Exogenous gallic acid induces changes in the metabolome of Mentha arvensis L. regulating the expression of key genes involved in menthol biosynthesis

Mentha arvensis L. (corn mint) is an aromatic plant of great economic interest. Nowadays, one of the most important goals of the 2030 Agenda is to identify new phytostimulants able to substitute the common fertilizers and pesticides, in promoting sustainable crop productions. Thus, the objective of the present contribution was to evaluate the potential effect of several concentrations (1–750 μM) of gallic acid (GA) as phystostimulant for corn mint plants treated for 15 and 21 days. Here, we demonstrated that GA was able to stimulate the accumulation of total phenolics, flavonoids and terpenoids. In addition, 21 phenolics were quantified by HPLC-DAD analysis, demonstrating how GA was able to increase some specific metabolites, such as kaempferol and quercetin known to exert roles in cellular signalling. By GC–MS technique, instead, the lipophilic fraction was investigated, putting in evidence that GA caused a rise in the levels of menthol, the key molecule of the mint chemotype used in this work, and a decrease of its precursor, the limonene. This result suggested a putative effect of GA on the biosynthetic steps of menthol; therefore, the gene expression of 5 enzymes involved in its biochemical pathway was measured through RT-qPCR analysis, highlighting a substantial increase in the transcript of Menthone reductase and a simultaneous decrease in that of Limonene synthase. All together these data proved that GA, especially after 21 days of treatment at a medium-low dose (50 μM), was able to exert on M. arvensis a positive modulation of the metabolome.