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

Tobacco (Nicotiana tabacum var Petit Havana) ndhB-inactivated mutants (ndhB (-)) obtained by plastid transformation (E.M. Horvath, S.O. Peter, T. Joet, D. Rumeau, L. Cournac, G.V. Horvath, T.A. Kavanagh, C. Schafer, G. Peltier , P. Medgyesy-Horvath [2000] Plant Physiol 123: 1337-1350) were used to stu dy the role of the NADH-dehydrogenase complex (NDH) during photosynthesis a nd particularly the involvement of this complex in cyclic electron flow aro und photosystem I (PSI). Photosynthetic activity was determined on Leaf dis cs by measuring CO, exchange and chlorophyll fluorescence quenchings during a dark-to-light transition. In the absence of treatment, both non-photoche mical and photochemical fluorescence quenchings were similar in ndh(-) and wild type (WT). When leaf discs were treated with 5 muM antimycin A, an inh ibitor of cyclic electron flow around PSI, both quenchings were strongly af fected. At steady state, maximum photosynthetic electron transport activity was inhibited by 20% in WT and by 50% in ndhB(-). Under non-photorespirato ry conditions (2% O-2, 2,500 muL L-1 CO2), antimycin A had no effect on pho tosynthetic activity of WT, whereas a 30% inhibition was observed both on q uantum yield of photosynthesis assayed by chlorophyll fluorescence and on C O2 assimilation in ndhB(-). The effect of antimycin A on ndhB- could not be mimicked by myxothiazol, an inhibitor of the mitochondrial cytochrome he, complex, therefore showing that it is not related to an inhibition of the m itochondrial electron transport chain but rather to an inhibition of cyclic electron flow around PSI. We conclude to the existence of two different pa thways of cyclic electron flow operating around PSI in higher plant chlorop lasts. One of these pathways, sensitive to antimycin A, probably involves f erredoxin plastoquinone reductase, whereas the other involves the NDH compl ex. The absence of visible phenotype in ndhB(-) plants under normal conditi ons is explained by the complement of these two pathways in the supply of e xtra-ATP for photosynthesis.