Effect of methyl jasmonate on phenolic acids accumulation and the expression profile of their biosynthesis-related genes in Mentha spicata hairy root cultures

Mentha spicata L., as a rich source of phenolic acids, is known for its therapeutic importance. Elicitors play a crucial role in biosynthetic pathways to improve plant secondary metabolites production. The purpose of this study was to investigate the effect of methyl jasmonate (MeJA) as a potent elicitor, on phenolic acids production and the relative changes in expression of their biosynthesis-related key genes (PAL, TAT, C4H, HPPR, and 4CL) in M. spicata hairy root cultures, to elucidate the interrelation between phenolic acids accumulation and the regulation mechanism of gene expression. The results showed that the relative expression levels of phenylpropanoid pathway genes, i.e., PAL, C4H, 4CL, and HPPR in the tyrosine-derived pathway increased 4.04-fold, 3.62-fold, 1.75-fold, and 1.45-fold in comparison to untreated controls, respectively. High-performance liquid chromatography analysis indicated that MeJA dramatically increased rosmarinic acid content (55.44 µg g−1 dry wt, about 11.84-fold) after 6 h exposure to elicitor. Moreover, caffeic acid, chlorogenic acid, and cinnamic acid contents were also enhanced significantly (p < 0.05) in response to MeJA treatment. On the other hand, the application of MeJA had a negative effect on both TAT expression level and lithospermic acid B accumulation. Our results implied that MeJA has a significant impact on RA, CA, CGA, and CIA accumulation, which might be the consequence of gene activation from the phenylpropanoid pathway (PAL, C4H, and 4CL) and HPPR, a key regulatory enzyme of the tyrosine-derived pathway in the elicitor-treated hairy root cultures of M. spicata. MeJA is an effective elicitor for stimulating valuable phenolic acids production in M. spicata hairy root cultures.