Triacylglycerol stability limits futile cycles and inhibition of carbon capture in oil-accumulating tobacco leaves

Authors: JOHNSON, BRANDON - Washington State University; Allen, Douglas - Doug; BATES, PHILIP - Washington State University

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plant lipids are a crucial source of calories for the human diet and vegetable oils can also serve as renewable feed stocks for biodiesel and to replace other petroleum-derived products. Leaves that have been engineered to produce high levels of lipids provide a unique opportunity to consider the altered metabolism of a plant tissue. In the future, it may be that leaves are harvested for lipids as a biofuel feed stock analogous to current efforts to produce ethanol from plant biomass. We used a series of isotopes in this study to track metabolism in the leaves when different genes were expressed. We found changes in the biomass composition could be explained by lipid production and some turnover of lipids diurnally as a tradeoff with starch in some instances. The studies are important because lipid production by plants in different tissues will need to be stable and maintained if it is to be harvested and add value to the crop. As we develop crops with added lipids, this improves their value for agriculture and contributes to sustainability.

Technical Abstract: Engineering plant vegetative tissue to accumulate triacylglycerols (TAG, e.g., oil) can increase the amount of oil harvested per acre to levels that exceed current oilseed crops. Engineered Nicotiana tabacum (tobacco) lines that accumulate 15% to 30% oil of leaf dry weight were studied and resulted in starkly different metabolic phenotypes. In-depth analysis of the leaf lipid accumulation and 14CO2 metabolic tracing mechanistically described metabolic adaptations to the leaf oil engineering. An oil-for-membrane lipid tradeoff in the 15% oil line (referred to as HO) was surprisingly not further exacerbated when lipid production was enhanced to 30% (LEC2 line). The HO line exhibited a futile cycle that limited TAG yield through exchange with starch, altered carbon flux into various metabolite pools and end products, and indicated overlapping pathways of the glyoxylate cycle and photorespiration that limited CO2 assimilation by 50%. In contrast, inclusion of the LEC2 transcription factor in tobacco improved TAG stability, alleviated the TAG-to-starch futile cycle, and recovered CO2 assimilation and plant growth that were comparable to wild type but with much higher lipid levels in the leaves. Thus, the unstable production of storage reserves and futile cycling limit vegetative oil engineering approaches. The capacity to overcome futile cycles and maintain enhanced stable TAG levels in LEC2 demonstrated the importance of considering unanticipated metabolic adaptations while engineering vegetative oil crops.