Does MYC2 really play a negative role in jasmonic acid-induced indolic glucosinolate biosynthesis in Arabidopsis thaliana?

Glucosinolates are a group of sulfur-rich thioglucosides and are known to play an important role in plant defense and benefit to human health. The accumulation of indolic glucosinolates (IGS) is strongly induced by jasmonic acid (JA); however, the underlying molecular mechanism remains only partly understood. MYC3 and MYC4, the homologs of MYC2 have been shown to act additively with MYC2 in the activation of some JA responses. In this study, quantitative RT-PCR revealed that the expression of MYC3 and MYC4 was induced to a higher level in myc2 mutants compared with WT plants after 6 h of methyl jasmonate (MeJA) treatment. Likewise, the transcript of MYB34, a known positive regulator of IGS biosynthesis, also showed a remarkable increase at 6 h in MeJA-treated myc2 mutants relative to that in MeJA-treated WT plants. Then, accordingly, the expression of tryptophan biosynthetic genes (ASA1, TSA1, and TSB1) was identified to be up-regulated at 6 h, but the induction of IGS biosynthetic genes (CYP79B2, CYP79B3, and CYP83B1) showed a temporally delayed increase at 24 h. Ultimately, the total IGS was detected to be increased at 24 h after MeJA treatment in myc2 mutants. Taken together, the compensatory activation of MYC3 and MYC4 in MeJA-treated myc2 mutants might occur temporally associated with the concurrent increase in the IGS biosynthesis, providing a new insight in the role of MYC2 in JA-induced IGS biosynthesis.