FLUENCE AND WAVELENGTH REQUIREMENTS FOR ARABIDOPSIS CAB GENE INDUCTION BY DIFFERENT PHYTOCHROMES

The roles of different phytochromes have been investigated in the phot oinduction of several chlorophyll a/b-binding protein genes (CAB) of A rabidopsis thaliana. Etiolated seedlings of the wild type, a phytochro me A (PhyA) null mutant (phyA), a phytochrome B (PhyB) null mutant (ph yB), and a phyA/phyB double mutant were exposed to monochromatic light to address the questions of the fluence and wavelength requirements f or CAB induction by different phytochromes. In the wild type and the p hyB mutant, PhyA photoirreversibly induced CAB expression upon irradia tion with very-low-fluence light of 350 to 750 nm. In contrast, using the phyA mutant, PhyB photoreversibly induced CAB expression with low- fluence red light. The threshold fluences of red light for PhyA- and P hyB-specific induction were about 10 nmol m(-2) and 10 mu mol m(-2) re spectively. In addition, CAB expression was photoreversibly induced wi th low-fluence red light in the phyA/phyB double mutant, revealing tha t another phytochrome(s) (PhyX) regulated CAB expression in a manner s imilar to PhyB. These data suggest that plants utilize different phyto chromes to perceive light of varying wavelengths and fluence, and begi n to explain how plants respond so exquisitely to changing light in th eir environment.