Enhancement of Non-Enzymatic Antioxidant Compounds and Expression of Rosmarinic Acid Biosynthesis-Related Genes in Melissa officinalis Using Cyanobacteria

Melissa officinalis L. (Lemon balm) is a medicinal plant rich in phenolic compounds, possessing numerous health benefits. To enhance the plant's medicinal properties and antioxidant activity, this study investigated the effect of two species of Nostoc, as nitrogen-fixing cyanobacteria, on the growth and phytochemical responses of M. officinalis. Also, several metabolites content including total phenol, total flavonoid, and rosmarinic acid (RA) were assessed. Additionally, alterations in the expression of RA biosynthesis-related genes, such as Rosmarinic acid synthase (RAS), Phenylalanine ammonia lyase (PAL), and Tyrosine amino transferase (TAT), were examined quantitatively. Pot cultivation was performed in a randomized complete block design in a greenhouse, with the treated plants irrigated with cyanobacterial suspensions at 21-day intervals for 84 days. The results demonstrated that cyanobacterial-treated plants exhibited significant improvements in root and shoot length, fresh and dry weights, leaf number, and ramification compared to the control. Additionally, there was a significant increase in the levels of hydrogen peroxide (H2O2), malondialdehyde (MDA), and proline content in treated plants, indicating an induction of oxidative stress and potential activation of antioxidant pathways. Moreover, cyanobacterial treatment significantly increased the content of total phenols and total flavonoids, as well as rosmarinic acid (RA). Also, cyanobacterial-treated plants exhibited higher expression levels of PAL and RAS genes compared to the control, suggesting an enhancement in the production of RA. In conclusion, the application of cyanobacterial elicitors positively influenced the growth and phytochemical responses of M. officinalis, leading to higher RA content and increased levels of antioxidants. This research highlights the potential of cyanobacterial bioelicitors as a cost-effective and sustainable approach to improve the medicinal quality and antioxidant activity of medicinal plants.