FLOWERING RESPONSES TO ALTERED EXPRESSION OF PHYTOCHROME IN MUTANTS AND TRANSGENIC LINES OF ARABIDOPSIS-THALIANA (L) HEYNH

The long-day plant Arabidopsis thaliana (L.) Heynh. flowers early in r esponse to brief end-of-day (EOD) exposures to far-red light (FR) foll owing a fluorescent short day of 8 h. FR promotion of flowering was nu llified by subsequent brief red light (R) EOD exposure, indicating phy tochrome involvement. The EOD response to R or FR is a robust measure of phytochrome action. Along with their wild-type (WT) parents, mutant s deficient in either phytochrome A or and responded similarly to the EOD treatments. Thus, neither phytochrome A nor B exclusively regulate d flowering, although phytochrome B controlled hypocotyl elongation. P erhaps a third phytochrome species is important for the EOD responses of the mutants and/or their flowering is regulated by the amount of th e FR-absorbing form of phytochrome, irrespective of the phytochrome sp ecies. Overexpression of phytochrome A or phytochrome 8 resulted in di ffering photoperiod and EOD responses among the genotypes. The day-neu tral overexpressor of phytochrome A had an EOD response similar to all of the mutants and WTs, whereas R EOD exposure promoted flowering in the overexpressor of phytochrome and and FR EOD exposure inhibited thi s promotion. The comparisons between relative flowering times and leaf numbers at flowering of the overexpressors and their WTs were not con sistent across photoperiods and light treatments, although both phytoc hromes A and B contributed to regulating flowering of the transgenic p lants.