ABA-Hypersensitive Germination3Encodes a Protein Phosphatase 2C (AtPP2CA) That Strongly Regulates Abscisic Acid Signaling during Germination among Arabidopsis Protein Phosphatase 2Cs

Abstract

The phytohormone abscisic acid (ABA) regulates physiologically important developmental processes and stress responses. Previously, we reported on Arabidopsis (Arabidopsis thaliana) L. Heynh. ahg mutants, which are hypersensitive to ABA during germination and early growth. Among them, ABA-hypersensitive germination3 (ahg3) showed the strongest ABA hypersensitivity. In this study, we found that the AHG3 gene is identical to AtPP2CA, which encodes a protein phosphatase 2C (PP2C). Although AtPP2CA has been reported to be involved in the ABA response on the basis of results obtained by reverse-genetics approaches, its physiological relevance in the ABA response has not been clarified yet. We demonstrate in vitro and in vivo that the ahg3-1 missense mutation causes the loss of PP2C activity, providing concrete confirmation that this PP2C functions as a negative regulator in ABA signaling. Furthermore, we compared the effects of disruption mutations of eight structurally related PP2C genes of Arabidopsis, including ABI1, ABI2, HAB1, and HAB2, and found that the disruptant mutant of AHG3/AtPP2CA had the strongest ABA hypersensitivity during germination, but it did not display any significant phenotypes in adult plants. Northern-blot analysis clearly showed that AHG3/AtPP2CA is the most active among those PP2C genes in seeds. These results suggest that AHG3/AtPP2CA plays a major role among PP2Cs in the ABA response in seeds and that the functions of those PP2Cs overlap, but their unique tissue- or development-specific expression confers distinct and indispensable physiological functions in the ABA response.

Enhancement of Abscisic Acid Sensitivity and Reduction of Water Consumption in Arabidopsis by Combined Inactivation of the Protein Phosphatases Type 2C ABI1 and HAB1

“…The results further suggest that PLDa1 is not needed for ABA-induced stomatal closing when ABI1 is deleted. These findings are interesting in light of these and other recent findings that several PP2Cs function as negative regulators of ABA signaling (Leonhardt et al, 2004;Saez et al, 2004;Kuhn et al, 2006;Yoshida et al, 2006), but deletion of the ABI1 PP2C is sufficient to restore PLDa1-independent ABAinduced stomatal closing in plda1 (Mishra et al, 2006). Finally, we show here that the abi1-2 and abi1-3 knockout lines show enhanced ABA-induced stomatal closing.…”

“…According to the dramatic effect of the combined loss-of-function phenotype, ABI1 and HAB1 must cooperate to negatively regulate ABA signaling at the seed and seedling stage. Another PP2C, PP2CA, was recently shown to strongly and negatively regulate ABA signaling during germination (Kuhn, et al, 2006;Yoshida et al, 2006). The ABA-mediated seed germination phenotype of pp2ca or hab1-1 abi1-2/hab1-1 abi1-3 mutants was apparent even though HAB1 and ABI1, or PP2CA, respectively, were functional.…”

“…Currently, at least four Arabidopsis (Arabidopsis thaliana) PP2Cs, ABI1, ABI2, PP2CA, and HAB1 (formerly named AtP2C-HA), are known to regulate ABA signaling. Evidence on their role as negative regulators of ABA signaling has been provided by genetic approaches (Gosti et al, 1999;Merlot et al, 2001;Tahtiharju and Palva, 2001;Gonzalez-Garcia et al, 2003;Leonhardt et al, 2004;Saez et al, 2004;Kuhn, et al, 2006;Yoshida et al, 2006). For instance, the recessive T-DNA insertion mutant hab1-1 shows ABAhypersensitive inhibition of seed germination and enhanced ABA-mediated stomatal closure (Leonhardt et al, 2004;Saez et al, 2004).…”

Overexpression of AtMYB44 Enhances Stomatal Closure to Confer Abiotic Stress Tolerance in Transgenic Arabidopsis

“…Instead, expression of genes that encode a group of Ser/Thr PP2Cs, such as ABI1, ABI2, AtPP2CA, HAB1, and HAB2, was suppressed in 35S:AtMYB44 plants and enhanced in atmyb44 knockout plants. These proteins belong to group A PP2Cs (Schweighofer et al, 2004) and have been described as negative regulators of the ABA signal transduction cascade (Gosti et al, 1999;Merlot et al, 2001;Tahtiharju and Palva, 2001;Sáez et al, 2004;Kuhn et al, 2006;Yoshida et al, 2006). The abi1 and abi2 mutations lead to phenotypic alterations in ABA-resistant seed germination and seedling growth, reduced seed dormancy, abnormal stomatal regulation, and defects in various responses to drought stress (Leung et al, 1997;Merlot et al, 2001).…”

Evolutionarily Conserved Regulatory Mechanisms of Abscisic Acid Signaling in Land Plants: Characterization of ABSCISIC ACID INSENSITIVE1-Like Type 2C Protein Phosphatase in the Liverwort Marchantia polymorpha

“…Among seventy-six PP2Cs encoded in the Arabidopsis genome, ABI1, ABI2, HAB1, HAB2, and AtPP2CA are members of group A PP2C, which possibly functions in the regulation of ABA signaling (Schweighofer et al, 2004). Transgenic plants overexpressing these PP2Cs typically show ABA-insensitive phenotypes such as slow stomata closure and enhanced seed germination by exogenously applied ABA, while the knockout lines of these genes exhibit ABA-hypersensitive phenotypes (Yoshida et al, 2006b). Recent studies indicated that negative regulation by the ABI1-related PP2Cs plays a central role in ABA signaling through signal perception and transduction mediated by ABA receptors (Ma et al, 2009;Park et al, 2009).…”