Two photosystems, termed I and II, absorb and convert light energy in photosynthesis in chloroplasts of green plants. The genes psbA and psaAB of the cytoplasmic, chloroplast genome encode core components of photosystem II and photosystem I, respectively. Here we show that the absolute amounts of photosystem I and photosystem II respond, in a complementary manner, to changes in light quality that preferentially excite each photosystem in mustard seedlings. We also show that the initial response to altered energy distribution is a change in the rates of transcription of psbA and psaAB. Changes in chlorophyll fluorescence emission in vivo suggest that the signal which initiates this change is the state of oxidation-reduction of plastoquinone, a component of the photosynthetic electron transport chain that connects photosystem I and photosystem II. The results are consistent with transcriptional effects observed previously with chloroplasts isolated in vitro, and demonstrate that redox control of chloroplast transcription initiates long-term adjustments that compensate for imbalance in energy distribution and adapt the whole plant to altered light environments.

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