Comprehensive Genome-Wide Identification and Expression Profiling of bHLH Transcription Factors in Areca catechu Under Abiotic Stress

The basic helix-loop-helix (<i>bHLH</i>) transcription factor (TF) family, the second-largest among eukaryotes, is known for its evolutionary and functional diversity across plant species. However, <i>bHLH</i> genes have not yet been characterized in <i>Areca catechu</i>. In this study, we identified 76 <i>AcbHLH</i> genes, which exhibit a variety of physicochemical properties. Phylogenetic analysis revealed evolutionary relationships between <i>Arabidopsis thaliana bHLH</i> genes (<i>AtbHLH</i>) and their counterparts in <i>A. catechu</i> (<i>AcbHLH</i>). These analyses also highlighted conserved amino acid motifs (S, R, K, P, L, A, G, and D), conserved domains, and evolutionary changes, such as insertions, deletions, and exon gains or losses. Promoter analysis of <i>AcbHLH</i> genes revealed 76 cis-elements related to growth, phytohormones, light, and stress. Gene duplication analysis revealed four tandem duplications and twenty-three segmental duplications, while <i>AcbHLH63</i> in the Areca genome exhibited significant synteny with <i>bHLH</i> genes from <i>A. thaliana</i>, <i>Vitis vinifera</i>, <i>Solanum lycopersicum</i>, <i>Brachypodium distachyon</i>, <i>Oryza sativa</i>, and <i>Zea mays</i>. Furthermore, relative expression analysis showed that under drought stress (DS), <i>AcbHLH22</i>, <i>AcbHLH39</i>, <i>AcbHLH45</i>, and <i>AcbHLH58</i> showed distinct upregulation in leaves at specific time points, while all nine <i>AcbHLH</i> genes were upregulated in roots. Under salt stress (SS), <i>AcbHLH22</i>, <i>AcbHLH39</i>, <i>AcbHLH45</i>, and <i>AcbHLH58</i> were upregulated in leaves, and <i>AcbHLH22</i>, <i>AcbHLH34</i>, and <i>AcbHLH39</i> exhibited differential expression in roots at various time points. This study provides valuable insights into the bHLH superfamily in <i>A. catechu</i>, offering a solid foundation for further investigation into its role in responding to abiotic stresses.