Natural Variation within a Species for Traits Underpinning C4 Photosynthesis

Engineering C-4 photosynthesis into C-3 crops could substantially increase their yield by alleviating photorespiratory losses. This objective is challenging because the C-4 pathway involves complex modifications to the biochemistry, cell biology, and anatomy of leaves. Forward genetics has provided limited insight into the mechanistic basis of these properties, and there have been no reports of significant quantitative intraspecific variation of C-4 attributes that would allow trait mapping. Here, we show that accessions of the C-4 species Gynandropsis gynandra collected from locations across Africa and Asia exhibit natural variation in key characteristics of C-4 photosynthesis. Variable traits include bundle sheath size and vein density, gas-exchange parameters, and carbon isotope discrimination associated with the C-4 state. The abundance of transcripts encoding core enzymes of the C-4 cycle also showed significant variation. Traits relating to water use showed more quantitative variation than those associated with carbon assimilation. We propose that variation in these traits likely adapted the hydraulic system for increased water use efficiency rather than improving carbon fixation, indicating that selection pressure may drive C-4 diversity in G. gynandra by modifying water use rather than photosynthesis. The accessions analyzed can be easily crossed and produce fertile offspring. Our findings, therefore, indicate that natural variation within this C-4 species is sufficiently large to allow genetic mapping of key C-4 traits and regulators.