No abstract

SummaryPTLF, the Populus trichocarpa homolog of LEAFY (LFY) and FLORICAULA, was cloned to assess its function in a dioecious tree species. In situ hybridization studies showed that the gene was expressed most strongly in developing in¯orescences. Expression was also seen in leaf primordia and very young leaves, most notably in apical vegetative buds near in¯orescences, but also in seedlings. Although ectopic expression of the PTLF cDNA in Arabidopsis accelerated¯owering, only one of the many tested transgenic lines of Populus¯owered precociously. The majority of trees within a population of 3-year-old transgenic hybrid Populus lines with PTLF constitutively expressed showed few differences when compared to controls. However, phenotypic effects on growth rate and crown development, but not owering, were seen in some trees with strong PTLF expression and became manifest only as the trees aged. Competence to respond to overexpression of LFY varied widely among Populus genotypes, giving consistent early¯owering in only a single male P. tremula 3 P. tremuloides hybrid and causing gender change in another hybrid genotype. PTLF activity appears to be subject to regulation that does not affect heterologously expressed LFY, and is dependent upon tree maturation. Both genes provide tools for probing the mechanisms of delayed competence to¯ower in woody plants.