Genome-Wide Identification of the Maize ARR-B Gene Family and Expression Analysis Response to Low-Phosphorus in Maize (Zea mays L.)

Cytokinin signaling is essential for plant growth, development, and responses to environmental stresses. As key transcription factors in this pathway, type-B response regulators (ARR-Bs) activate the expression of primary cytokinin-responsive genes. However, the systematic identification of the ARR-B family in maize and its functional roles in responses to phosphorus (P) nutritional stress remain largely unclear. In this study, the maize ARR-B (ZmARR-B) transcription factor family was systematically identified and comprehensively characterized at the genome-wide level. A total of 12 ZmARR-B genes were identified and comprehensively characterized in terms of chromosomal localization, physicochemical properties, protein structure, phylogenetic relationships, gene organization, conserved motifs, promoter cis-regulatory elements, and collinearity. Based on systematic measurements of leaf inorganic phosphate (Pi) concentrations in 70 maize inbred lines from southwestern China, the lines 082, B73, and Ye107 were selected as representative materials. By integrating long-term (14 d) and short-term (0.5–12 h) low-P treatments with transcriptome sequencing and qRT-PCR validation, we systematically characterized the expression dynamics of ZmARR-B family members under P deficiency. The results showed that ZmARR-B genes exhibited markedly differentiated low-P-responsive expression patterns among distinct genetic backgrounds. ZmARR-B03 was transiently induced during the early phase of low-P treatment (0–6 h), followed by a rapid decline in transcript abundance. This characteristic pulse-like expression pattern suggests that ZmARR-B03 may function as a rapid-response regulator during early P-starvation signal transduction. In contrast, ZmARR-B06 exhibited sustained transcriptional upregulation throughout the low-P treatment period, suggesting a potential role in long-term adaptive regulation under P-deficient conditions. Notably, the number of low-P-responsive ZmARR-B members was significantly greater in the P-efficient inbred line 082 than in the P-sensitive line Ye107, indicating that ZmARR-B-mediated P-starvation responses are strongly genotype dependent. This study provides an important foundation for elucidating the molecular mechanisms by which cytokinin signaling, through type-B ARRs, regulates phosphate nutritional adaptation in maize. It also identifies potential candidate genes for the genetic improvement of P-efficient maize germplasm.