The effects of drought, salinity, and temperature stresses on the expression of menthone menthol reductase gene in Peppermint (Mentha piperita L.)

Peppermint (Mentha piperita) is one of the most important medicinal plants producing secondary metabolites. Menthol is an important component of peppermint essential oil monoterpenes, which is widely used in pharmaceutical and industrial applications. This monoterpene is produced by the Menthone Menthol Reductase (MMR) enzyme. Plant growth as well as the expression of this gene are affected by environmental stresses. In this study, peppermint rhizomes were first surface-sterilized and cultured in MS medium supplemented with sodium chloride (zero, 50, and 100 mM) and mannitol (zero, 50, 100, and 150 mM). Then they were placed in growth chambers at 23, 26, and 29°C. Three weeks after applying stresses, the expression of this gene in the leaves of the plants was measured by Real-Time PCR, and the obtained data were analyzed. According to the results, the expression of the MMR gene decreased at low concentrations of sodium chloride but increased by 97% at high concentration of 100 mM compared to the control. However, the expression of this gene has reached a minimum level of expression at high concentrations of mannitol as well as at high temperatures. Due to the importance of peppermint in the pharmaceutical industry, increasing the production of menthol through applying different treatments is important. Based on the obtained results, the treatment with mannitol (zero mM) and sodium chloride (50 mM) at 23°C increased the expression of the MMR gene by 93% compared to the control and it seems that the menthol production has reached its maximum level.