AFP is a novel negative regulator of ABA signaling that promotes ABI5 protein degradation

Plants have evolved protective mechanisms to ensure their survival when threatened by adverse environmental conditions during their transition to autotrophic growth. During germination, there is a 2- to 3-d period during which a plant can execute growth arrest when challenged by water deficit. This postgermination developmental checkpoint is signaled by the stress hormone abscisic acid (ABA), which induces the expression of the bZIP transcription activator ABI5. The growth arrest efficiency depends on ABI5 levels, and abi5 mutants are ABA-insensitive and unable to execute the ABA-mediated growth arrest. Here we show that a novel ABI5-interacting protein, designated as AFP, can form high molecular weight (Mr) complexes with ABI5 in embryo-derived extracts. Like ABI5, ABI five binding protein (AFP) mRNA and protein levels are induced by ABA during seed germination. Two different afp mutant alleles (afp-1 and afp-2) are hypersensitive to ABA, whereas transgenic plants overexpressing AFP are resistant; in these plants, AFP and ABI5 protein levels are inversely correlated. Genetic analysis shows that abi5-4 is epistatic to afp-1, indicating the ABA hypersensitivity of afp mutants requires ABI5. Proteasome inhibitor studies show that ABI5 stability is regulated by ABA through ubiquitin-related events. When expressed together, AFP and ABI5 are colocalized in nuclear bodies, which also contain COP1, a RING motif protein. Our results suggest that AFP attenuates ABA signals by targeting ABI5 for ubiquitin-mediated degradation in nuclear bodies.