Increased Sensitivity of Photosynthesis to Anaerobic Conditions Induced by Targeted Inactivation of the Chloroplast ndhB Gene

Abstract

The plastid genome of higher plants has been reported to contain 11 ndh genes which have been recently shown to encode a NADH dehydrogenase (NDH) complex (1, 2). The NDH complex has been suggested to be involved in the reduction of plastoquinones occurring during cyclic electron flow around PSI (3) or during chlororespiration. Inactivation of ndh genes has been achieved by plastid transformation in tobacco (4). Inactivation of ndhC, ndhK and ndhJ genes revealed that this complex is functional and catalyzes the post-illumination reduction of the PQ pool (4). However, a possible participation of the NDH complex during light reactions and particularly an involvement in cyclic electron flow was not shown by the authors. It has been previously reported that cyclic electron flow was stimulated under anaerobic conditions (5) due to an adequate redox poise of the NAD(P)/NAD(P)H ratio (6) and also to the lack of electrons draining by the Mehler reaction (7). We hypothesized that differences in photosynthetic activity might be observed in ndh-disrupted plants in anaerobiosis conditions where cyclic electron flow is expected to be stimulated. Targeted inactivation of the plastidic ndhB gene was achieved by plastid transformation of tobacco. Analysis of homoplasmic transformants showed that ndhB inactivation leads to a disappearance of the NDH complex. Although no difference in chlorophyll fluorescence was observed between control and inactivated ndhB transformants (ndhB-) during a dark to light transition performed under aerobic conditions, we observed a significant reduction of electron transfer activity under anaerobic conditions in ndhB- leaves.