Reevaluation of Abscisic Acid-Binding Assays Shows That G-Protein-Coupled Receptor2 Does Not Bind Abscisic Acid

The phytohormone abscisic acid (ABA) plays an integral role in plant growth and development, but only recently have putative receptors been identified (Razem et al., 2006; Shen et al., 2006; Liu et al., 2007; Pandey et al., 2009). Our results, however, suggest that the putative extracellular ABA receptor, G-Protein-Coupled Receptor2 (GCR2), does not bind ABA. We propose that an improved method for measuring ABA binding is required and that the search for ABA receptors should continue. ABA regulates dormancy, seed germination, and maturation as well as promoting adaptation to abiotic stress and regulating stomatal aperture (for review, see Leung and Giraudat, 1998; Finkelstein et al., 2002; Himmelbach et al., 2003). Genetic approaches have led to the isolation of genes involved in the ABA biosynthetic and signal transduction pathways (Finkelstein et al., 2002; Nambara and Marion-Poll, 2005). Yet, only recently have supposed ABA receptors been identified in Arabidopsis (Arabidopsis thaliana; Verslues and Zhu, 2007). These are the nuclear flowering-time protein FCA (Razem et al., 2006), the plastid-associated magnesium-protoporphyrin IX chelatase H subunit (CHLH; Shen et al., 2006), a protein originally identified as a membrane-bound GCR2 (Liu et al., 2007), and, recently, two novel G-protein coupled receptors (GPCRs), GPCR-Type G-Protein1 (GTG1) and GTG2 (Pandey et al., 2009). The identification of multiple ABA receptors was expected, as both extracellular and intracellular ABA perception occurs (for review, see Finkelstein et al., 2002) and different ABA analogs induce distinct gene expression patterns (Huang et al., 2007). Moreover, ABA-binding activities have been found in discrete cellular fractions, including the plasma membrane, cytosol, and endomembranes, suggesting that multiple receptors exist (Wan and Hasenstein, 1996; Pedron et al., 1998; Zhang et al., 1999, 2001; Kitahataa et al., 2005).