Sumoylation of transcription factor MYB30 by the small ubiquitin-like modifier E3 ligase SIZ1 mediates abscisic acid response in Arabidopsis thaliana

The phytohormone abscisic acid (ABA) plays an essential role in plant development and during the response of the plant to abiotic stress. In this study, we report that the R2R3-type transcription factor MYB30 is involved in the regulation of ABA signaling. Arabidopsis mutants lacking MYB30 are hypersensitive to ABA during germination and seedling growth. A K283R substitution in MYB30 blocks its SUMO E3 ligase SIZ1-mediated sumoylation in Arabidopsis protoplasts, indicating that MYB30 is sumoylated by SIZ1 and that K283 is the principal site for small ubiquitin-like modifier conjugation. Expression of MYB30(K283R) in myb30 partially rescues the mutant ABA-hypersensitive phenotype, but expression of wild-type MYB30 complements the mutant phenotype. Overexpression of MYB30 in wild-type results in an ABA-insensitive phenotype, whereas overexpression of MYB30 in the siz1 mutant does not alter siz1 hypersensitivity to ABA. The siz1-2 myb30-2 double-mutant exhibits greater ABA sensitivity than either single mutant, but a mutation in the SIZ1-sumoylated ABI5 transcription factor suppresses the ABA hypersensitivity of myb30-2 to wild-type levels. Our results suggest that coordination of ABI5 and MYB30 sumoylation by SIZ1 may balance gene expression, which is required for regulation of ABA signaling during seed germination.