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.