To investigate the homeotic systems underlying floral development in a dioecious tree, and to provide tools for the manipulation of floral development, we have isolated two Populus trichocarpa genes, PTAG1 and PTAG2, homologous to the Arabidopsis floral homeotic gene AGAMOUS (AG). PTAG1 and PTAG2 are located on separate linkage groups, but their non-coding regions are highly similar, consistent with a phylogenetically recent duplication. Intron/exon structure is conserved in relation to AG and the Antirrhinum AG orthologue, PLENA (PLE), and low-stringency Southern analysis demonstrated the absence of additional genes in the poplar genome with significant PTAG1/2 homology. PTAG1 and PTAG2 exhibit an AG-like floral expression pattern, and phylogenetic analysis of the AG subfamily strongly supports evolutionary orthology to C-class organ identity genes. The high degree of similarity shared by PTAG1 and PTAG2 in both sequence (89% amino acid identity) and expression indicates that they are unlikely to be functionally associated with specification of tree gender. Unexpectedly, PTAG transcripts were consistently detected in vegetative tissues.
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