ACTION SPECTRA FOR PHYTOCHROME A-SPECIFIC AND B-SPECIFIC PHOTOINDUCTION OF SEED-GERMINATION IN ARABIDOPSIS-THALIANA

We have examined the seed germination in Arabidopsis thaliana of wild type (wt), and phytochrome A (PhyA)- and B (PhyB)-mutants in terms of incubation time and environmental light effects. Seed germination of t he wt and PhyA-null mutant (phyA) was photoreversibly regulated by red and far-red lights of 10-1,000 mu mol m(-2) when incubated in darknes s for 1-14 hr, but no germination occurred in PhyB-null mutant (phyB). When wt seeds and the phyB mutant seeds were incubated in darkness fo r 48 hr, they synthesized PhyA during dark incubation and germinated u pon exposure to red light of 1-100 nmol m(-2) and far-red light of 0.5 -10 mu mol m(-2), whereas the phyA mutant showed no such response, The results indicate that the seed germination is regulated by PhyA and P hyB but not by other phytochromes, and the effects of PhyA and PhyB ar e separable in this assay. We determined action spectra separately for PhyA- and PhyB-specific induction of seed germination at Okazaki larg e spectrograph. Action spectra for the PhyA response show that monochr omatic 300-780 nm lights of very low fluence induced the germination, and this induction was not photoreversible in the range examined. Acti on spectra for the PhyB response show that germination was photorevers ibly regulated by alternate irradiations with light of 0.01-1 mmol m(- 2) at wavelengths of 540-690 nm and 695-780 nm. The present work clear ly demonstrated that PhyA photoirreversibly triggers the germination u pon irradiations with ultraviolet, visible and far-red light of very l ow fluence, while PhyB controls the photoreversible effects of low flu ence.