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The phytohormone abscisic acid (ABA) is well known for its regulatory roles in integrating environmental constraints with the developmental programs of plants. Here, we characterize the biological function of the Arabidopsis (Arabidopsis thaliana) RING-H2 protein RHA2a in ABA signaling. The rha2a mutant is less sensitive to ABA than the wild type during seed germination and early seedling development, whereas transgenic plants overexpressing RHA2a are hypersensitive, indicating that RHA2a positively regulates ABA-mediated control of seed germination and early seedling development. Double mutant analyses of rha2a with several known ABA-insensitive mutants suggest that the action of RHA2a in ABA signaling is independent of that of the transcription factors ABI3, ABI4, and ABI5. We provide evidence showing that RHA2a also positively regulates plant responses to salt and osmotic stresses during seed germination and early seedling development. RHA2a is a functional E3 ubiquitin ligase, and its conserved RING domain is likely important for the biological function of RHA2a in ABA signaling. Together, these results suggest that the E3 ligase RHA2a is an important regulator of ABA signaling during seed germination and early seedling development.
The phytohormone abscisic acid (ABA) is well known for its regulatory roles in integrating environmental constraints with the developmental programs of plants (for review, see Leung and Giraudat, 1998; Finkelstein et al., 2002; Zhu, 2002; Assmann, 2003; Himmelbach et al., 2003; Nambara and Marion-Poll, 2003; Chow and McCourt, 2004; Christmann et al., 2006; Yamaguchi-Shinozaki and Shinozaki, 2006). ABA affects a broad range of physiological processes during different developmental stages. For example, ABA helps maintain seed dormancy to ensure that seeds germinate under favorable conditions. Immediately after germination, ABA may inhibit the establishment and subsequent development of young seedlings, with this postgerminative arrest representing an early developmental checkpoint to slow seedling growth until better conditions arise (Lopez-Molina et al., 2001; Finkelstein et al., 2002; Nambara and Marion-Poll, 2003). During more advanced developmental stages, ABA also regulates plant responses to various abiotic stresses, largely by directing guard cell functioning (Finkelstein et al., 2002; Assmann, 2003; Christmann et al., 2006; Yamaguchi-Shinozaki and Shinozaki, 2006). Therefore, ABA-regulated processes are generally divided into two broad and overlapping categories: ABA signaling in seeds (maintenance of seed dormancy and control of early seedling development) and ABA signaling in guard cells...