Temporal and spatial pattern of light-dependent acquisition of competence for shoot formation in tomato hypocotyl. I Light pulse conditions

Abstract

In the present paper, we compare, in photomorphogenic mutants of tomato, the effect of a single exposure to light of etiolated seedlings on acquisition of competence for subsequent adventitious shoot formation in hypocotyl segments cultured in vitro under continuous white it light. The results indicate that the efficiency of 1 hr white light is maximal and constant in time in apical segments, while decreases with age in middle and basal segments. In apical segments of wild type (WT), au and hp-1 mutant of tomato the promoting effect of 1 hr pulse of far red light was similar to that of red and blue light, while it was substantial but reduced in phyA,phyB1, and phyA/phyB1 mutants. The residual response displayed by phyA,phyB1 and phyA/phyB1 background indicates that besides phyA and phyBl some other phytochromes are involved in the far-red mediated response. Pre-irradiation with fluence of red light inducing very low fluence responses (VLFR, 10 - 8 - 10 - 6 mol m 2 ) as well as low fluence responses (LFR, > 10 - 6 mol m 2 ) enhanced the frequency of shoot formation in apical segments of all the genotypes, whereas it was ineffective in middle and basal segments of phyA, phyB1 and phyA/phyB1 mutants. Only in WT tomato VLFR induced substantial enhancement of capability for shoot regeneration in middle and basal segments. Thus presence of phyB1 and phyA appears to be absolutely necessary for LFR- and VLFR-induced acquisition of competence for shoot formation in middle and basal segments of tomato hypocotyl, whereas in apical segments the remaining phytochromes, besides phyA and phyBl, are sufficient.