• Energy Research

SummaryHigh biomass crops have recently attracted significant attention as an alternative platform for the renewable production of high energy storage lipids such as triacylglycerol (TAG). While TAG typically accumulates in seeds as storage compounds fuelling subsequent germination, levels in vegetative tissues are generally low. Here, we report the accumulation of more than 15% TAG (17.7% total lipids) by dry weight in Nicotiana tabacum (tobacco) leaves by the co‐expression of three genes involved in different aspects of TAG production without severely impacting plant development. These yields far exceed the levels found in wild‐type leaf tissue as well as previously reported engineered TAG yields in vegetative tissues of Arabidopsis thaliana and N. tabacum. When translated to a high biomass crop, the current levels would translate to an oil yield per hectare that exceeds those of most cultivated oilseed crops. Confocal fluorescence microscopy and mass spectrometry imaging confirmed the accumulation of TAG within leaf mesophyll cells. In addition, we explored the applicability of several existing oil‐processing methods using fresh leaf tissue. Our results demonstrate the technical feasibility of a vegetative plant oil production platform and provide for a step change in the bioenergy landscape, opening new prospects for sustainable food, high energy forage, biofuel and biomaterial applications.

Abstract Plants are being engineered for enhanced ethanol production; however, challenges remain in meeting the demand for bioenergy that is expected to double by 2030. Therefore, targeting carbon accumulation in the form of energy-dense oils in nonseed biomass is considered a superior alternative for bioenergy production. Although oils in the form of triacylglycerols (TAGs) are typically stored in seed tissues, various nonseed tissues such as mesocarp, tubers, stems and leaves also serve as storage tissues for TAG accumulation in plants. Moreover, the biomass of these tissues is generally far greater than seed biomass. In order to increase oil content in nonseed biomass for bioenergy and nutritional purposes, it is important to understand how such plants naturally accumulate TAG in nonseed tissues. Several molecular approaches, including transcriptomics, have been undertaken to elucidate the metabolic and regulatory mechanisms of carbon partitioning in oil-rich nonseed tissues. Such studies are expected to generate important transgenic tools that can be used to alter fatty acid metabolism and engineer plants to produce oil-rich biomass successfully. This review focuses on the potential of different oil-rich nonseed tissues and the strategies developed for enhancing oil biomass.

Abstract Metabolically engineered high-leaf oil plants have been developed to meet the increasing demand for plant oils. Oil production of these plants under controlled conditions is promising; however, their performance under field-like conditions with abiotic stresses remains uncertain. In this study, wild-type (WT) and high-leaf oil (HLO) transgenic tobacco (Nicotiana tabacum) plants were exposed to moderate and sustained water stress to mimic field conditions. The effects of water stress on biomass and lipid accumulation were investigated at the physiological, biochemical, and transcriptional levels. The presence of transgenes increased leaf triacylglycerol (TAG) levels in HLO plants by upregulating endogenous genes involved in lipid biosynthesis at the expense of biomass reduction, altered leaf lipid content and profile, and a decrease in unsaturation levels of membrane lipids compared to WT plants. Moreover, the biomass penalty in HLO plants could reduce canopy transpiration, contributing to their better performance under water-limited environments. Furthermore, WT and HLO plants exhibited enhanced TAG accumulation under water stress but via different mechanisms. In WT plants, water stress induced lipid remodeling, upregulated genes encoding phosphatidic acid phosphatase (PAP), diacylglycerol o-acyltransferase (DGAT2), and lipid droplet-associated proteins (LDAP1), but downregulated genes encoding Gly-Asp-Ser-Leu (GDSL) lipases. In contrast, HLO plants showed increased TAG accumulation primarily through upregulation of OLEOSINS and downregulation of GDSLs under water stress. In conclusion, moderate water stress promoted oil production in HLO plants, demonstrating the robustness of HLO technology for sustainable oil production in the field under water deficit conditions which may be more prevalent in the future due to climate change.