MODE OF PHYTOCHROME-B ACTION IN THE PHOTOREGULATION OF SEED-GERMINATION IN ARABIDOPSIS-THALIANA

Arabidopsis thaliana seeds imbibed for a short duration show phytochro me B (PhyB) specific photo-induction of germination. Using this system , the relationship was determined between the amount of PhyB in seeds and photon energy required for PhyB-specific germination in two transg enic Arabidopsis lines transformed with either the Arabidopsis PhyB cD NA (ABO) or the rice PhyB cDNA (RBO). Immunochemical detection of PhyB apoprotein (PHYB) showed that the expression level of PHYB in ABO see ds was at least two times higher than that in the wild-type seeds, but in RBO seeds the PHYB level was indistinguishable from that in wild-t ype seeds. The photon fluence required for induction and photoreversib le inhibition of germination was examined using the Okazaki large spec trograph. At the wavelengths of 400-710 nm, the ABO seeds required sig nificantly less photon fluence than wildtype seeds for induction of ge rmination, whereas the RBO seeds required similar fluence to wild-type seeds. A critical threshold wavelength for either induction or inhibi tion of germination of ABO seeds shifted towards the longer wavelength s relative to wild-type seeds. By assuming that PhyA and PhyB are simi lar in their photochemical parameters, amounts of P-fr at each wavelen gth were calculated. The photon fluence required for 50% germination w as equivalent to the fluence generating a P-fr/P-tot ratio of 0.21-0.4 3 in wild-type seeds, and of 0.035-0.056 in ABO seeds. These results i ndicate that PhyB-specific seed germination is not strictly a function of the P-fr/P-tot ratio, but is probably a function of the absolute P -fr concentration.