Copper ion induced production of rosmarinic acid in lemon balm (Melissa officinalis L.) seedlings

Rosmarinic acid (RA), one of the most important active ingredients of lemon balm ( Melissa officinalis L.), exhibits antiviral, antibacterial, antioxidative and anticancer properties. Furthermore, it can improve functionality in baking process. Cu acts as a cofactor of several proteins and plays a key role in photosynthesis, respiration, lignin synthesis, response to oxidative stress and cell wall metabolism, but can be toxic to plants in high concentrations. We hypothesized that abiotic stresses, as one of the external factors inducing the defense mechanism of plants, may contribute to the production of secondary metabolites, especially RA, in representatives of the Lamiaceae family. In current study, RA accumulation, expression of tyrosine aminotransferase gene (TAT), contents of flavonoid and anthocyanin as well as antioxidant enzymes activities were investigated in 45-day-old M. officinalis seedlings after treatment with different concentrations of Cu2+ (0, 5, 10, 20, and 30 mM). Samples were collected and analyzed after 8 and 16 hours of treatment. Lower concentrations of Cu2+ positively affected RA accumulation at both aforementioned treatment times, which is consistent with the increase in TAT gene expression profile. Flavonoid, anthocyanin and soluble protein contents of the seedlings significantly decreased (except at 20 and 30 µM Cu2+-treated seedlings after 8 hours). RA content and expression of TAT gene decreased significantly at the highest concentration of Cu2+ for 16 hours. Concurrently, elevated levels of superoxide dismutase and peroxidase activities were measured in these seedlings. Latter can indicate that lower concentrations of Cu2+ cause oxidative stress. Reactive oxygen species (ROS), which act as signal molecules, are accumulated and due to their positive effects on the expression of TAT gene more RA is produced. In contrast, at the highest concentration of copper ions, ROS suppressed TAT gene expression and prevented the degradation of the gene product.