A DEFICIENS homolog from the dioecious tree black cottonwood is expressed in female and male floral meristems of the two-whorled, unisexual flowers

We isolated PTD, a member of the DEFICIENS (DEF) family of MADS box transcr iption factors, from the dioecious tree, black cottonwood (Populus trichoca rpa). In females, in situ hybridization experiments showed that PTD mRNA wa s first detectable in cells on the flanks of the inflorescence meristem, be fore differentiation of individual flowers was visually detectable. In male s, the onset of PTD expression was delayed until after individual flower di fferentiation had begun and floral meristems were developing. Although PTD was initially expressed throughout the inner whorl meristem in female and m ale flowers, its spatial expression pattern became sex-specific as reproduc tive primordia began to form. PTD expression was maintained in stamen primo rdia, but excluded from carpel primordia, as well as vegetative tissues. Al though PTD is phylogenetically most closely related to the largely uncharac terized TM6 subfamily of the DEF/APETELA3(AP3)/TM6 group, its spatio-tempor al expression patterns are more similar to that of DEF and AP3 than to othe r members of the TM6 subfamily.