Faster responses of photosynthesis to light transitions increase biomass and grain yield in transgenicSorghum bicoloroverexpressing Rieske FeS

Sorghum is one of the most important crops providing food and feed in many of the world’s harsher environments. Sorghum utilises the C 4 pathway of photosynthesis in which a biochemical carbon concentrating mechanism results in high CO 2 assimilation rates. Overexpressing the Rieske subunit of the Cytochrome b 6 f complex was previously shown to increase the rate of photosynthetic electron transport and stimulate CO 2 assimilation in the model C 4 plant Setaria viridis . To test whether productivity of C 4 crops could be improved by Rieske overexpression, we created transgenic Sorghum bicolor plants with increased Rieske content. The transgenic plants showed no marked changes in abundance of other photosynthetic proteins or chlorophyll content. Increases in yield of Photosystem II and CO 2 assimilation rate as well as faster responses of non-photochemical quenching during transient photosynthetic responses were observed as a result of an elevated in vivo Cytochrome b 6 f activity in plants overexpressing Rieske. The steady-state rates of electron transport and CO 2 assimilation did not differ between transgenic and control plants, suggesting that Cytochrome b 6 f is not the only factor limiting electron transport in sorghum at high light and high CO 2 . Nevertheless, more agile responses of photosynthesis to light transitions led to increases in biomass and grain yield in plants overexpressing Rieske. Our results indicate that increasing Rieske content could boost productivity of C 4 crops by improving the efficiency of light utilisation and conversion to biomass.