bHLH19 and bHLH20 repress jasmonate‐mediated plant defense against insect herbivores in Arabidopsis

SUMMARY Plants are attacked by various insect herbivores. Upon attack‐triggered biosynthesis of the phytohormone jasmonates (JAs), the JA receptor CORONATINE INSENSITIVE 1 recruits the JA‐ZIM domain (JAZ) repressors for ubiquitination, releases the MYC‐MYB transcription factor (TF) complexes, and enhances glucosinolates (GSs) biosynthesis to promote defense against insects in Arabidopsis. However, the negative regulation of JA‐regulated defense remains largely unclear. Here, we found that Arabidopsis IVa bHLH TFs bHLH19 and bHLH20 interacted with JAZs. The bhlh19/20 mutations enhanced defense against the insects Spodoptera frugiperda and S. exigua , while their overexpression inhibited defense. bHLH19/20 repressed defense via at least two layers of regulation: first, bHLH19/20 interacted with the members MYC2/3/4/5 and MYB34/51/122 of MYC‐MYB complexes, and inhibited the interaction/transcription activity of MYC2‐MYB34; second, bHLH19/20 activated the RNA level of nitrile‐specifier protein 1 , which converts GSs into the less toxic nitriles. bhlh19/20 exhibited no penalty in JA‐regulated growth inhibition. Collectively, our findings reveal the molecular mechanism for negatively regulating JA‐mediated defense against insects in Arabidopsis without growth penalty by the pair of bHLH19/20 TFs.