The roles of ATP synthase and the cytochrome b₆/f complexes in limiting chloroplast electron transport and determining photosynthetic capacity

In C₃ plants, CO₂ assimilation is limited by ribulose 1,5-bisphosphate (RuBP) regeneration rate at high CO₂. RuBP regeneration rate in turn is determined by either the chloroplast electron transport capacity to generate NADPH and ATP or the activity of Calvin cycle enzymes involved in regeneration of RuBP. Here, transgenic tobacco (Nicotiana tabacum 'W38') expressing an antisense gene directed at the transcript of either the Rieske iron-sulfur protein of the cytochrome (Cyt) b₆/f complex or the d-subunit of chloroplast ATP synthase have been used to investigate the effect of a reduction of these complexes on chloroplast electron transport rate (ETR). Reductions in d-subunit of ATP synthase content did not alter chlorophyll, Cyt b₆/f complex, or Rubisco content, but reduced ETR estimated either from measurements of chlorophyll fluorescence or CO₂ assimilation rates at high CO₂. Plants with low ATP synthase content exhibited higher nonphotochemical quenching and achieved higher ETR per ATP synthase than the wild type. The proportional increase in ETR per ATP synthase complex was greatest at 35°C, showing that the ATP synthase activity can vary in vivo. In comparison, there was no difference in the ETR per Cyt b₆/f complex in plants with reduced Cyt b₆/f content and the wild type. The ETR decreased more drastically with reductions in Cyt b₆/f complex than ATP synthase content. This suggests that chloroplast ETR is more limited by Cyt b₆/f than ATP synthase content and is a potential target for enhancing photosynthetic capacity in crops.