Functional conservation and diversification of APETALA1/FRUITFULL genes in Brachypodium distachyon

The duplicated grass APETALA1/FRUITFULL (AP1/FUL) genes have distinct but overlapping patterns of expression, suggesting their discrete roles in transition to flowering, specification of spikelet meristem identity and specification of floral organ identity. In this study, we analyzed the expression patterns and functions of four AP1/FUL paralogs (BdVRN1, BdFUL2, BdFUL3 and BdFUL4) in Brachypodium distachyon, a model plant for the temperate cereals and related grasses. Among the four genes tested, only BdVRN1 could remember the prolonged cold treatment. The recently duplicated BdVRN1 and BdFUL2 genes were expressed in a highly consistent manner and ectopic expressions of them caused similar phenotypes such as extremely early flowering and severe morphological alterations of floral organs, indicating their redundant roles in floral transition, inflorescence development and floral organ identity. In comparison, ectopic expressions of BdFUL3 and BdFUL4 only caused a moderate early flowering phenotype, suggesting their divergent function. In yeast two-hybrid assay, both BdVRN1 and BdFUL2 physically interact with SEP proteins but only BdFUL2 is able to form a homodimer. BdVRN1 also interacts weakly with BdFUL2. Our results indicate that, since the separation of AP1/FUL genes in grasses, the process of sub- or neo-functionalization has occurred and paralogs function redundantly and/or separately in flowering competence and inflorescence development.