Discovering Transcriptional Regulators of Photosynthesis in Energy Sorghum to Improve Productivity
Laurie Leonelli1, Elena Pelech1* (firstname.lastname@example.org), Atinder Singh1, Kithmee De Silva1, Matthew Brooks2, and Steve Long1
1University of Illinois at Urbana–Champaign; and 2Global Change and Photosynthesis Research Unit, United States Department of Agriculture Agricultural Research Service
This research aims to identify and investigate the transcription factors involved in the regulation of photosynthesis in energy sorghum. The major goal of this project is to model and validate gene regulatory networks that reveal the relationship between transcription factors and photosynthesis, particularly those that cause a loss of efficiency in lower leaf canopy leaves. This information will allow researchers to rank transcription factors by importance and thus, will guide future design strategies for developing energy sorghum cultivars with improved photosynthetic light-use efficiency in overall productivity.
C4 grasses such as annual energy sorghum hybrids (Sorghum bicolor) have great potential for both carbon sequestration and as feedstocks for biofuels and building materials. Sorghum is also exceptionally drought tolerant, which allows cultivation on land that is marginal for most food crops and as a vegetative crop, it also avoids the problems faced by grain crops during the water deficit sensitive reproductive phase (Mullet et al. 2014). However, in contrast to most plants, sorghum belongs to a clade of C4 species that has undergone a maladaptive loss of photosynthetic efficiency in self-shaded leaves within the canopy and current models predict that this loss results in a 15-20% reduction in potential productivity (Pignon et al. 2017). Specifically, most plants have evolved to dynamically tune their photosynthetic machinery by shifting the stoichiometry of proteins involved in the light reactions of photosynthesis to maintain a high maximum absolute quantum efficiency of CO2 assimilation (𝛷𝐶𝑂2,𝑚𝑎𝑥) in the shade. Seminal work has shown that the lower self-shaded leaves from C4 bioenergy crops (bioenergy sorghum, Miscanthus and maize) do not retain a high 𝛷𝐶𝑂2,𝑚𝑎𝑥 compared to their upper sun-exposed leaves, which is due to the change in light environment, not leaf age (Pignon et al. 2017; Collison et al. 2020). Variation in the severity of this 𝛷𝐶𝑂2,𝑚𝑎𝑥 loss between sorghum cultivars suggests that this maladaptive trait may be the result of difference in the expression of one or more genes (Jaikumar et al. 2021). Since transcription factors (TFs) are key regulators of gene expression in response to environmental stimuli such as changes in light intensity and quality, researchers hypothesize that key TFs cause the observed maladaptive loss of photosynthetic efficiency in energy sorghum and optimizing their expression will restore photosynthetic efficiency and alleviate suboptimal 𝛷𝐶𝑂2,𝑚𝑎𝑥 in the shaded canopy. Researchers further hypothesize that genes influencing 𝛷𝐶𝑂2,𝑚𝑎𝑥 will have expression patterns that correspond to measurable changes in photosynthetic traits and that researchers will be able to identify these genes by comparing changes in expression in response to the light environment across energy sorghum cultivars and canopy positions. Therefore, researchers will identify these key transcription factors by analyzing variations in gene expression and photosynthetic traits such as 𝛷𝐶𝑂2,𝑚𝑎𝑥 across light conditions and sorghum cultivars. Researchers will also use in planta validation of TF gene targets to model a gene regulatory network to describe the regulation of photosynthesis in sorghum. Identifying the cause of photosynthetic inefficiency in shaded energy sorghum canopies and engineering solutions to restore the 15-20% loss in productivity and enhance yield will improve the overall potential of this bioenergy crop to meet the growing needs for energy security.
Mullet, J., et al. 2014. “Energy Sorghum—A Genetic Model for the Design of C4 Grass Bioenergy Crops.” Journal of Experimental Botany 65(13), 3479–89. DOI: https://doi.org/10.1093/jxb/eru229.
Pignon, C. P., et al. 2017. “Loss of Photosynthetic Efficiency in the Shade. An Achilles Heel for the Dense Modern Stands of Our Most Productive C4 Crops?” Journal of Experimental Biology, 68(2), 335–45. DOI: https://doi.org/10.1093/jxb/erw456.
Collison, R. F., et al. 2020. “Not Age, Underlies the Maladaptation of Maize and Miscanthus Photosynthesis to Self-Shading.” Frontiers in Plant Science 11, DOI: https://doi.org/10.3389/fpls.2020.00783.
Jaikumar, N. S. et al. 2021. “Can Improved Canopy Light Transmission Ameliorate Loss of Photosynthetic Efficiency in the Shade? An Investigation of Natural Variation in Sorghum Bicolor.” Journal of Experimental Botany 72(13), 4965–80. DOI: https://doi.org/10.1093/jxb/erab176.
This research is supported by the DOE Office of Science, Office of Biological and Environmental Research (BER), grant no. DE-SC0023107. This program is supported by the U. S. Department of Energy, Office of Science, through the Genomic Science program, Office of Biological and Environmental Research, under FWP ERKP123.