Comparative Genomic Analysis of the Poaceae Cytokinin Response Regulator RRB Gene Family and Functional Characterization of OsRRB5 in Drought Stress Tolerance in Rice

The cytokinin (CK) type B response regulator (<i>RRB</i>) gene is involved in the CK signaling pathway and performs a key function for mediating reactions to amounts of abiotic stresses. Nevertheless, the <i>RRB</i> gene family remains to be characterized in Poaceae (also known as Gramineae or grasses). Here, we performed a comprehensive analysis encompassing phylogenetic relationships, evolutionary pressures, and expression patterns of the <i>RRB</i> gene family in six Poaceae species, including rice, <i>Panicum</i>, <i>Sorghum</i>, <i>Setaria</i>, maize, and wheat. Phylogenetic tree and syntenic analyses revealed that the <i>RRB</i> genes were divided into seven orthologous gene clusters (OGCs), indicating that the common ancestor of these Poaceae species possessed at least seven <i>RRB</i> genes. Further analysis revealed that the evolution of the Poaceae <i>RRB</i> gene family was primarily driven by purifying selection. The expression pattern of rice <i>OsRRB</i> toward phytohormonal and abiotic stresses was also investigated. The findings revealed that several phytohormones, including cytokinin (CK), abscisic acid (ABA), and methyl jasmonate (MeJA), as well as abiotic factors such as drought and cold, significantly increased the expression levels of these genes. Importantly, haplotype analysis identified four crucial variation sites within the <i>OsRRB5</i> genomic regions that may contribute to drought resistance in rice. Our findings lay the groundwork for further elucidating the biological function of <i>OsRRB</i> genes and provide a promising new target for developing stress-resistant rice varieties.