Trichome-specific and developmentally regulated biosynthesis of nepetalactones in leaves of cultivated Nepeta rtanjensis plants
Nepetalactones are accumulated as the main secondary metabolites in the majority of Nepeta species; however, their biosynthetic pathway remains to a great extent unknown, as well as their relationship to the overall physiology of these plants. Nepetalactones most certainly arise through general iridoid biosynthetic pathway leading to nepetalactol, a common precursor of all plant-derived iridoids that is formed in a reaction catalysed by iridoid synthase (IS, EC 18.104.22.168). In this paper, we aimed to localize the sites of nepetalactone biosynthesis and accumulation by comparatively analysing the nepetalactones content and the expression of putative iridoid synthase (NrIS) in leaves of Nepeta rtanjensis Diklić & Milojević, a trans,cis-nepetalactone and dehydronepetalactone rich species. Another aim was to give an insight into the developmental regulation of nepetalactone biosynthesis in leaves of cultivated N. rtanjensis plants. Knowledge on the spatial-, and developmental- regulation of nepetalactone metabolism could greatly benefit potential growers by enabling them to adequately plan and manage plant cultivation, harvesting, and handling and thus increase yield of bioactive compounds. The expression profile of putative NrIS1 is highly correlated with trans,cis-nepetalactone and dehydronepetalactone content/ratio in leaves of different developmental stages. The results strongly indicate that developmental regulation of nepetalactone biosynthesis in leaves resides largely at the level of gene expression, and suggest an important role of NrIS1. Young leaves exhibited the most intensive nepetalactone biosynthesis and accumulation, but also its conversion to dehydronepetalactone. Thus, the pattern of NL accumulation in leaves can be explained by both the rate of its biosynthesis and metabolic degradation. The gene expression patterns of NrIS1 and NrIS2 transcripts in trichomes and abraded leaves revealed trichomes as the main site of their activity. UHPLC/qqqMS profiling of nepetalactones in trichomes and abraded leaves, complemented with Raman spectroscopy and dichloromethane dipping experiment, supported glandular trichome-specific localization of nepetalactone biosynthesis and accumulation.
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