Abstract
BackgroundEremophila R.Br. (Scrophulariaceae) is a diverse genus of plants with species distributed across semi-arid and arid Australia. It is an ecologically important genus that also holds cultural significance for many Indigenous Australians who traditionally use several species as sources of medicines. Structurally unusual diterpenoids, particularly serrulatane and viscidane-types, feature prominently in the chemical profile of many species and recent studies indicate that these compounds are responsible for much of the reported bioactivity. We have investigated the biosynthesis of diterpenoids in three species: Eremophila lucida, Eremophila drummondii and Eremophila denticulata subsp. trisulcata.ResultsIn all studied species diterpenoids were localised to the leaf surface and associated with the occurrence of glandular trichomes. Trichome-enriched transcriptome databases were generated and mined for candidate terpene synthases (TPS). Four TPSs with diterpene biosynthesis activity were identified: ElTPS31 and ElTPS3 from E. lucida were found to produce (3Z,7Z,11Z)-cembratrien-15-ol and 5-hydroxyviscidane, respectively, and EdTPS22 and EdtTPS4, from E. drummondii and E. denticulata subsp. trisulcata, respectively, were found to produce 8,9-dihydroserrulat-14-ene which readily aromatized to serrulat-14-ene. In all cases, the identified TPSs used the cisoid substrate, nerylneryl diphosphate (NNPP), to form the observed products. Subsequently, cis-prenyl transferases (CPTs) capable of making NNPP were identified in each species.ConclusionsWe have elucidated two biosynthetic steps towards three of the major diterpene backbones found in this genus. Serrulatane and viscidane-type diterpenoids are promising candidates for new drug leads. The identification of an enzymatic route to their synthesis opens up the possibility of biotechnological production, making accessible a ready source of scaffolds for further modification and bioactivity testing.
Highlights
Eremophila R.Br. (Scrophulariaceae) is a diverse genus of plants with species distributed across semi-arid and arid Australia
Liquid chromatography-high resolution mass spectrometry (LC-HRMS) analysis of this resinous layer showed the presence of m/z values expected of diterpenoids reported from these species (Fig. 1 and Additional file 2: Figure S1) [18, 19, 54]
Based on our results indicating surfacelocalisation of diterpenoids and in combination with the well-established role of glandular trichomes in terpenoid biosynthesis in numerous species [55], we speculated that they would have this function in Eremophila
Summary
Eremophila R.Br. (Scrophulariaceae) is a diverse genus of plants with species distributed across semi-arid and arid Australia. (Scrophulariaceae) is a diverse genus of plants with species distributed across semi-arid and arid Australia. (Scrophulariaceae) is a large and diverse genus of plants endemic to mainland Australia. Members of this genus occur across the continent with. Further reports on the bioactivity of structurally related diterpenoids isolated from Leucophyllum frutescens ( Scrophulariaceae) [20] and several marine gorgonian coral species [21] support this group of molecules as a potential source of new drug leads. Many of the diterpenoids appear to be C20 analogues of sesquiterpenes with an uncyclized fourth prenyl unit Because of their unusual structures and potential as drug leads, we set out to identify the enzymes involved in Eremophila diterpenoid biosynthesis
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