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, ATPδ 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.

The electrochromic signal, redox reactions in the cytochrome bf complex and photosystem functionality in photoinhibited tobacco leaf segments

Photosynthetic electron transport in vivo was investigated in tobacco leaves pre-illuminated with strong light under conditions where Photosystem (PS) II repair was inhibited by lincomycin. Flash-induced redox changes of cytochrome b563, cytochrome f and plastocyanin, and the electrochromic (EC) signal (caused by a carotenoid band-shift due to charge separation across thylakoid membranes) from leaf segments were measured by deconvoluting absorbance changes at 520, 554, 564 and 575 nm. The EC signal was composed of easily separable fast and slow components. The fast EC signal decreased linearly with the loss of functional PS II centres, but there was a residual fast EC phase which was attributable to PS I centres alone. Inactivation of PS II centres by photoinhibitory light was also well-correlated with the quenching of variable fluorescence measured as the ratio of variable to maximum fluorescence, Fv/Fm. On complete photoinactivation of PS II centres, the slow rise of the flash-induced EC signal became more prominent, suggesting enhanced electrogenic charge transfer across the cytochrome bf complex as part of a path of electron flow involving PS I. Thus, both PS I and the cytochrome bf complex appeared to be fully functional after treatment of tobacco leaves with photoinhibitory light at room temperature. In totally photoinhibited leaf segments, the rate coefficients of cyt fIII re-reduction increased from 59 s-1 (+ lincomycin, no photoinhibitory light) to 130 s-1, and that of cytochrome b563 reduction also increased, from 270 s-1 to 500 s-1, suggesting that the prevailing plastoquinol concentration was higher after photoinhibitory light treatment. The source of the electrons entering the pool under these conditions was probably a high concentration of NADPH and reduced ferredoxin.