The APETALA1 and FRUITFUL homologs in Camellia japonica and their roles in double flower domestication

The APETALA1/FRUITFUL (AP1/FUL) family genes encode MADS-box transcription factors, which are broadly involved in many aspects of floral development in higher plants. Gene duplication in the core eudicots has produced the euAP1 and euFUL clades. It remains unclear how the functional divergence of this gene family occurred. Camellia japonica is a famous ornamental species which belongs to the Theaceae (Ericales) group. Artificial selection for aesthetic flowers in Camellia has resulted in a remarkable diversity of floral forms, and double flower is one of the most important traits which provides a valuable resource for studying the underlying domestication mechanism. Here we isolated two homologs of the AP1/FUL family, named as CjAPL1 and CjAPL2, from C. japonica. Sequence and phylogenic analyses revealed that they were orthologs of FUL and AP1, respectively. We showed by gene expression profiling and ectopic expression in Arabidopsis that CjAPL1 and CjAPL2 potentially played different roles during floral development. Overexpression of CjAPL1/2 displayed similar phenotypes in Arabidopsis including early flowering, formation of terminal flowers, and increase in stamen and pistil numbers, but only in plants overexpressing CjAPL2 was the petal number increased. This therefore indicates that duplication of AP1- and FUL-like genes was functionally divergent in Ericales. Furthermore, higher expression levels of CjAPL1/2 were identified in four different double-flower varieties compared to wild single-flower camellias. Our results provide evidence for functional diversification of AP1-like and FUL-like genes in core eudicot species and point to their roles in double flower domestication.