Three SnRK2 Protein Kinases are the Main Positive Regulators of Abscisic Acid Signaling in Response to Water Stress in Arabidopsis

Abstract

Responses to water stress are thought to be mediated by transcriptional regulation of gene expression via reversible protein phosphorylation events. Previously, we reported that bZIP (basic-domain leucine zipper)-type AREB/ABF (ABA-responsive element-binding protein/factor) transcription factors are involved in ABA signaling under water stress conditions in Arabidopsis. The AREB1 protein is phosphorylated in vitro by ABA-activated SNF1-related protein kinase 2s (SnRK2s) such as SRK2D/SnRK2.2, SRK2E/SnRK2.6 and SRK2I/SnRK2.3 (SRK2D/E/I). Consistent with this, we now show that SRK2D/E/I and AREB1 co-localize and interact in nuclei in planta. Our results show that unlike srk2d, srk2e and srk2i single and double mutants, srk2d srk2e srk2i (srk2d/e/i) triple mutants exhibit greatly reduced tolerance to drought stress and highly enhanced insensitivity to ABA. Under water stress conditions, ABA- and water stress-dependent gene expression, including that of transcription factors, is globally and drastically impaired, and jasmonic acid (JA)-responsive and flowering genes are up-regulated in srk2d/e/i triple mutants, but not in other single and double mutants. The down-regulated genes in srk2d/e/i and areb/abf triple mutants largely overlap in ABA-dependent expression, supporting the view that SRK2D/E/I regulate AREB/ABFs in ABA signaling in response to water stress. Almost all dehydration-responsive LEA (late embryogenesis abundant) protein genes and group-A PP2C (protein phosphatase 2C) genes are strongly down-regulated in the srk2d/e/i triple mutants. Further, our data show that these group-A PP2Cs, such as HAI1 and ABI1, interact with SRK2D. Together, our results indicate that SRK2D/E/I function as main positive regulators, and suggest that ABA signaling is controlled by the dual modulation of SRK2D/E/I and group-A PP2Cs.

Protein kinase CK2α subunits constitutively activate ABA signaling in Arabidopsis

“…snrk2.2/2.3/2.6 ) triple mutant plants, which harbor triple knockout mutations of subclass III SnRK2s. 11,28 ck2α1/2/3-4i mutant plants display reduced sensitivity to ABA during germination compared with WT plants (Figure 2B and C) as reported previously, 27 indicating that CK2 α s are positive modulators of ABA signaling during germination. Interestingly, compared between ck2α1/2/3-4i mutant and srk2d/e/i triple mutant plants, we found that the effect of CK2αs on ABA insensitivity during germination is much smaller than that of the subclass III SnRK2s (Figure 2B and C).…”

Regulation of Leaf Starch Degradation by Abscisic Acid Is Important for Osmotic Stress Tolerance in Plants

“…The transcriptional activation of BAM1 and AMY3 in response to exogenously applied ABA was abolished in mutants lacking three SnRK2 kinases, snrk2.2 snrk2.3 snrk2.6, or greatly reduced in mutants lacking three AREB/ABF TFs, areb1 areb2 abf3 ( Figure 7A), all known positive regulators of ABA signaling in the leaves (Nakashima et al, 2009;Umezawa et al, 2009;Fujita et al, 2009;Fujii and Zhu, 2009;Yoshida et al, 2010). The residual transcriptional activation of BAM1 in the areb1 areb2 abf3 mutant was most likely due to the remaining activity of another bZIP TF, ABF1, which is also part of the AREB/ABF-Snrk2 signaling pathways in leaves under osmotic stress (Yoshida et al, 2015).…”

Reconstitution of Abscisic Acid Signaling from the Receptor to DNA via bHLH Transcription Factors

“…The triple knock-out plants of these isoforms were found to exhibit dramatic insensitivity to ABA, but the double and single mutants showed less ABA sensitivity than the triple mutants Fujita et al, 2009;Nakashima et al, 2009;Umezawa et al, 2009). We tested whether these three SnRK2s also redundantly phosphorylate AKSs.…”