Elucidation of Triacylglycerol Overproduction in the C4 Bioenergy Crop Sorghum bicolor by Constraint-Based Analysis

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.

The expression of genes encoding novel Sesame oleosin variants facilitates enhanced triacylglycerol accumulation in Arabidopsis leaves and seeds

“…9% (DW) was achieved 4 d after induction provides support for rapid TAG accumulation at or close to the end of vegetative growth (Liang et al ., 2023). Further support for the idea of inducing TAG accumulation late in development comes from metabolic modeling in which it is estimated that Sorghum could accumulate 20% TAG DW in its vegetative tissues in 28 d at the end of the growing season if only 5% of net carbon assimilation was consumed for oil synthesis (Clark & Schwender, 2022). In addition to achieving the optimal balance of expression of lipogenic factors, other approaches may help reduce yield drag, such as the expression of purple acid phosphatase2 (Cai et al ., 2022).…”

Accelerating the development of a sustainable bioenergy portfolio through stable isotopes

“…Therefore, the engineering strategy could be to divert the C flow away from sucrose export towards oil synthesis (Figure 3). A recent theoretical modeling study on Sorghum bicolor found that, based on daily rates of net CO 2 photo-assimilation observed for mature leaves under field conditions, TAG could accumulate to a 20% (w/dw) level in less than a month (Clark & Schwender, 2022). However, this goal could only be accomplished if 5% of the photo-assimilated C was diverted towards oil accumulation, and futile cycles of lipid biosynthesis and degradation operated on a massive scale, which would significantly impede the overall photo-assimilation (Clark & Schwender, 2022).…”

“…A recent theoretical modeling study on Sorghum bicolor found that, based on daily rates of net CO 2 photo‐assimilation observed for mature leaves under field conditions, TAG could accumulate to a 20% (w/dw) level in less than a month (Clark & Schwender, 2022). However, this goal could only be accomplished if 5% of the photo‐assimilated C was diverted towards oil accumulation, and futile cycles of lipid biosynthesis and degradation operated on a massive scale, which would significantly impede the overall photo‐assimilation (Clark & Schwender, 2022). In many cases, reported TAG levels are substantially below a 20% w/dw level, although some plants such as Nicotiana tabacum have been engineered to produce TAG levels above 30% w/dw level (Box S4).…”