Reorganization of acyl flux through the lipid metabolic network in oil-accumulating tobacco leaves

Authors: ZHOU, XUE-RONG - Csiro European Laboratory; BHANDARI, SAJINA - Washington State University; JOHNSON, BRANDON - Washington State University; KOTAPATI, HARI - Washington State University; Allen, Douglas - Doug; VANHERCKE, THOMAS - Csiro European Laboratory; BATES, PHILIP - Washington State University

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2019
Publication Date: 2/1/2020
Citation: Zhou, X., Bhandari, S., Johnson, B.S., Kotapati, H., Allen, D.K., Vanhercke, T., Bates, P.D. 2020. Reorganization of acyl flux through the lipid metabolic network in oil-accumulating tobacco leaves. Plant Physiology. 182(2):739-755. https://doi.org/10.1104/pp.19.00667.
DOI: https://doi.org/10.1104/pp.19.00667

Interpretive Summary: A finite supply of petroleum and a growing demand for energy to power increasingly industrialized nations are global factors that emphasize the vital need to develop renewable and sustainable sources of energy dense liquid fuels. Seed-derived vegetative oil, mainly consisting of triacylglycerol (TAG), provides a sustainable alternative to petroleum. TAG-based plant oils are one of the most energy-dense compounds found in nature, but are found predominantly in seeds. Non-seed derived plant oils, which can accumulate more lipid per acre of land are an attractive strategy, including the production of oils in vegetative tissues of high biomass crops. Attempts to engineer oil in non-seed tissues have recently demonstrated increased TAG levels by targeting different aspects of lipid biosynthesis, storage and protection. However, the relationship between TAG synthesis and the underlying leaf lipid metabolism, including effects on the accumulation of essential leaf photosynthetic membranes is unknown. Using isotopes to trace metabolism along with enzymatic assays, we characterized the underlying changes necessary to accommodate enhanced lipid production in leaves. The studies provide biochemical insights about regulation of metabolism and how it might be further engineered to produce food and feed stocks for fuel or other traditionally petroleum-derived products.

Technical Abstract: The triacylglycerols (TAGs; i.e. oils) that accumulate in plants represent the most energy-dense form of biological carbon storage, and are used for food, fuels, and chemicals. The increasing human population and decreasing amount of arable land have amplified the need to produce plant oil more efficiently. Engineering plants to accumulate oils in vegetative tissues is a novel strategy, because most plants only accumulate large amounts of lipids in the seeds. Recently, tobacco (Nicotiana tabacum) leaves were engineered to accumulate oil at 15% of dry weight due to a push (increased fatty acid synthesis)-and-pull (increased final step of TAG biosynthesis) engineering strategy. However, to accumulate both TAG and essential membrane lipids, fatty acid flux through nonengineered reactions of the endogenous metabolic network must also adapt, which is not evident from total oil analysis. To increase our understanding of endogenous leaf lipid metabolism and its ability to adapt to metabolic engineering, we utilized a series of in vitro and in vivo experiments to characterize the path of acyl flux in wild-type and transgenic oil-accumulating tobacco leaves. Acyl flux around the phosphatidylcholine acyl editing cycle was the largest acyl flux reaction in wild-type and engineered tobacco leaves. In oil-accumulating leaves, acyl flux into the eukaryotic pathway of glycerolipid assembly was enhanced at the expense of the prokaryotic pathway. However, a direct Kennedy pathway of TAG biosynthesis was not detected, as acyl flux through phosphatidylcholine preceded the incorporation into TAG. These results provide insight into the plasticity and control of acyl lipid metabolism in leaves.