A novel mechanism of nuclear photosynthesis gene regulation by redox signals from the chloroplast during photosystem stoichiometry adjustment

Photosynthetic organisms acclimate to long term changes in the environmenta l light quality by an adjustment of their photosystem stoichiometry to main tain photosynthetic efficiency. By using light sources that predominantly e xcite either photosystem I (PSI) or photosystem II (PSII), we studied the e ffects of excitation imbalances between both photosystems on nuclear PSI ge ne transcription in transgenic tobacco seedlings with promoter::beta -glucu ronidase gene fusions. Shifts from PSI to PSII light sources (and vice vers a) induced changes in the reduction/oxidation state of intersystem redox co mponents, and acclimation of tobacco seedlings to such changes were monitor ed by changes in chlorophyll alb ratios and in vivo chlorophyll a fluoresce nce. The ferredoxin-NADP(+)-oxidoreductase gene promoter did not respond to these treatments, those from the genes for subunits PsaD and PsaF of PSI a re activated by a reduction signal, and the plastocyanin promoter responded to both reduction and oxidation signals. Additional experiments with photo synthetic electron transport inhibitors 3-(3 ' ,4 ' -dichlorophenyl)-1,1 ' -dimethyl urea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone demonstr ated that the redox state of the plastoquinone pool controls the activity o f the plastocyanin promoter, whereas subunit PsaD and PsaF gene transcripti on is regulated by other photosynthesis-derived signals. Thus, the expressi on of nuclear-encoded PSI genes is controlled by diverse light quality-depe ndent redox signals from the plastids during photosystem stoichiometry adju stment.