Location: Commodity Utilization Research
Title: Identification, classification and differential expression of oleosin genes in tung tree (Vernicia fordii) Authors
Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 6, 2014
Publication Date: February 6, 2014
Citation: Cao, H., Zhang, L., Tan, X., Long, H., Shockey, J.M. 2014. Identification, classification and differential expression of oleosin genes in tung tree (Vernicia fordii). PLoS One. 9(2):1-21. Interpretive Summary: Tung tree is an economically important tree with a very limited growing area in the United States. Tung seeds contain approximately 50-60% oil (dry weight basis) with about 80 mole % a-eleostearic acid. Tung oil is readily oxidized because of the three unique conjugated double bonds in eleostearic acid. Dried tung oil is impervious to heat, moisture, dust and many chemical challenges. Tung oil, unlike other drying oils, does not darken with age. These properties of tung oil make it a widely used drying ingredient in paints, varnishes, coatings and finishes. Recently, tung oil has been explored as a raw material to produce biodiesel, polyurethane and wood flour composites, thermosetting polymer and repairing agent for self-healing epoxy coatings. Our project focuses on alternative ways of producing tung oil-like fatty acids and other high-value industrial oils by engineering tung oil biosynthetic genes into oilseed crops. Tung oil is one type of triacylglycerols that are packed in subcellular structures called oil bodies or lipid droplets. Oleosins (OLE) are the major proteins in plant oil bodies. They may function to stabilize oil bodies at low water potential and/or regulate the sizes of oil bodies. The objectives of this study were to identify OLE genes, classify OLE proteins and analyze OLE gene expression in tung trees. We identified five OLE genes in tung tree. We performed genome-wide phylogenetic analysis and multiple sequence alignment and classified the five tung OLE genes based on 65 OLE from 19 tree species including the sequenced genomes of peach, poplar, castor bean, cacao and grapevine. Finally, we used TaqMan and SYBR Green qPCR assays to evaluate the relative abundance and tissue distribution of the five OLE mRNA in the seeds, leaves and flowers of tung trees. The information on OLE expression profiles suggests that OLE1, OLE2 and OLE3 genes may play major roles in tung oil biosynthesis and/or tung oil body development. Therefore, they might be preferred targets for tung oil engineering in transgenic plants.
Technical Abstract: Triacylglycerols (TAG) are the major molecules of energy storage in eukaryotes. TAG are packed in subcellular structures called oil bodies or lipid droplets. Oleosins (OLE) are the major proteins in plant oil bodies. Multiple isoforms of OLE are present in plants such as tung tree (Vernicia fordii), whose seeds are rich in novel TAG with a wide range of industrial applications. The objectives of this study were to identify OLE genes, classify OLE proteins and analyze OLE gene expression in tung trees. We identified five tung tree OLE genes coding for small hydrophobic proteins. Genome-wide phylogenetic analysis and multiple sequence alignment demonstrated that the five tung OLE genes represented the five OLE subfamilies and all contained the “proline knot” motif (PX5SPX3P) shared among 65 OLE from 19 tree species, including the sequenced genomes of Prunus persica (peach), Populus trichocarpa (poplar), Ricinus communis (castor bean), Theobroma cacao (cacao) and Vitis vinifera (grapevine). TaqMan and SYBR Green qPCR methods were used to study the differential expression of OLE genes in tung tree tissues under optimized conditions. Expression results demonstrated that 1) All five OLE genes were expressed in developing tung seeds, leaves and flowers; 2) OLE mRNA levels were much higher in the seeds than leaves or flowers; 3) OLE1, OLE2 and OLE3 genes were expressed in tung seeds at much higher levels than OLE4 and OLE5 genes; 4) the amounts of OLE mRNA rapidly increased during seed development; and 5) the expression levels of OLE genes were well-coordinated with tung oil accumulation in the seeds. These results suggest that tung OLE genes 1-3 probably play major roles in tung oil accumulation and/or oil body development and that tung OLE genes 4-5 may contribute to tung oil accumulation in the mid stage of seed development.