Characterisation of a Leafy Homologue, a Gene Regulating Floral Meristem Identity, from the Long Day Plant Silene coeli-rosa.

The LFY gene, which has now been isolated in at least 17 species, is important in the transition from vegetative to reproductive growth. The floral meristems of lfy mutants exhibited increased inflorescence characteristics, and constitutive expression of the gene was sufficient to convert lateral inflorescence meristems to solitary flowers in Arabidopsis, tobacco and Populus. Previous work on the determinate plant, Silene coeli-rosa, which required 7 LD for 100% flowering, concentrated on changes to the cell cycle and peptide composition of the shoot apex during floral evocation. A partial cDNA clone of a Silene LFY homologue (SFL) has been isolated. SFL shows strong homology to other LFY homologue proteins within the two conserved domains, with up to 88% identical amino acids. It contains highly acidic and basic domains, a glutamine rich region and leucine repeats: all putat5ive transcriptional activation domains. Expression studies using quantitative PCR show that SFL was not induced by non-inductive SD conditions, or by a continuous light treatment that inhibited flowering. This is in contrast to the expression patterns observed in vegetative Arabidopsis, pea, petunia, Impatiens, tobacco and tomato, but consistent with the expression of Antirrhinum homologue which is restricted to the floral meristems. During the 7 LD induction period, SFL transcripts were first detected after 5 LD, a treatment which resulted in 81% of plants flowering, under the conditions used. Fewer than 5 LD failed to induce flowering or SFL expression. SFL was also expressed in apices subjected to an inductive 7 LD treatment followed by 48h darkness, which delayed flowering and suppressed the synchronisation of the cell cycle which occurs immediately prior to floral initiation. In situ hybridisation revealed the spatial expression of SFL in Silene. No SFL was detected prior to D7 during floral induction or in non-inductive SD controls. On experimental D7, SFL mRNA was restricted to the flanks of the primary apical dome and in D8 apices expression had spread throughout the dome. Importantly, this pattern of expression differs to that observed in the other two determinate species in which LFY has been studied, namely tobacco and Impatiens.