MULTIPLE PHYTOCHROMES ARE INVOLVED IN RED-LIGHT-INDUCED ENHANCEMENT OF FIRST-POSITIVE PHOTOTROPISM IN ARABIDOPSIS-THALIANA

The amplitude of phototropic curvature to blue light is enhanced by a prior exposure of seedlings to red light. This enhancement is mediated by phytochrome. Fluence-response relationships have been constructed for red-light-induced enhancement in the phytochrome A (phyA) null mut ant, the phytochrome B- (phyB) deficient mutant, and in two transgenic lines of Arabidopsis thaliana that overexpress either phyA or phyB. T hese fluence-response relationships demonstrate the existence of two r esponses in enhancement, a response in the very-low-to-low-fluence ran ge, and a response in the high-fluence range. Only the response in the high-fluence range is present in the phyA null mutant. In contrast, t he phyB-deficient mutant is indistinguishable from the wild-type paren t in red-light responsiveness. These data indicate that phyA is necess ary for the very-low-to-low but not the high-fluence response, and tha t phyB is not necessary for either response range. Based on these resu lts, the high-fluence response, if controlled by a single phytochrome, must be controlled by a phytochrome other than phyA or phyB. Overexpr ession of phyA has a negative effect and overexpression of phyB has an enhancing effect in the high-fluence range. These results suggest tha t overexpression of either phytochrome perturbs the function of the en dogenous photoreceptor system in an unpredictable fashion.