Author : Uy, Lawrence Yves Cheng

Summary : Using the C4 model grass Setaria viridis, a novel mutant (MT) line was identified through a forward genetics approach. The mutants exhibit severe photosynthetic defects, including drastic reduction of Fv/Fm in both ambient and high CO2 conditions, stunted growth, and altered cellular anatomy. Physiological analysis revealed a non-stomatal limitation to photosynthesis, characterized by impaired carboxylation efficiency and reduced RuBisCO levels, while maintaining wild type-like (WTL) carbon isotope discrimination values. This primary biochemical defect led to a cascade of downstream effects, including compromised sugar and starch accumulation and smaller chloroplasts. Using backcrossing and bulked segregant analysis coupled with whole-genome sequencing, the possible causal mutations were mapped to chromosome 7: genes for a wall­associated kinase (SvWAK5) and an elongation factor G (SvEFG). Structural modeling predicted that both mutations caused significant conformational changes in their respective proteins. The pleiotropic phenotype, particularly the reduction in the amount of RuBisCO protein, strongly suggests that the chloroplast protein synthesis was highly affected. This implies that SvEFG has a strong likelihood of causing the smaller chloroplast phenotype affecting photosynthesis, while SvWAK contributed to the stunting, lower yield, and delayed flowering. This work not only identifies key genetic components required for healthy C4 photosynthesis but also provides a valuable genetic resource for translating these genetic insights into C3 crops like rice, bringing us closer to the goal of enhancing yield potential and ensuring food security for a growing global population.