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  • Experimentally measured DGAT activity was first reported

    2020-11-19

    Experimentally measured DGAT activity was first reported by Weiss et al. (1960) and several different types of DGAT enzyme have since been described in plants [12,41,42]. As recently as 2011 there appeared to be just two DGAT enzymes both in plants and in other eukaryotes, namely DGAT1 and DGAT2 [43]. Both DGAT1 and DGAT2 are membrane-bound (normally on the ER) enzymes but they are otherwise structurally very distinct from each other. It therefore seems likely that these two enzymes originally evolved separately but became functionally convergent as they acquired similar types acyltransferase activity involving DAG substrates [43], albeit possibly with different roles in ER-based TAG formation in different plant tissues. More recently, a third putative DGAT isoform, a soluble enzyme termed DGAT3, was discovered and there are preliminary reports that this enzyme has DGAT activity and may also participate in a cytosolic pathway of TAG biosynthesis [25,44,45]. Finally, a fourth DGAT activity, a bi-functional DGAT/wax ester synthase (WS/DGAT) has been described in a wide range of organisms from bacteria to plants [46]. The primary function of WS/DGAT is believed to be the formation of surface wax esters, although it has been suggested that this enzyme is also responsible for making small amounts of TAG [25,46,47]. Interestingly WS/DGAT is a very diverse protein family with some members shown to be soluble in Napabucasin while others are membrane-bound [[48], [49], [50]]. As with the DGAT1 and DGAT2 genes, both DGAT3 and WS/DGAT have very distinct evolutionary pathways and appear to have originated independently of each other [25]. In all of the land plant genomes and at least one algal genome analysed to date some or all the four DGAT gene families have multiple copies, implying that the duplication events responsible for this probably occurred prior to Streptophyte diversification [25,51]. Genetically speaking, oil accumulation in plant tissues is a complex quantitative trait that involves numerous genes. Efforts to increase oil yields in commercially valuable crops, such as E. guineensis and oilseeds like soybean, require the identification of the specific genes that regulate this highly desirable agronomic trait so that breeders can focus on variation involving those key genes [52] in a similar manner to efforts to manipulate the acyl quality of the oil [53]. It is becoming increasingly apparent that DGAT activity is pivotal to increasing oil yield in seeds, as demonstrated by the significant increases in TAG accumulation when DGAT genes are over-expressed in transgenic plants [12,25,54]. E. guineensis, as the world’s most productive edible (and industrial) oil crop, is an important contributor to global food security with several studies indicating that demand for the oil will continue to increase substantially over the coming decades [2]. This has led to concerns that the forecast increased demand for palm oil might lead to further conversion of sensitive tropical habitats to E. guineensis plantations [2]. However, an alternative strategy would be to increase oil yields in E. guineensis fruits themselves so that more oil can be produced from exisiting smallholder and commercial plantations. This would reduce the requirement for the conversion of additional land for E. guineensis cultivation [2]. A further challenge is the improvement of oil quality in order to expand markets for palm oil, e.g. by reducing the saturate content and increasing oleic acid levels in order to compete more effectively with premium edible vegetable oils such as olive and sunflower oils and also to reduce free fatty acid levels by inhibiting or removing lipase gene expression in freshly picked palm fruits [55]. In order to fulfil the strategy of improving palm oil yield and quality, it is necessary to improve our understanding of the regulation of TAG accumulation and especially the role of DGAT in the non-seed tissues of E. guineensis fruits, namely the mesocarp, where relatively few studies have been performed to date [[56], [57], [58]]. In this study, we describe the genomic architecture of the various isoforms within the four classes of DGAT genes in E. guineensis, namely DGAT1, DGAT2, DGAT3 and WS/DGAT as compared with 12 other plant species. We have also evaluated DGAT gene expression in a range of tissues, including mesocarp, seed kernel, and selected vegetative tissues, as well as at different developmental stages.