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 nuclear-encoded Rieske FeS protein of cytochrome bf complex [Price et al. (1995) Aust J Plant Physiol 22: 285-297]. FeS phenotypes ranging from intermediate to low were obtained which had 69% and 26% of the Rieske FeS protein of wild type. Similar reductions in the other subunits of cytochrome bf complex, cytochrome f, cytochrome b(563) and the 17 kDa subunit, were demonstrated in the thylakoids of intermediate and low FeS phenotypes. Confirmation that the levels of assembled cytochrome bf in leaves matched the levels of the FeS protein was demonstrated by laser flash-induced redox absorbance changes in leaves, with the extents of cytochrome f oxidation and cytochrome b(563) reduction being equivalent to the decreased amounts of the subunits in isolated thylakoids of the antisense plants. Despite greatly enhanced photochemical reduction of Q(A) and the plastoquinone pool in the antisense plants, light acclimation of the FeS phenotypes to irradiance did not occur. Furthermore, the state 1-state 2 transitions were identical in wild type and antisense plants. Our results suggest that neither Q(A) nor the plastoquinone pool acts alone in either the redox control of gene expression or the regulation of light energy distribution between the photosystems. We suggest rather that reduced plastoquinone acting at the inner Q(p) site of cytochrome bf complex is involved in molecular redox signalling.