SEED-GERMINATION OF ARABIDOPSIS-THALIANA PHYA PHYB DOUBLE MUTANTS IS UNDER PHYTOCHROME CONTROL/

We examined the photocontrol of seed germination in the phyA/ phyB dou ble mutants of Arabidopsis thaliana seeds. Dormant phyA/ phyB seeds sh owed a red/far-red light (R/FR)-reversible induction of seed germinati on. This suggests the involvement of at least one other phytochrome, p hyC, D, and/or E, in controlling seed germination. We designated this spectrally active phytochrome in phyA/ phyB as phyX. The full reversib ility of the R-induced germination by subsequent FR pulses, and the ob servation that the response is reversible by FR, even after a 3-h R tr eatment, indicates that this phyX response belongs to the low-fluence- response type. Thus, this phyX response is functionally related to phy B-mediated responses. However, in contrast to phyB-controlled seed ger mination, this phyX-mediated response needs a prolonged imbibition per iod and exhibits reversibility kinetics different from that needed for phyB. Furthermore, this phyX response requires a prolonged irradiatio n time and shows a fluence rate response dependency, showing a similar ity to the high irradiance response of photomorphogenesis. Thus, phyX, with regard to its control of seed germination, is a functionally new phytochrome that shares some characteristics of both phyA- and phyB-m ediated responses.