Conserved C-terminal motifs of the Arabidopsis proteins APETALA3 and PISTILLATA are dispensable for floral organ identity function

The B-class genes APETALA3 (AP3) and PISTILLATA (PI) in Arabidopsis (Arabidopsis thaliana) and their orthologs in other species have been the focus of studies to elucidate the development of petals and stamens in angiosperm flowers. Evolutionary analysis indicates that B-class genes have undergone multiple gene duplication events in angiosperms. The resultant B-class lineages are characterized by short, conserved amino acid sequences at the extreme C-terminal end of the B-class proteins. AP3 is a member of the euAP3 lineage that contains both the euAP3 and PI-derived motifs at the C terminus. PI is a member of the PI lineage that contains the C-terminal PI motif at the C terminus. Despite conservation over a wide evolutionary distance, the function of C-terminal motifs is not well understood. In this study, we demonstrate that truncated forms of AP3 and PI, which lack the conserved C-terminal motifs, function to direct floral organ identity specification in Arabidopsis plants. By contrast, larger truncations, which remove the third putative amphipathic alpha-helix in the K domain of AP3 or PI, are nonfunctional. We conclude that the euAP3 and PI-derived motifs of AP3 and the PI motif of PI are not essential for floral organ identity function of AP3 and PI in Arabidopsis.