Genome‑wide Identification and Stress‑Responsive Expression Analysis of the ARR‑B Gene Family in Watermelon (Citrullus lanatus)

Background Type-B response regulator (ARR-B) genes act as positive regulators of cytokinin signal transduction and exert essential functions in plant growth, development, and abiotic stress tolerance. Nevertheless, systematic genome-wide identification and functional characterization of the ARR-B gene family have not been comprehensively performed in watermelon, an economically important crop vulnerable to diverse abiotic stresses. Results In this study, a total of 10 ARR-B family genes were identified from the watermelon reference genome, and their physicochemical properties, gene structures, conserved cis-elements, tissue expression patterns and abiotic stress response characteristics were systematically analyzed using bioinformatics approaches. Subcellular localization prediction showed that nine ClARR-B proteins were localized in the nucleus, while only ClARR - B4 was distributed in the cytoplasm. Chromosome mapping revealed that ClARR-B genes were unevenly distributed on watermelon chromosomes, with five members located on chromosome 6, accounting for the largest number of family members. Multiple cis-acting elements related to growth and development, hormone response, biotic and abiotic stresses were identified in the promoter regions of ClARR-B genes. The qRT-PCR results indicated that ClARR - B1 and ClARR - B2 were highly expressed in watermelon roots, stems, leaves, flowers and fruits, suggesting their critical roles in the growth and development of major watermelon organs. Compared with the control, ClARR-B4 , ClARR-B7 and ClARR-B9 were markedly upregulated under drought stress, while ClARR-B2 and ClARR-B6 displayed the strongest upregulation under salt stress. ClARR-B4 was the most significantly induced by cold stress. Collectively, ClARR-B4 is involved in the responses of watermelon to drought and cold stresses, laying a solid basis for subsequent studies on abiotic stress tolerance. Conclusion A total of ten ARR-B transcription factor family members were identified in watermelon, which are extensively involved in watermelon growth, development and abiotic stress response. This study provides a theoretical basis for further exploring the molecular functions of the ARR-B gene family in watermelon development and stress resistance.