Expression patterns of AP1, FUL, FT and LEAFY orthologs in Vitaceae support the homology of tendrils and inflorescences throughout the grape family

The leaf-opposed tendril, a characteristic organ in Vitaceae (grape family), is likely a morphological key innovation for the family. It has been considered as the homologous organ of the inflorescence. Expression of floral related genes has been studied extensively in the model species, grapevine (Vitis vinifera), to uncover molecular mechanisms that determine the development of a common uncommitted primordium (or an anlage) into an inflorescence or a tendril. However, to investigate the homology of tendrils and inflorescences in Vitaceae, evidence only from the highly derived grapevine is insufficient. Therefore, gene sequences of four key floral meristem genes, i.e., FUL, AP1, FT and LEAFY orthologs were obtained from transcriptome data of 14 Vitaceae species, the grapevine genome and the outgroup Leea guineensis. Additionally, expression patterns of these four genes were studied in leaves, tendrils, and inflorescences of five phylogenetically distinct Vitaceae species. Expression of the AP1 ortholog was only detected in the tendril and the inflorescence but not in the leaf for all species, indicating that the tendril is more like the inflorescence than the leaf and that the tendrils from these six species including grapevine are likely homologous. Meanwhile, expression of the LEAFY ortholog was found in the inflorescence but not in the tendril and leaf, suggesting that the LEAFY ortholog expression might play a role in determining whether an anlage develops into a tendril or an inflorescence. Based on combined evidence from the expression patterns of these four genes, the possible mechanisms on the evolution of tendrils are discussed.