The MIK region rather than the C-terminal domain of AP3-like class B floral homeotic proteins determines functional specificity in the development and evolution of petals

Abstract

• In core eudicots, euAP3-type MADS-box genes encode a PISTILLATA (PI)-derived motif, as well as a C-terminal euAP3 motif that originated from a paleoAP3 motif of an ancestral APETALA3 (AP3)-like protein through a translational frameshift mutation. To determine the functional and evolutionary relevance of these motifs, a series of point mutation and domain-swap constructs were generated, involving CsAP3, a paleoAP3-type gene from the basal angiosperm Chloranthus spicatus encoding a truncated paleoAP3 motif, and AtAP3, a euAP3-type gene from the core eudicot Arabidopsis thaliana. • The chimeric constructs were expressed in A. thaliana under the control of the AP3 promoter or the CaMV 35S promoter in an ap3 mutant or wild-type background, respectively. • Significant recovery of AP3 function was obtained in both complementation and ectopic expression experiments whenever the region upstream of the C-terminal motifs (MIK region) from A. thaliana was taken, even when the PI-derived motif and the truncated paleoAP3 motif of CsAP3 substituted for the corresponding sequences from AtAP3. However, no or very weak complementation or gain-of-function was seen when the MIK region was from CsAP3. • Our data suggest that changes in the MIK region rather than mutations in the C-terminal domain were of crucial importance for the evolution of the functional specificity of euAP3-type proteins in stamen and petal development. © The Authors (2008).