They do not seem to be involved in the regulation of stomata aperture. They play a role especially during germination and seedling development. The importance of all SnRK2s as global positive regulators can be deduced from extremely ABA-insensitive phenotype of the triple mutant (Cutler et al. 2010).

4.2 Second Messengers Involved in Stomata Aperture Regulation

SnRK2.6 also interacts and phosphorylates respiratory burst oxidase homolog F (RBOHF), plasma membrane NADPH oxidase, which is involved in the production of ROS, especially of hydrogen peroxide and superoxide. In this way, ABA is able to induce rapid burst of H2O2 (Pei et al.

2000). Hydrogen peroxide reversibly inhibits ABI1 through the oxidation of cysteine residues in catalytic center (Meinhard et al. 2002). Thus, hydrogen peroxide generated during the signaling temporarily inactivates negative regulators, which results in amplification of the ABA signal.

Hydrogen peroxide was also found to stimulate activation of calcium channels (Kwak et al. 2003), which in the consequence results in elevation of cytoplasmic calcium level and activation of Ca-dependent protein kinases. Also other second messengers are involved in triggering of Ca2+ signal - inositol 1,4,5 triphosphate, produced by phospholipase C, myo-inositol hexakis phosphate, or cyclic ADP-ribose (Schroeder et al.

2001). Another ABA second messenger is phos-phatidic acid, produced by phospholipase D (Zhang et al. 2005).

ABA signal transduction involves also nitrogen oxide (NO, Meinhard et al. 2002). Experiments with H2O2 scavenger (catalase) and NO synthase inhibitor (L-NAME) showed that catalase was able to abolish production of both ROS and NO while L-NAME prevented only the rise in NO levels (Srivastava et al. 2009). These data indicated that H2O2 production preceded NO

elevation. NO downregulation was, however, sufficient to suppress stomata closure. NO was reported to exert its effect via cyclic ADP-ribose (Neil et al. 2002) .

Another kinase activated by ABA is sphingosine kinase (SPHK1), which produces sphingosine-1-phosphate (Coursol et al. 2003). Overexpression of SPHK1 led to the increase of stomata sensitivity to ABA, as well as to the decrease of the rate of germination (Worrall et al. 2008). Sphingosine-1-phosphate, thus, functions as another second messenger in ABA signaling. It requires GPA1 for its action.

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