Plant oils containing hydroxy fatty acids (HFAs) are valuable alternatives to petroleum for making cosmetics, lubricants, polymers, resins, and biofuels. The major source of HFAs is Castor (Ricinus communis) but US cultivation is banned due to toxic ricin content. Lesquerella (Physaria fendleri) is native to southwest US, non-toxic, and produces HFA. However, Lesquerella is not a crop and needs to be further bred or engineered to be a high yielding alternative crop. To enhance the development of Lesquerella, there is a need to understand the metabolic pathways involved in HFA accumulation in seed oil (triacylglycerol, TAG). Castor uses a linear pathway to make HFA-TAG directly, whereas other common oilseed crops produce non-HFA-TAG using a pathway where the membrane lipid phosphatidylcholine (PC) is used as the substrate for TAG assembly. Most plants actively keep unusual fatty acids like HFA out of membrane lipids, thus it was thought Lesquerella would also use the linear pathway like Castor. However, transcriptomics in Lesquerella indicated that expression of genes involved in TAG assembly was similar to plants utilizing the PC pathway, making the oil biosynthetic pathway in Lesquerella unclear. Therefore, a developing seed isotopic labeling approach was used to identify the oil biosynthetic pathway. The metabolic tracing indicated a PC pathway producing TAG containing 0-1 HFA, from PC that did not contain HFA. Over time TAG containing 0-1 HFA was converted to 2-HFA-containing TAG. We propose Lesquerella uses a novel TAG remodeling pathway that changes the oil composition after initial TAG assembly. Further characterization of enzymes involved in TAG remodeling will not only aid in developing Lesquerella into a crop, but will also provide new tools for engineering valuable unusual fatty acids into current oilseed crops.
Coauthors: Philip Bates – Washington State University