Physiological drought alters nepetalactone metabolisam in Nepeta rtanjensis leaves

Abstract

Major constituents of Nepeta rtanjensis Diklić & Milojević: trans,cis-nepetalactone and its dehydrogenation product dehydronepetalactone, are synthetized and accumulated in glandular trichomes. Interestingly, dehydronepetalactone is the major monoterpenoid in fresh leaves of N. rtanjensis, while the amount of this compound dramatically decreases in dry leaves. Furthermore, dehydronepetalactone has been previously identified in Nepeta species mainly in the cases when essential oils and extracts prepared from fresh plant material were analysed. All this lead us to presume that nepetalactone metabolism is reprogrammed during the process of leaf dehydration. Here we present for the first time an insight into the molecular background of constitutive nepetalactone biosynthesis in leaves of N. rtanjensis and its alterations under physiological drought stress, which was experimentally induced in vitro by exposing plants to PEG 8000 (3 MPa) for 1, 3 and 6 days. Leaves of PEG-treated and of non-treated plants were collected and subjected to gene expression analysis and to metabolic profiling. Putative genes encoding enzymes for intermediate steps of nepetalactone biosynthetic pathway (GPPS, GES, G8O, 8HGO, IS1 and IS2) were mined from N. rtanjensis leaf transcriptome. Although majority of analysed genes were significantly down-regulated during the process of leaf dehydration, PEG-induced physiological drought induced no significant changes in nepetalactone content, while dehydronepetalactone content was slightly decreased. The possible key enzymes controlling the nepetalactone biosynthetic-flux could be GPPS, G8O, and IS1, which showed stable expression levels during dehydration. Stressed plants most likely maintain nepetalactone content stable by lowering both its biosynthesis and degradation, which results in decreased dehydronepetalactone content in leaves, and thus in altered nepetalactone/dehydronepetalactone ratio.