REDUCED LEVELS OF CYTOCHROME BF COMPLEX IN TRANSGENIC TOBACCO LEADS TO MARKED PHOTOCHEMICAL REDUCTION OF THE PLASTOQUINONE POOL, WITHOUT SIGNIFICANT CHANGE IN ACCLIMATION TO IRRADIANCE

Tobacco transgenics with decreased amounts of the FeS apoprotein were generated using an antisense RNA construct targeted against the nuclea r-encoded Rieske FeS protein of cytochrome bf complex [Price et al. (1 995) Aust J Plant Physiol 22: 285-297]. FeS phenotypes ranging from in termediate to low were obtained which had 69% and 26% of the Rieske Fe S protein of wild type. Similar reductions in the other subunits of cy tochrome bf complex, cytochrome f, cytochrome b(563) and the 17 kDa su bunit, were demonstrated in the thylakoids of intermediate and low FeS phenotypes. Confirmation that the levels of assembled cytochrome bf i n leaves matched the levels of the FeS protein was demonstrated by las er flash-induced redox absorbance changes in leaves, with the extents of cytochrome f oxidation and cytochrome b(563) reduction being equiva lent to the decreased amounts of the subunits in isolated thylakoids o f the antisense plants. Despite greatly enhanced photochemical reducti on of Q(A) and the plastoquinone pool in the antisense plants, light a cclimation of the FeS phenotypes to irradiance did not occur. Furtherm ore, the state 1-state 2 transitions were identical in wild type and a ntisense plants. Our results suggest that neither Q(A) nor the plastoq uinone pool acts alone in either the redox control of gene expression or the regulation of light energy distribution between the photosystem s. We suggest rather that reduced plastoquinone acting at the inner Q( p) site of cytochrome bf complex is involved in molecular redox signal ling.