The role of the jasmonate signalling transcription factors MYC2/3/4 in circadian clock mediated regulation of immunity in Arabidopsis

Plants are exposed to pathogens at specific, yet predictable times of the day-night cycle. In Arabidopsis the circadian clock influences temporal differences in susceptibility to the necrotrophic pathogen, Botrytis cinerea. The jasmonic acid (JA) pathway regulates immune responses against B. cinerea. The paralogous basic helix-loop-helix transcription factors, MYC2, MYC3 and MYC4 are primary regulators of the JA pathway but their role in regulating temporal variation in immunity is untested. This study aimed to investigate the roles of the MYC transcription factors in the temporal defence response to B. cinerea. We inoculated leaves from wild-type, myc single-, double- and triple-knockout mutants, and lines overexpressing MYC2, MYC3 or MYC4, with B. cinerea at two times of day in constant light, and compared lesion sizes. The presence of MYC2, MYC3 or MYC4 alone was sufficient to maintain temporal variation in susceptibility, but this was abolished in the myc234 tripleknockout mutant. Constitutive expression of MYC2, MYC3 or MYC4 abolished time-of-day differences in susceptibility. The data suggest that MYC2, MYC3 and MYC4 function redundantly in regulating temporal defence responses against B. cinerea and are a point of convergence between the JA pathway and the circadian clock in Arabidopsis.