CORRELATION OF BLUE LIGHT-INDUCED PHOSPHORYLATION TO PHOTOTROPISM IN ZEA-MAYS L

The physiology of light-induced phototropic curvature has been studied extensively in coleoptiles of grasses, particularly in Avena and Zea mays L. In Z. mays L., we have found that, in addition to curvature, b lue light also induces rapid phosphorylation of a 114-kD protein in th e tips of coleoptiles, and, in a previous report, we reported several characteristics of the phosphorylated substrate protein and kinase (J. M. Palmer, T.W. Short, S. Gallagher, W.R. Briggs [1993] Plant Physiol 102: 1211-1218). Here, we compare the phosphorylation response to seve ral known aspects of phototropism physiology. Blue light-induced phosp horylation occurs only in the upper portion of the coleoptile and is a bsent from the node and mesocotyl. The specific activity of phosphoryl ation is highest in the extreme apical portion of the tip, which is al so the site of maximal sensitivity to phototropic stimuli (A. W. Galst on [1959] In Physiology of Movements, Encyclopedia of Plant Physiology , Springer, Berlin). Fluence-response determinations indicate that lig ht dosage levels that stimulate curvature also stimulate phosphorylati on. However, the threshold for inducing detectable phosphorylation in maize cannot be matched to the threshold for curvature induction. The recovery of sensitivity to phototropic stimuli after exposure to high fluences of light occurs with kinetics that are very similar to those for recovery of the phosphorylation response after a previous high-flu ence light exposure. In addition, wavelengths of light in the blue and near-ultraviolet regions of the spectrum that maximally stimulate pho totropic curvature also maximally stimulate in vitro phosphorylation i n maize. The pattern of stimulation matches the absorption spectra of flavoproteins, which have been proposed as candidates for blue light p hotoreceptors.