Photosynthesis is strongly reduced by antisense suppression of chloroplastic cytochrome bf complex in transgenic tobacco

We have used transgenic tobacco (Nicotiana tabacum L. cv. W38) plants expressing an antisense gene directed against the transcript of the Rieske FeS protein of the chloroplast bf complex to examine the effect a reduction in chloroplast Rieske FeS content on leaf gas exchange and photosynthetic metabolite pools. Plants with chloroplast Rieske FeS protein content ranging from 5 to 80% of wild-type were analysed. CO2 assimilation rate declined linearly with the reduction in Rieske FeS content when expressed on a leaf area basis. In contrast to photosynthesis, there was no change in stomatal conductance except for plants with less than 10% of wild-type Rieske FeS content. There was a close correlation between Rieske FeS content and cytochrome f content, showing that the reduction in Rieske FeS content lead to a similar reduction in other components of the cytochrome bf complex. While lower Rieske FeS content was associated with declines in chlorophyll content, ATP delta subunit and ribulosebisphosphate carboxylase-oxygenase (Rubisco) contents, these declines were considerably smaller than the reduction in cytochrome bf content. As Rieske FeS content was reduced, there was a slight lowering of the chlorophyll a/b ratio. Lower CO2 assimilation rates at ambient CO2 and high light were associated with dramatic reductions in ribulose bisphosphate (RuBP) and modest declines in 3-phosphoglycerate (PGA). Rubisco carbamylation declined to 40-50% in plants with Rieske FeS content lower than 20% of wild-type. We conclude that, at high light, a reduction in chloroplast Rieske FeS protein content leads to inhibition of CO2 assimilation rate via reductions in the rate of RuBP regeneration caused by a restriction in electron transport and via effects on in vivo Rubisco activity.