Short-term salicylic acid application induces the expression of GPPS and NEPS1 in catnip

The catnip plant, Nepeta cataria L., produces bioactive iridoid monoterpenes known as nepetalactones. These monoterpenes follow a noncanonical biosynthetic route starting as isoprene monomers in the methylerythritol (MEP) pathway after condensation via geranyl pyrophosphate synthase (GPPS) and later cyclization through the iridoid pathway via nepetalactol-related short-chain dehydrogenase enzymes (NEPS). Nepetalactones exhibit insect repellant properties against disease-carrying vectors such as mosquitoes, bed bugs and ticks. Chemical elicitors such as salicylic acid have been used to induce biochemical changes in medicinal and aromatic plant species including catnip that can temporarily increase the yield and quality of secondary metabolites. This is the first study to correlate the effects of short-term salicylic acid application on the genes in the nepetalactone biosynthetic pathway. Both 0.5 mM SA and 1.0 mM SA resulted in upregulation of GPPS and NEPS1. However, within the 24-h time course of the study significant changes in nepetalactone yield were not observed. Correlation analysis revealed a significant relationship between the mRNA transcript abundance for GPPS and NEPS1 demonstrating a low positive co-expression strength for the two genes.