Genome-Wide Identification and Expression Analysis of the ClHMGB Gene Family in Watermelon Under Abiotic Stress and Fusarium oxysporum Infection

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 <i>ClHMGB</i> genes in watermelon using genome-wide search. Phylogenetic and homology analyses classified them into four distinct classes. Synteny analysis revealed that <i>ClHMGB</i> genes share closer evolutionary relationships with dicots than with monocots. Tissue-specific expression profiling showed that eight <i>ClHMGB</i> 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, <i>ClHMGB</i> members displayed distinct expression patterns. For instance, <i>ClHMGB4</i> and <i>ClHMGB8</i> 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 <i>Fusarium oxysporum</i> f. sp. <i>niveum</i> race 2 (<i>Fon</i> R2) induced the upregulation of more <i>ClHMGB</i> genes than the less virulent race 1 (<i>Fon</i> R1). Four members (<i>ClHMGB1</i>, <i>4</i>, <i>6</i>, and <i>7</i>) 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 <i>ClHMGB</i> genes in growth, development, and stress responses of watermelon.