Phytochromes and the photocontrol of flowering.

Analysis of photomorphogenic mutants and transgenic plants provides further insights into the roles of individual phytochrome species. The presence of a significant early-flowering response to low R/FR ratio has been reveled in Arabidopsis phyB mutants, that are also homozygous for a late-flowering mutation. This firstly, implicates at least one other novel phytochrome species, in addition to phytochrome B, in the low R/FR ratio-mediated early- flowering response. Secondly, identifies features that are likely to represent a loss-of-function mutant in this novel phytochrome species. Examination of the Arabidopsis elg mutant, a putative novel phytochrome loss-of- function mutant, defines ELG as a novel gene that influences elongation growth. What is more, ELG appears to act independently of phytochrome and GA. However, H4S seedlings overexpress the Arabidopsis HAT4 transgene, which is proposed to be down-regulated by a novel phytochrome. Thus, physiological analysis of Arabidopsis H4S seedlings reveal characteristics that may be representative of seedlings with a deficiency in a novel phytoclirome species. The physiological effects of phytochrome overexpression have also been examined in DN and SD N. tabacum, cv. Hicks. Allelic series overexpressing phytochrome A, phytochrome B and phytochrome C were generated for this purpose. These analyses provide a full characterisation of a phytochrome B-overexpression phenotype in tobacco, previously unreported. They also provide the first evidence that phytochrome C is a functional photoreceptor, and the first description of a phytochrome C-overexpression phenotype. DN and SD N. tabacum differ only with respect to the MM locus, which confers SD photoperiodicity. Hence, these plants are ideally suited for the comparative analysis of phytoclirome overexpression on photoperiodic perception. Alterations in the NB-mediated flowering response in transgenic plants suggests that phytochrome A- and B-overexpression disrupt photoperiodic timimg. However, the behaviour of N. tabacum overexpressing phytochrome C suggests that the phytochrome C transgene may specifically affect MM transduction.