Phytochrome A and phytochrome B1 control the acquisition of competence forshoot regeneration in tomato hypocotyl

We analysed the light-dependent acquisition of competence for adventitious shoot formation in hypocotyls of phytochrome A (fri) and phytochrome B1 (tr i) mutants of tomato and their wild type by pregrowing the seedlings under different light quality. The regenerative response in vitro of explants fro m etiolated seedlings was reduced in comparison to that displayed by light- grown ones. Our results indicate that the light-dependent acquisition of co mpetence for shoot regeneration in the tomato hypocotyl is regulated by phy tochrome and antagonistically by a blue-light receptor. By using phytochrom e mutants and narrow wave band light we showed that it is mediated at least by two distinct phytochrome species: phytochrome B1 and phytochrome A. The action of phytochrome B1 during seedling growth was sufficient to induce t he full capacity of the subsequent regenerative response in vitro in explan ts from all positions along the hypocotyls. In contrast far-red light actin g through phytochrome A did not induce the full capability of shoot regener ation from middle and basal segments of the hypocotyl when phytochrome B1 w as absent (tri mutant). A few middle and basal hypocotyl explants pre-grown in blue light regenerated shoots.