THE ARABIDOPSIS ABSCISIC ACID-INSENSITIVE2 (ABI2) AND ABI1 GENES ENCODE HOMOLOGOUS PROTEIN PHOSPHATASES 2C INVOLVED IN ABSCISIC-ACID SIGNAL-TRANSDUCTION

Abscisic acid (ABA) mediates seed maturation and adaptive responses to environmental stress. In Arabidopsis, the ABA-INSENSITIVE1 (ABI1) pro tein phosphatase 2C is required for proper ABA responsiveness both in seeds and in vegetative tissues. To determine whether the lack of rece ssive alleles at the corresponding locus could be explained by the exi stence of redundant genes, we initiated a search for ABI1 homologs. On e such homolog turned out to be the ABI2 locus, whose abi2-1 mutation was previously known to decrease ABA sensitivity. Whereas abi1-1 is (s emi)dominant, abi2-1 has been described as recessive and maternally co ntrolled at the germination stage. Unexpectedly, the sequence of the a bi2-1 mutation showed that it converts Gly-168 to Asp, which is precis ely the same amino acid substitution found in abi1-1 and at the coinci dental position within the ABI1 phosphatase domain (Gly-180 to Asp). I n vitro assays and functional complementation studies in yeast confirm ed that the AB12 protein is an active protein phosphatase 2C and that the abi2-1 mutation reduced phosphatase activity as well as affinity t o Mg2+. Although a number of differences between the two mutants in ad aptive responses to stress have been reported, quantitative comparison s of other major phenotypes showed that the effects of both abi1-1 and abi2-1 on these processes are nearly indistinguishable. Thus, the hom ologous ABI1 and ABI2 phosphatases appear to assume partially redundan t functions in ABA signaling, which may provide a mechanism to maintai n informational homeostasis.