Molecular characterization of a mutation affecting abscisic acid biosynthesis and consequently stomatal responses to humidity in an agriculturally important species

Mutants deficient in the phytohormone abscisic acid (ABA) have been instrumental in determining not only the biosynthetic pathway for this hormone, but also its physiological role in land plants. The wilty mutant of Pisum sativum is one of the classic, well-studied ABA deficient mutants; however this mutant remains uncharacterised at a molecular level. Using a candidate gene approach, we show that the wilty mutation affects the xanthoxin dehydrogenase step in ABA biosynthesis. To date, this step has only been represented by mutants in the ABA2 gene of Arabidopsis thaliana. Functional ABA biosynthesis appears to be critical for normal stomatal responses to changes in humidity in angiosperms, with wilty mutant plants having no increase in foliar ABA levels in response to a doubling in vapour pressure deficit and no closure of stomata. Phylogenetic analysis of the ABA2 gene family from diverse land plants indicates that the evolution of an ABA-biosynthetic-specific short-chain dehydrogenase (ABA2) evolved in the earliest angiosperms. The relatively recent evolutionary origin of specificity in this step has important implications for both the evolution of ABA biosynthesis and action in land plants.