Abstract
SUMO (small ubiquitin-related modifier) conjugation (sumoylation) to protein substrates is a reversible posttranslational modification that regulates transcription factor activity. The siz1 mutation, impaired in SUMO E3, caused ABA-hypersensitive to seed germination arrest and seedling growth inhibition. Further, expression of ABA-responsive genes was hyper-induced by ABA in siz1 seedlings. abi5 suppressed the ABA sensitivity caused by siz1, revealing an epistatic genetic interaction between SIZ1 and ABI5. A K391R substitution in ABI5 (ABI5K391R) blocked SIZ1-mediated sumoylation of ABI5 in vitro and in protoplasts, indicating that ABI5 is sumoylated through SIZ1 and that K391 is the principal site for SUMO conjugation. ABI5K391R expression in abi5 plants resulted in ABA-responsive gene expression, seed germination arrest and primary root growth inhibition greater than that observed in complemented lines (ABI5 expression in abi5). Together, these results demonstrate that SIZ1-dependent sumoylation of ABI5 negatively regulates ABA signaling.
