EXPOSURE OF OAT SEEDLINGS TO BLUE-LIGHT RESULTS IN AMPLIFIED PHOSPHORYLATION OF THE PUTATIVE PHOTORECEPTOR FOR PHOTOTROPISM AND IN HIGHER SENSITIVITY OF THE PLANTS TO PHOTOTROPIC STIMULATION

Dark recovery of blue light-induced in vitro phosphorylation in oat (A vena sativa L.) seedlings after in vivo preirradiation with blue light revealed different recovery kinetics for the coleoptile base and tip. Although, in both cases, maximum in vitro phosphorylation was observe d 90 min after in vivo blue light treatment, the phosphorylation level s for the entire base were about 3-fold higher than those found in non preirradiated plants. The tip response only slightly exceeded that of the dark controls. The fluence applied during preirradiation determine d the extent of the increase in phosphorylation. Consequently, unilate ral irradiation and subsequent dark incubation resulted in a more pron ounced increase in phosphorylation in the irradiated than in the shade d side of the coleoptile base. Furthermore, blue light-irradiation con ditions, known to induce neither first-nor second-positive curvature i n nonpreirradiated plants, stimulated both asymmetric distribution of protein phosphorylation and second-positive phototropic curvature in t he coleoptile base when administered to blue light-pretreated plants. Based on these data, we conclude that photosensitivity of the coleopti le base increases upon exposure to blue light in a time-and fluence-de pendent manner, providing an excellent explanation of the invalidity o f the Bunsen-Roscoe reciprocity law for second-positive phototropism.