DIRECTED DISRUPTION OF THE TOBACCO NDHB GENE IMPAIRS CYCLIC ELECTRON FLOW AROUND PHOTOSYSTEM-I

To evaluate the physiological significance of cyclic electron flow aro und photosystem (PS) I, we used a reverse genetic approach to focus on 11 chloroplast genes that encode homologs of mitochondrial complex I subunits (ndhA-K), Since their discovery, the exact function of the re spiratory components in plant chloroplasts has been a matter of discus sion. We disrupted one of these genes (ndhB) in tobacco by chloroplast transformation. Analysis of the transient increase in chlorophyll flu orescence after actinic light illumination and the redox kinetics of P 700 (reaction center chlorophylls of PS I) suggest that the cyclic ele ctron flow around PS I is impaired in the ndhB-deficient transformants . Transformants grew normally in a greenhouse, suggesting that the cyc lic electron flow around PS I mediated by ndh gene products is dispens able in tobacco lander mild environmental conditions.