Abscisic acid (ABA) is a plant hormone of particular interest because it promotes stress tolerance such as resistance to drought. Such influences on plant hardiness are likely to be important in efforts to maximize or improve the efficiency of food production from crops. Efforts to identify a receptor for ABA have been unsuccessful, so Hill and colleagues made an anti-idiotypic antibody (AB2) that recognized a monoclonal antibody to ABA itself. They then used AB2 to identify a protein from barley and confirmed that it bound ABA with high affinity. The group now reports cloning and characterization of a related protein from Arabidopsis that reveals a new mechanism for hormone receptor signaling. The Arabidopsis protein is known as FCA and is a nuclear RNA-binding protein that functions by preventing the accumulation of an mRNA that represses flowering. FCA functions with a protein partner known as FY, and in vitro binding assays showed that binding of ABA to FCA disrupted the interaction of FCA and FY. Treatment of plants with ABA produced a similar effect on flowering time as did loss-of-function mutants of FCA or FY, indicating the biological relevance of regulation of FCA by ABA. However, the classic stress-induced responses, such as closing of stomatal pores or inhibition of seed germination, were not impaired in FCA mutants. Thus, FCA appears to mediate only a subset of ABA's actions, and there are likely other ABA receptors to be found. Uncharacterized mRNA-binding proteins are plentiful in plants and animals, and the findings suggest that some of these could be hormone receptors.
F. A. Razem, A. El-Kereamy, S. R. Abrams, R. D. Hill, The RNA-binding protein FCA is an abscisic acid receptor. Nature 439, 290-294 (2006). [PubMed]