Transcriptome Analyses to Assess the Determinants of Essential Oil Composition in Mint Species

Various mint species are cultivated across the globe for their essential oils.  The most abundant constituent of spearmint (Mentha spicata) oil is the monoterpene (-)-carvone.  Water mint (Mentha aquatica), in contrast, accumulates primarily (+)-menthofuran in the oil.  A cross between water mint and spearmint led to the development of peppermint (Mentha x piperita cv. Black Mitcham), which produces an oil rich in (-)-menthone and (-)-menthol (although the oils of the parents do not contain these compounds above trace levels).  Japanese mint (Mentha arvensis), which is not directly related to peppermint, forms an oil with very high amounts of (-)-menthol.  The selfing of spearmint resulted in a progeny from which one line, termed Erospicata, was selected that contained high amounts of (-)-menthone but was essentially devoid of (-)-menthol.  To evaluate the causes for these compositional differences between cultivars, we performed Illumina-based transcriptome analyses with glandular trichomes, the specialized anatomical structures responsible for the biosynthesis of essential oils at different developmental stages.  The transcripts for specific branch-point enzymes in the monoterpene pathway were differentially expressed in mint cultivars.  For example, the gene encoding (-)-limonene 6-hydroxylase was expressed at very high levels in spearmint (which accumulates C6-oxygenated (-)-carvone), while (-)-limonene 3-hydroxylase transcript was highly abundant in all other cultivars (which produce C3-oxygenated monoterpenes).  The accumulation of (-)-menthol in the oil appears to occur only in cultivars in which the expression of the (-)-menthone-(-)-menthol reductase gene is high at later stages of glandular trichome development.  The implications of these findings will be discussed.