A BLUE-LIGHT PHOTORECEPTOR MEDIATES THE FLUENCE-RATE-DEPENDENT EXPRESSION OF GENES ENCODING THE SMALL-SUBUNIT OF RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE IN LIGHT-GROWN PHASEOLUS-VULGARIS PRIMARY LEAVES

The level of transcripts of genes encoding the small subunit of ribulo se 1,5-bisphosphate carboxylase/oxygenase (rbcS genes) in the primary leaves of Phaseolus vulgaris L. increases substantially when plants gr own in a low fluence rate of white light (15 mumol m-2 s-1; 400-700 nm ) are transferred to a tenfold higher fluence rate of identical spectr al quality. To investigate which photoreceptor acts as the fluence-rat e detector, plants grown for 16 d in low white light were transferred to blue-enriched or red light environments of various fluence rates. T he results indicate that the fluence-rate-dependent increase in rbcS e xpression is mediated specifically by blue light. Red light of the sam e fluence rate, which was found to be equally effective in driving pho tosynthesis, had much less effect on expression, indicating that light absorbed by the photosynthetic pigments does not mediate the response . Moreover, there is no correlation of the transcript levels with eith er the cycling rate or photoequilibrium of the phytochrome system. Run -on assays with isolated nuclei indicate that blue light substantially increases the rate of rbcS transcription. Experiments with gene-speci fic probes show that individual members of the P. vulgaris rbcS gene f amily exhibit the fluence-rate-dependent, blue-light-mediated increase in expression.