Expression and functional divergence of a type-A response regulator paralog pair formed by dispersed duplication during Populus deltoides evolution.

Gene duplication and divergence are essential to plant evolution. The Arabidopsis type-A response regulator (ARR) family, negative regulators in cytokinin signaling, exemplifies gene expansion and differential retention. Despite extensive research, the understanding of type-A RR homologs in woody plants remains limited. In this study, the evolution history of type-A RR gene families across four rosids and one monocot has been comprehensively investigated. Focusing on Populus deltoides, a unique pair of dispersed duplicates, PdRR8 and PdFERR, is identified, and their duplication is estimated to have occurred in the common ancestor of the four rosids. The duplication remnants corresponding to PdRR8 have been retained in all rosids but the counterpart of PdFERR has been lost. In poplar, PdRR8 shows the highest expression levels in leaves, while PdFERR is specifically expressed in female floral buds. Among various external stimuli, cold strongly represses PdRR8 promoter activity, whereas 6-BA markedly inhibits that of PdFERR. Overexpression of PdRR8 in the Arabidopsis arr16arr17 double-mutant fully complements the reduced hydrotropic response. In contrast, PdFERR fails to rescue the hydrotropic defects of the mutant. Results of evolutionary, expression and functional analyses indicate that PdRR8, rather than PdFERR, is the true ortholog of the ARR16-ARR17 paralogs. Though PdRR8 and PdFERR originate from a common ancestral gene and evolve under strong negative selection, these two dispersed duplicates have exhibited differential expression and some degree of functional divergence.