UV-B radiation-elicitated biosynthesis of iridoids in leaves of in vitro grown Nepeta nuda L.

Abstract

Because of their sessile lifestyle, plants have been constantly facing ultraviolet (UV-B) radiation, a type of abiotic stress which affects their overall growth, development, physiology, and metabolism. Qualitative and quantitative content of specialized metabolites, chemical compounds that allow plants to adapt to specific conditions of the environment, is also affected. Nepeta nuda L. (fam. Lamiaceae) is characterized by a significant amount of iridoids, monoterpenoid compounds with prominent pharmacological effects. The aim of this study was to detect changes in the content of major iridoids (1,5,9-epideoxyloganic acid (1,5,9-eDLA), nepetalactone (NL), and dehydronepetalactone (DNL)) in N. nuda plants after exposure to UV-B radiation (during 30 and 60 min), in parallel with expression analysis of genes participating in the biosynthetic pathway of iridoids (NnGAPDH, NnGPPS, NnGES, NnG8O, Nn8HGO, NnIS, NnNEPS). After both UV-B treatments, the gene GPPS showed the highest increase in gene expression, relative to control plants, while the gene G8O showed the highest decrease in expression level. At the same time, UHPLC/DAD/(±)HESI-MS2 analysis revealed an increase in the concentration of 1,5,9-eDLA and DNL following the exposure of plants to UV-B radiation for 60 minutes. In the case of a 30-minute treatment, there was an increase in the concentration of eDLA, while the concentration of DNL was reduced, relative to non-treated plants. The present study brings novel insights into the molecular background of iridoid biosynthesis in UV-B stressed plants, which could help in optimizing the elicitation procedures and thus accumulation of bioactive compounds in N. nuda.