Compartmentalization of Jojoba Seed Lipid Metabolites

Seeds from the desert shrub Simmondsia chinensis (jojoba) are one of the only known natural plant sources to store a majority of its oil in the form of liquid wax esters (WE) instead of triacylglycerols (TAGs) and these oils account for ~55% of the seed weight. Jojoba oil is highly valued as cosmetic additives and mechanical lubricants, yet despite its value much is still unknown about its neutral lipid biosynthetic pathways and lipid droplet packaging machinery. Here, we have used a multi-"omics" approach to study how spatial differences in lipid metabolites, gene expression, and lipid droplet proteins influence the synthesis and storage of jojoba lipids. Through these studies mass spectrometry analyses revealed that WEs are compartmentalized primarily in the cotyledonary tissues, whereas TAGs are, surprisingly, localized to the embryonic axis tissues. To study the differences in gene expression between these two tissues, a de novo transcriptome was assembled from high throughput RNAseq data. Differential gene expression analysis revealed that the Jojoba Wax Synthase, which catalyzes the formation of wax esters, and the Diacylglycerol O-Acyltransferase1, which catalyzes the final acylation of triacylglycerol synthesis, were differentially expressed in the cotyledons and embryonic axis tissues, respectively. Furthermore, through proteomic analysis of lipid droplet proteins from lipid droplets of the cotyledons and embryonic axis, it was estimated that each of these tissues contains a different proportion of the major lipid droplet proteins, oleosins, steroleosins, caleosins, and lipid droplet associated proteins. The Jojoba Olesosin1, Lipid Droplet Associated Protein 1, and Lipid Droplet Associated Protein 3, were identified as potential lipid droplet proteins that could be important for storage of wax esters. The coding sequences of these genes were transiently expressed in N. benthamiana leaves individually, and with co-expression of Mus musculus diacylglycerol acyltransferase 2, and in all cases were able to induce neutral lipid accumulation. These data also suggest a Lipid Droplet Associated Protein 1 has a specialized role for wax ester accumulation in the cotyledons, whereas Lipid Droplet Associated Protein 3 may have a more generalized role for the storage of triacylglycerols. These differences in compartmentation suggests that the cotyledons and embryonic axis of jojoba have evolved tissue-specific sets of genes for neutral lipid assembly and lipid droplet accumulation. It may be important to consider this tissue context for genetic engineering strategies designed to introduce genes from jojoba into other oilseed crops.