RESTORATION OF PHOTOTROPIC RESPONSIVENESS IN DECAPITATED MAIZE COLEOPTILES

The literature indicates that the tip of maize (Zea mays L.) coleoptil es has the localized functions of producing auxin for growth and perce iving unilateral light stimuli and translocating auxin laterally for p hototropism. There is evidence that the auxin-producing function of th e tip is restored in decapitated coleoptiles. We examined whether the functions for phototropism are also restored by using blue-light condi tions that induced a first pulse-induced positive phototropism (fPIPP) and a time-dependent phototropism (TDP). When the apical 5 mm, in whi ch photosensing predominantly takes place, was removed, no detectable fPIPP occurred even if indole-3-acetic acid (lanolin mixture) was appl ied to the cut end. However, when the blue-light stimulation was delay ed after decapitation, fPIPP became inducible in the coleoptile stumps supplied with indole-3-acetic-acid/lanolin (0.01 mg g(-1)), indicatin g that phototropic responsiveness was restored. This restoration progr essed 1 to 2 h after decapitation, and the curvature response became c omparable to that of intact coleoptiles. The results for TDP were qual itatively similar, but some quantitative differences were observed. It appeared that the overall TDP was based on a major photosensing mecha nism specific to the tip and on at least one additional mechanism not specific to the tip, and that the tip-specific TDP was restored in dec apitated coleoptiles with kinetics similar to that for fPIPP. It is su ggested that the photoreceptor system, which accounts for fPIPP and a substantial part of TDP, is regenerated in decapitated coleoptiles, pe rhaps together with the mechanism for lateral auxin translocation.