In Vitro Cyclic Electron Transport in Barley Thylakoids follows Two Independent Pathways

Abstract

In vitro cyclic electron transport around PSI was studied in thylakoids isolated from barley (Hordeum vulgare L.). Redox poising was obtained by using anaerobic conditions, preillumination, and the addition of 3-(3,4-dichlorophenyI)-l,l-dimethylurea. Postillumination rates of P700+ re-reduction of 1 to 5 electrons s-' were observed, depending on the conditions. The thylakoids supported two parallel paths of cyclic electron transport that were distinguishable by differences in antimycin sensitivity, saturation characteristics, and substrate specificity. l h e pathway most sensitive to antimycin was not saturated at ferredoxin concentrations up to 50 PM, whereas the more insensitive pathway was saturated at 5 PM ferredoxin. At the lower concentration of reduced ferredoxin, the antimycin-sensitive rate of P700+ re-reduction was lower than the antimycin-insensitive rate. The lower range of reduced ferredoxin concentrations are closer to in vivo conditions. Flavodoxin is shown to mediate cyclic electron transport. Flavodoxin was less efficient in mediating the antimycin-sensitive pathway but mediated the antimycin-insensitive pathway as efficiently as ferredoxin. Antibodies raised against ferredoxin:NADP+ oxidoreductase had no effect on either pathway for re-reduction of P700+. However, the ferredoxin: NADP+ oxidoreductase inhibitor 2'-monophosphoadenosine-5'-diphosphoribose was able to inhibit the antimycin-sensitive as well as the antimycin-insensitive pathway.Traditionally, cyclic electron transport has been considered a means of providing ATP for CO, fixation and other energy-requiring processes in addition to the ATP provided by linear electron flow. The importance of cyclic electron transport in ATP synthesis is rather clear in the bundle-sheath cells of C, plants (Bendall and Manasse, 1995). In contrast, the contribution of cyclic electron transport to overall photophosphorylation has been reported to be very small in C, plants and cyanobacteria (Maxwell and

The NAD(P)H Dehydrogenase in Barley Thylakoids Is Photoactivatable and Uses NADPH as well as NADH1

“…PCC 6803 mutant deficient in NDH was also deficient in Fd-catalyzed cyclic electron transport. In contrast to this result, cyclic electron transport of barley thylakoids could not be inhibited by antibodies against FNR (Scheller, 1996).…”

“…The reaction medium contained (in a total volume of 3 mL): 33 mm Tricine (pH 7.5), 83 mm NaCl, 10 mm MgCl 2 , 0.1 mm MeV, 6.7 mm NH 4 Cl, 0.3 mm NaN 3 , and thylakoids corresponding to 60 g of Chl. Fd-catalyzed cyclic electron transport was measured as rates of P700 ϩ reduction under anaerobic and redox-poised conditions as described previously (Scheller, 1996).…”

Increased Sensitivity of Photosynthesis to Antimycin A Induced by Inactivation of the Chloroplast ndhB Gene. Evidence for a Participation of the NADH-Dehydrogenase Complex to Cyclic Electron Flow around Photosystem I

“…One pathway was shown to be sensitive to antimycin A, whereas the other would involve a NAD(P)H dehydrogenase activity (Ravenel et al, 1994). The existence of an antimycin A-insensitive cyclic electron pathways around PSI was also proposed in C 3 plants from experiments performed in vitro (Hosler and Yocum, 1987;Scheller, 1996).…”

“…Based on photoacoustic measurements performed in vivo in C. reinhardtii cells, Ravenel et al (1994) observed that antimycin A and N-ethyl-maleimide could inhibit PSI energy storage in vivo when added together, these compounds having no effect when added alone. More recently, based on P700 ϩ rereduction measurements performed in barley thylakoids, Scheller (1996) proposed the existence of an antimycin-insensitive cyclic electron transport around PSI. The involvement of the NDH complex in cyclic electron flow in association with other pathways was shown in cyanobacteria (Mi et al, 1992;Yu et al, 1993) and recently suggested in higher plants from in vitro experiments performed on broken chloroplasts (Endo et al, 1998).…”

“…Most studies concluding to an involvement of cyclic electron flow during photosynthesis in C 3 plants have been based on the effect of this compound on photosynthetic reactions such as photophosphorylation (Cleland and Bendall, 1992), rereduction of P700 ϩ (Scheller, 1996), CO 2 -dependent O 2 evolution (Furbank and Horton, 1987), 14 CO 2 fixation (Heber et al, 1978;Woo, 1983), or chlorophyll fluorescence (Ivanov et al, 1998). It was suggested that inhibition of photosynthetic reactions by antimycin A was related to the involvement of an antimycin A-sensitive ferredoxin plastoquinone reductase activity in cyclic reactions (Moss and Bendall, 1984;Cleland and Bendall, 1992).…”