Metabolomic and transcriptomic responses to waterlogging stress and hydrogen peroxide elicitation: impact on the production of iridoids and phenolics in leaves of two Nepeta species

Abstract

Nepeta species, known for their diverse bioactive metabolites, particularly iridoid and phenolic compounds, have recently attracted growing interest due to their biotechnological potential and ability to cope with various environmental stresses. This study was aimed at leveraging the specialized metabolism of chemically distinct Nepeta nuda and N. grandiflora by applying waterlogging (W), hydrogen peroxide (H₂O₂), or a combination of both stresses. These treatments are anticipated to trigger H<inf>2</inf>O<inf>2</inf>-responsive signaling pathways that regulate the biosynthesis of terpenes and phenolics. Following the exposure to W and/or H<inf>2</inf>O<inf>2</inf> treatments, species-specific patterns of changes in leaf metabolomes and transcriptomes were recorded. Prominent increase of nepetalactone (NL) content in N. nuda (up to 12-fold) and slight increase of 1,5,9-epi-deoxyloganic acid (1,5,9-eDLA) in leaves of NL-lacking N. grandiflora (up to 1.5 fold) were conditioned by upregulation of iridoid biosynthesis-related genes, especially those encoding for NAD-dependent nepetalactol-related short-chain-dehydrogenase/reductases (NEPSs) responsible for setting the NL stereochemistry and for regulating metabolic flux through the iridoid pathway. NL content is apparently more susceptible to oxidative stress-induced alterations than that of iridoid glycosides. Although H<inf>2</inf>O<inf>2</inf> and W/H<inf>2</inf>O<inf>2</inf> treatments significantly stimulated the accumulation of rosmarinic acid, especially in leaves of N. nuda, the expression levels of the majority of the corresponding biosynthetic genes were only slightly upregulated. N. nuda leaves also displayed higher antioxidant potential than those of N. grandiflora, suggesting either superior capacity of the former species to mitigate oxidative stress, or higher tolerance of N. grandiflora. Although overlapping response mechanisms to the two applied stresses were expected to exert an additive burden on Nepeta specialized metabolism, this was not observed as the H<inf>2</inf>O<inf>2</inf> and W/H<inf>2</inf>O<inf>2</inf> treatments proved equally effective. While the mechanistic basis of the targeted stress elicitation requires further investigation, the present study underscores its biotechnological potential for upscaling the production of iridoids and phenolics in Nepeta species.