A wheat MYB transcription factor activates defense by repressing TaSIZ1 in response to branched-chain amino acid accumulation.

Branched-chain amino acid (BCAA) accumulation has been linked to the induction of salicylic acid (SA)-related defense responses in wheat (Triticum aestivum). Here, we explored whether the underlying mechanism might involve a wheat ortholog of SAP AND MIZ1 DOMAIN-CONTAINING LIGASE1 (SIZ1), a critical SA regulator in other systems. We found that TaSIZ1 was significantly repressed in mutant plants disrupted in the BCAA aminotransferase gene TaBCAT1; these plants accumulate enhanced levels of BCAAs and SA. Overexpressing TaSIZ1 suppressed SA-hyperaccumulation in TaBCAT1 mutants, and treating wild-type plants with the BCAA Leu repressed TaSIZ1 expression. Thus, TaSIZ1 is responsive to BCAA levels and influences SA accumulation, consistent with a role linking BCAA and SA in defense. Nuclear proteomic analysis of the TaBCAT1 mutant identified transcriptional regulators that could be modifying BCAA-responsive TaSIZ1 expression. This included an ortholog of the Arabidopsis thaliana trihelix transcription repressor 6b-INTERACTING PROTEIN-LIKE1 (ASIL1) with a potential binding site in the TaSIZ1 promoter. Further work showed that TaASIL1 bound to the TaSIZ1 promoter. Also, disrupting TaASIL1 function inhibited Leu-dependent TaSIZ1 repression. Based on these data, we propose that elevated BCAA levels, such as those arising during pathogen attack, activate TaASIL1, which represses TaSIZ1, thereby promoting SA accumulation and SA-mediated defense in wheat.