IDENTIFICATION OF A FUNCTIONAL RESPIRATORY COMPLEX IN CHLOROPLASTS THROUGH ANALYSIS OF TOBACCO MUTANTS CONTAINING DISRUPTED PLASTID NDH GENES

The plastid genomes of several plants contain homologues, termed ndh g enes, of genes encoding subunits of the NADH:ubiquinone oxidoreductase or complex I of mitochondria and eubacteria. The functional significa nce of the Ndh proteins in higher plants is uncertain, We show here th at tobacco chloroplasts contain a protein complex of 550 kDa consistin g of at least three of the ndh gene products: NdhI, NdhJ and NdhK, We have constructed mutant tobacco plants with disrupted ndhC ndhK and nd hJ plastid genes, indicating that the Ndh complex is dispensible for p lant growth under optimal growth conditions, Chlorophyll fluorescence analysis shows that in vivo the Ndh complex catalyses the post-illumin ation reduction of the plastoquinone pool and in the light optimizes t he induction of photosynthesis under conditions of mater stress, We co nclude that the Ndh complex catalyses the reduction of the plastoquino ne pool using stromal reductant and so acts as a respiratory complex, Overall, our data are compatible with the participation of the Ndh com plex in cyclic electron how around the photosystem I complex in the li ght and possibly in a chloroplast respiratory chain in the dark.