The Roles of ATP Synthase and the Cytochrome b  6/f Complexes in Limiting Chloroplast Electron Transport and Determining Photosynthetic Capacity    

Abstract

In C 3 plants, CO 2 assimilation is limited by ribulose 1,5-bisphosphate (RuBP) regeneration rate at high CO 2 . 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) b 6 /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 b 6 /f complex, or Rubisco content, but reduced ETR estimated either from measurements of chlorophyll fluorescence or CO 2 assimilation rates at high CO 2 . 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 b 6 /f complex in plants with reduced Cyt b 6 /f content and the wild type. The ETR decreased more drastically with reductions in Cyt b 6 /f complex than ATP synthase content. This suggests that chloroplast ETR is more limited by Cyt b 6 /f than ATP synthase content and is a potential target for enhancing photosynthetic capacity in crops.Plants capture light energy with their light-harvesting systems, including chlorophylls (Chls) and carotenoids, and drive photosynthetic electron transport through the thylakoid membranes of the chloroplasts. Electrons excised from water in PSII are ultimately transferred to NADP + via PSI, resulting in production of NADPH. This process is known as linear electron transport. At the same time, this linear electron transport that passes through the cytochrome (Cyt) b 6 /f complex generates a proton gradient across the thylakoid membrane (DpH;Allen, 2003). Together with the proton gradient generated by the water-splitting complex associated with PSII, these proton gradients enable ATP production by the ATP synthase complex and help to regulate nonphotochemical quenching (NPQ) of excitation energy (Mü ller et al., 2001). There is also a cyclic electron transport that depends on the PSI photochemical reactions and also passes through the Cyt b 6 /f complex. The cyclic electron transport can generate a DpH and drives ATP synthesis by ATP synthase without concomitant generation of NADPH (Shikanai, 2007). ATP and NADPH generated by light reactions are utilized primarily in the Calvin cycle and photorespiratory cycle. The activity and regulation of the Cyt b 6 /f complex and the ATP synthase are thus key components determining the rate of NADPH and ATP production for CO 2 fixation. Photosynthetic CO 2 assimilation rate can be viewed as being limited either by the capacity o...

Proton Gradient Regulation 5-Mediated Cyclic Electron Flow under ATP- or Redox-Limited Conditions: A Study of ƊATPase pgr5 and ƊrbcL pgr5 Mutants in the Green Alga Chlamydomonas reinhardtii

“…In tobacco (Nicotiana spp.) knockdowns of chloroplast ATPase (Rott et al, 2011;Yamori et al, 2011), a decrease in linear electron flow revealed the negative feedback of an excess proton motive force on cytochrome b 6 f activity. In C. reinhardtii, an overacidification of the lumen in DATPase would result in an exaggerated photosynthetic control and be alleviated in DATPase pgr5.…”

Inducible Repression of Nuclear-Encoded Subunits of the Cytochrome b6f Complex in Tobacco Reveals an Extraordinarily Long Lifetime of the Complex

“…These searches did not reveal any homologous sequences with more than 18-bp length; therefore, off-target silencing effects of the RNAi construct are unlikely to occur. Furthermore, this sequence is part of the 645-bp-long sequence used to generate the well-characterized tobacco Rieske-2Fe2S antisense plants (Price et al, 1995(Price et al, , 1998Anderson et al, 1997;Yamori et al, 2011).…”

“…It is noteworthy that in investigations published before 1997, cyt-bf contents may have been overestimated by 30%, because the difference extinction coefficient of cytochrome f has been substantially revised since then (Metzger et al, 1997), making a limiting function of the cyt-bf even more likely. A predominant role of the cyt-bf in photosynthetic flux control was ultimately confirmed by the specific inhibition of cyt-bf activity (Kirchhoff et al, 2000) and by an antisense approach against the Rieske protein (Price et al, 1995(Price et al, , 1998Anderson et al, 1997;Yamori et al, 2011).…”

Recruitment of a Ribosomal Release Factor for Light- and Stress-Dependent Regulation of petB Transcript Stability in Arabidopsis Chloroplasts

“…It appeared that the abundance of MCA1 limits the amount of petA mRNA and accordingly levels of the encoded protein, cytochrome f (Raynaud et al, 2007). This and the fact that the activity of the cytochrome b 6 f complex is rate limiting for the light reaction in photosynthesis (Yamori et al, 2011) prompted us to investigate whether PrfB3 also has a regulatory function in petB gene expression. First, we have shown that downregulation of PrfB3 below a threshold of ;25% goes along with a severe reduction in the amount of petB RNA and consequently in the abundance of cytochrome b 6 protein levels ( Figure 8A).…”