Control of hypocotyl elongation in Arabidopsis thaliana by photoreceptor interaction

In order to test the interaction of different phytochromes and blue-light r eceptors, etiolated seedlings of wild-type Arabidopsis thaliana (L.) Heynh. , a phytochrome (phy) B-overexpressor line (ABO), and the photoreceptor mut ants phyA-201, phyB-5, hy4-2.23n, fha-1, phyA-201/phyB-5, and phyA-201/hy4- 2.23n were exposed to red and far-red light pulses after various preirradia tions. The responsiveness to the inductive red pulses is primarily mediated by phyB which is rather stable in its far-red-absorbing form as demonstrat ed by a very slow loss of reversibility. Without preirradiation the red pul ses had an impact on hypocotyl elongation only in PHYA mutants but not in t he wild type. This indicates a suppression of phyB function by the presence of phyA. Preirradiation with either far-red or blue light resulted in an i nhibition of hypocotyl elongation by red pulses in the wild type. Responsiv eness amplification by far-red light is mediated by phyA and disappears slo wly in the dark. The extent of responsiveness amplification by blue light w as identical in the wild type and in the absence of phyA, or the cryptochro mes cryl (hy4-2.23n) or cry2 (fha-1). Therefore, we conclude that stimulati on of phyB by blue light preirradiation is either mediated by an additional still-unidentified blue-light-absorbing pigment or that phyA, cryl and cry 2 substitute for each other completely. Both blue and red preirradiation es tablished responsiveness to red pulses in phyA-201/phyB-5 double mutants. T hese results demonstrate that inhibition of hypocotyl elongation by red pul ses is not only mediated by phyB but also by a phytochrome(s) other than ph yA and phyB.