Abstract
Upregulation of triacylglycerols (TAGs) in vegetative plant tissues such as leaves has the potential to drastically increase the energy density and biomass yield of bioenergy crops. In this context, constraint-based analysis has the promise to improve metabolic engineering strategies. Here we present a core metabolism model for the C4 biomass crop Sorghum bicolor (iTJC1414) along with a minimal model for photosynthetic CO2 assimilation, sucrose and TAG biosynthesis in C3 plants. Extending iTJC1414 to a four-cell diel model we simulate C4 photosynthesis in mature leaves with the principal photo-assimilatory product being replaced by TAG produced at different levels. Independent of specific pathways and per unit carbon assimilated, energy content and biosynthetic demands in reducing equivalents are about 1.3 to 1.4 times higher for TAG than for sucrose. For plant generic pathways, ATP- and NADPH-demands per CO2 assimilated are higher by 1.3- and 1.5-fold, respectively. If the photosynthetic supply in ATP and NADPH in iTJC1414 is adjusted to be balanced for sucrose as the sole photo-assimilatory product, overproduction of TAG is predicted to cause a substantial surplus in photosynthetic ATP. This means that if TAG synthesis was the sole photo-assimilatory process, there could be an energy imbalance that might impede the process. Adjusting iTJC1414 to a photo-assimilatory rate that approximates field conditions, we predict possible daily rates of TAG accumulation, dependent on varying ratios of carbon partitioning between exported assimilates and accumulated oil droplets (TAG, oleosin) and in dependence of activation of futile cycles of TAG synthesis and degradation. We find that, based on the capacity of leaves for photosynthetic synthesis of exported assimilates, mature leaves should be able to reach a 20% level of TAG per dry weight within one month if only 5% of the photosynthetic net assimilation can be allocated into oil droplets. From this we conclude that high TAG levels should be achievable if TAG synthesis is induced only during a final phase of the plant life cycle.
Highlights
Plant oils are valuable crop products because of their energy density and potential biofuel use
Cytosol/endoplasmic reticulum-localized lipid metabolism was set up as a sub-network specifying the biosynthesis of PC, DAG, and TAG with distinct molecular species that are composed of five acyl chain types, which are palmitate, stearate, oleate, and the polyunsaturated linoleic acid (C18:2) and linolenic acid (C18:3)
As an emerging concept for the development of high energy renewable plant resources, engineering bioenergy crops to accumulate TAG in their vegetative tissues holds the promise of achieving high overall yields
Summary
Plant oils are valuable crop products because of their energy density and potential biofuel use. While oilseeds can accumulate between 20 and 50% TAG by weight (Ohlrogge and Chapman, 2011; Wan et al, 2017), seeds typically make up only a fraction of the plant biomass produced during a growing cycle. It has been estimated that if the bulk of above ground harvested plant biomass would contain TAG at 10% (w/dw), yields per acre could be substantially higher than achievable for any seed oil crop (Ohlrogge and Chapman, 2011). As a by-product, vegetative TAG would be beneficial in high yielding bioenergy grasses that are already harvested for other compounds (Carpita and McCann, 2008; Weijde et al, 2013). For example, has been engineered to accumulate on average 4.3% (w/dw) TAG in its stems and up to 8% (w/dw) TAG in its leaves (Parajuli et al, 2020) and a techno-economic analysis determined that processing sugarcane containing 5% TAG per total dry weight for biodiesel in addition to converting its native sugars into ethanol would be economically advantageous, and 20% TAG could nearly double the potential profits compared to normal sugarcane (Huang et al, 2016)
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