The regulation of quantitative variation of foliar terpenes in medicinal tea tree Melaleuca alternifolia
© Webb et al; licensee BioMed Central Ltd. 2011
Published: 13 September 2011
Aims and methods
Melaleuca alternifolia(medicinal tea tree) is a small tree in the family Myrtaceae that is grown in plantations for its valuable terpene-rich leaf oil. We studied the expression of MVA and MEP pathway genes to identify those that are correlated with variation in foliar terpene concentration. Diagnostic primers were designed for seven MEP pathway genes, three MVA pathway genes and three genes downstream of both pathways. Transcript abundance for all 13 genes was then quantified from 48 individuals on the Fluidigm biomark platform.Foliar terpene concentrations were determined by GC of ethanol extracts containing an internal standard.
Several studies have shown that the expression of dxr and dxs has a strong influence on foliar terpene yield; however, this has only ever been demonstrated in transgenic plants and never on naturally occurring, quantitative variation in terpene concentrations. Our results show that a large proportion of the quantitative variation of foliar terpene concentration in M. alternifolia can be explained by quantitative variation of transcript abundance of genes in the early stages of terpenoid biosynthesis. A number of MEP pathway genes are correlated with variation in both the monoterpene terpinen-4-ol as well as the sesquiterpene bicyclogermacrene, which is also correlated with one MVA pathway gene. The multiple regression models for both terpinen-4-ol and bicyclogermacrene, each explained a significant amount of variation in the concentrations of each terpene.
The correlation between monoterpenes and transcripts from the MEP pathway was expected as they both occur in the plastid, however we also found a significant correlation between MEP pathway genes and bicyclogermacrene, a sesquiterpene synthesized in the cytosol. These correlations, as well as the results of the multiple regression analysis, strongly suggests that in M. alternifolia,the IPP from the plastid contributes significantly to the pool of IPP used for sesquiterpene biosynthesis.
This is the first study that shows the importance of variation in transcript abundance of terpene biosynthesis genes in determining naturally occurring variation in foliar terpene concentrations. The results also suggest that IPP originating in the plastid contributes significantly to the pool of precursors used for sesquiterpene biosynthesis in the cytosol in M. alternifolia. These results open the way for future studies to identify markers that can be used to improve early selection of plants with an enhanced yield and thus greater profitability.
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