Abstract
High-Mobility Group B (HMGB) proteins are conserved non-histone nuclear proteins involved in DNA replication, transcription, recombination, repair; plant growth and development; and stress responses. In this study, we identified nine ClHMGB genes in watermelon using genome-wide search. Phylogenetic and homology analyses classified them into four distinct classes. Synteny analysis revealed that ClHMGB genes share closer evolutionary relationships with dicots than with monocots. Tissue-specific expression profiling showed that eight ClHMGB members exhibit higher transcript levels in female and/or male flowers, suggesting that they play essential roles in floral organ development. Under drought, low-temperature, and salt stresses, ClHMGB members displayed distinct expression patterns. For instance, ClHMGB4 and ClHMGB8 were downregulated under drought and low-temperature stress but upregulated under salt stress, indicating potential functional specialization in response to different abiotic stresses. The highly virulent Fusarium oxysporum f. sp. niveum race 2 (Fon R2) induced the upregulation of more ClHMGB genes than the less virulent race 1 (Fon R1). Four members (ClHMGB1, 4, 6, and 7) were consistently upregulated by both races, suggesting that they may play fundamental roles in disease resistance. This study provides a foundation for further investigation into the roles of ClHMGB genes in growth, development, and stress responses of watermelon.