Flavonoid Compounds Are Enriched in Lemon Balm (Melissa officinalis) Leaves by a High Level of Sucrose and Confer Increased Antioxidant Activity

Medicinal plants are widely used in traditional medicine because plant secondary metabolites have been shown to benefit a broad spectrum of health conditions. Lemon balm, Melissa officinalis L., a member of the mint family, is native to Europe and is well known for its ability to reduce stress and anxiety, promote sleep, and ease pain and discomfort associated with digestion. In various plant species, strong anthocyanin induction is triggered by sucrose, but not by other sugars or osmotic stress; however, the mechanisms that induce anthocyanin accumulation in lemon balm leaves in response to sucrose and phytohormones remain unclear. In this study, we investigated the mechanisms that lead to increased levels of flavonoids in lemon balm plants. We observed that sucrose significantly increases the level of flavonoids in lemon balm plants and that sucrose induction appears to be mediated by the phytohormones abscisic acid and ethylene. We also identified delphinidin as the anthocyanidin that is primarily enriched in leaves grown in high-sucrose medium. Finally, we observed that reactive oxygen species levels are positively correlated with sucrose-mediated anthocyanin accumulation. Taken together, our results demonstrate that the level of flavonoids in lemon balm can be increased significantly and that plants such as lemon balm could potentially be used to prevent diseases that have been purported to be caused by free radical damage. Chemical abbreviations used: ABA, (+)-cis, transabscissic acid; ACC, 1-aminocyclopropanecarboxylic acid; CHI, chalcone isomerase; CHS, chalcone synthase; DPPH, 2, 2-diphenyl-1-picrylhydrazyl; GA, gibberellic acid; IAA, indole-3-acetic acid.