Long-term Exposure to Methyl Jasmonate Increases Myrosinases TGG1 and TGG2 in Arabidopsis coi1 and myc2,3,4 Mutants

Plants are constantly subjected to stresses such as wounding or herbivore attacks, which continuously activate the JA signaling pathway in nature. However, knowledge about the effect of long-term activation of the JA signaling pathway on plant defense response remains limited. THIOGLUCOSIDE GLUCOHYDROLASE 1 (TGG1) and TGG2 are enzymes that activate defensive metabolites, namely glucosinolates, which are involved in defense against herbivores and pathogens. Here, we show that prolonged exposure to the wounding hormone methyl jasmonate (MeJA) enhances TGG1 and TGG2 expression independent of the canonical jasmonic acid (JA) signaling pathway in Arabidopsis thaliana rosette leaves. We found that airborne MeJA treatment for up to 5 d enhanced both TGG1 and TGG2 gene expression and their protein levels in Arabidopsis leaves. Notably, TGG1 and TGG2 gene expressions significantly upregulated in two JA signaling pathway mutants, namely coi1-16 and myc2,3,4, following 5 d of MeJA treatment. TGG1 and TGG2 proteins accumulate in specialized myrosin cells of rosette leaves. Myrosin cell area expands in response to MeJA treatment in a leaf-age-dependent manner. Consistent with this, the expression of FAMA, a transcription factor known to regulate TGG1 and TGG2 gene expression, also increased in a leaf-age-dependent manner after 5 d of MeJA treatment. Taken together, our results suggest the existence of a non-canonical JA signaling pathway that is activated by long-term exposure to MeJA and regulates the expression of defense-related genes TGG1 and TGG2.