Transcriptional Modulation of Structural and Regulatory Genes Involved in Isoprene Biosynthesis and Their Relevance to Oil Yield and Menthol Content in Peppermint (Mentha piperita L.) upon MeJA and GA3 Treatments

Monoterpenes, the major components of the essential oils of the mint family, are synthesized in glandular trichomes. In peppermint (Mentha piperita L.) precursors for the biosynthesis of monoterpenes are provided by plastidial methyl-erythritol-phosphate (MEP) pathways. Knowledge regarding regulatory elements modulating isoprenoid biosynthetic pathway and trichome differentiation is rudimentary in mint plant. In this study, we aimed to find dynamic transcriptional responses of MEP pathway and regulatory genes to methyl jasmonate (MeJA) and gibberellic acid (GA3) in M. piperita by quantitative real time-PCR (qRT-PCR). Moreover, the effects of altered precursor availability on oil composition and yield were evaluated after treatments. Consistent with the up-regulation of most of the MEP pathway genes, induced expression of AP2, WRKY and MYB genes resulted in an increase in oil yield and menthol content, suggesting combinatorial action of the JA-modulated transcription factors (TFs) in elicitation of MEP pathway gene expression. In addition to enhancing expression of some of the MEP pathway genes, GA probably affected trichome formation by induction of a C2H2-type zinc finger protein as a key trichome initiation regulator resulting in a rise in oil and menthol yield. Reduced expression of the C2H2 related TF coincided with the induction of MEP pathway genes, suggesting that the C2H2 TF may act as potential transcriptional repressor in the MEP pathway. Moreover, WRKY may regulate C2H2 and AP2 genes expression to control JA-elicited synthesis of terpenoids in M. piperita. These results provide insights related to the underlying mechanism of transcriptional regulation of terpenoids and trichome formation.