Identification, Classification and Differential Expression of Oleosin Genes in Tung Tree (Vernicia fordii)

Heping Cao,Jay M. Shockey,Hongxu Long,Xiaofeng Tan,Lin Zhang,Marie-Joelle Virolle

doi:10.1371/journal.pone.0088409

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). Tung OLE1, OLE2 and OLE3 belong to the S type and OLE4 and OLE5 belong to the SM type of Arabidopsis OLE. TaqMan and SYBR Green qPCR methods were used to study the differential expression of OLE genes in tung tree tissues. 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 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) OLE mRNA levels rapidly increased during seed development; and 5) OLE gene expression was 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. Therefore, they might be preferred targets for tung oil engineering in transgenic plants.

Highlights

Tung tree (Vernicia fordii) is an economically important tree with a very limited growing area in the United States [1,2]
Tung OLE1, OLE2 and OLE3 belong to the S type and OLE4 and OLE5 belong to the SM type of Arabidopsis OLE
Expression results demonstrated that all five OLE genes were expressed in developing tung seeds, leaves and flowers but their expression levels were much higher in the seeds than leaves or flowers

Summary

Introduction

Tung tree (Vernicia fordii) is an economically important tree with a very limited growing area in the United States [1,2]. Tung seeds contain approximately 50–60% oil (dry weight basis) with about 80 mole % a-eleostearic acid (9cis, 11trans, 13trans octadecatrienoic acid) [3]. Tung oil is readily oxidized because of the three unique conjugated double bonds in eleostearic acid. 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 [4,5]. Tung oil has been explored as a raw material to produce biodiesel [6,7,8], polyurethane and wood flour composites [9], thermosetting polymer [10] and repairing agent for self-healing epoxy coatings [11]

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