Diverse effects of overexpression of LEAFY and PTLF, a poplar (Populus) homolog of LEAFY/FLORICAULA, in transgenic poplar and Arabidopsis

PTLF, the Populus trichocarpa homolog of LEAFY (LFY) and FLORICAULA, was cl oned to assess its function in a dioecious tree species. In situ hybridizat ion studies showed that the gene was expressed most strongly in developing inflorescences. Expression was also seen in leaf primordia and very young l eaves, most notably in apical vegetative buds near inflorescences, but also in seedlings. Although ectopic expression of the PTLF cDNA in Arabidopsis accelerated flowering, only one of the many tested transgenic lines of Popu lus flowered precociously, The majority of trees within a population of 3-y ear-old transgenic hybrid Populus lines with PTLF constitutively expressed showed few differences when compared to controls. However, phenotypic effec ts on growth rate and crown development, but not flowering, were seen in so me trees with strong PTLF expression and became manifest only as the trees aged. Competence to respond to overexpression of LFY varied widely among Po pulus genotypes, giving consistent early flowering in only a single male P. tremula x P. tremuloides hybrid and causing gender change in another hybri d 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 flower in woody plants.