Abstract
Medicinal tea tree (Melaleuca alternifolia) leaves contain large amounts of an essential oil, dominated by monoterpenes. Several enzymes of the chloroplastic methylerythritol phosphate (MEP) pathway are hypothesised to act as bottlenecks to the production of monoterpenes. We investigated, whether transcript abundance of genes encoding for enzymes of the MEP pathway were correlated with foliar terpenes in M. alternifolia using a population of 48 individuals that ranged in their oil concentration from 39 -122 mg.g DM−1. Our study shows that most genes in the MEP pathway are co-regulated and that the expression of multiple genes within the MEP pathway is correlated with oil yield. Using multiple regression analysis, variation in expression of MEP pathway genes explained 87% of variation in foliar monoterpene concentrations. The data also suggest that sesquiterpenes in M. alternifolia are synthesised, at least in part, from isopentenyl pyrophosphate originating from the plastid via the MEP pathway.
Highlights
Plant essential oils are an important crop in many parts of the world and their profitability is closely linked to both the profile and concentration of the oil in each plant
Six essential oil chemotypes have been identified in medicinal tea tree [3,6], but the only one sought by the tea tree industry is that dominated by the monoterpene terpinen-4-ol, which is derived from the spontaneous rearrangement of sabinene hydrate, which in turn is produced by a single terpene synthase [3]
Terpene yield is under strong genetic control in woody plants [35,36]
Summary
Plant essential oils are an important crop in many parts of the world and their profitability is closely linked to both the profile and concentration of the oil in each plant. Improving the oil yield of essential oil crops relies on a long process of traditional breeding and in the case of tree crops, this can require many years before production. Medicinal tea tree (Melaleuca alternifolia Cheel) is a small Myrtaceous tree with sub-dermal foliar oil glands [2] containing a valuable essential oil dominated by monoterpenes [3]. Six essential oil chemotypes have been identified in medicinal tea tree [3,6], but the only one sought by the tea tree industry is that dominated by the monoterpene terpinen-4-ol, which is derived from the spontaneous rearrangement of sabinene hydrate, which in turn is produced by a single terpene synthase [3]. In M. alternifolia, the narrow-sense heritability of foliar oil concentration is high (,0.7) suggesting that genetic control of oil yield is significant [8]
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