Genome-Wide Identification of the Double B-Box (DBB) Family in Three Cotton Species and Functional Analysis of GhDBB22 Under Salt Stress

Salt stress causes harm to plants through multiple aspects, such as osmotic pressure, ion poisoning, nutrient imbalance, and oxidative damage. Zinc finger proteins harboring two B-box domains, known as double B-box (DBB) proteins, constitute the DBB family. While <i>DBB</i> genes have been implicated in regulating circadian rhythms and stress responses in various plant species, their functions in cotton remain largely unexplored. The present study characterized the <i>DBB</i> gene family across the genomes of <i>Gossypium hirsutum</i> L., <i>Gossypium raimondii</i> L., and <i>Gossypium arboreum</i> L., revealing a complement of 58 members. These <i>DBB</i> genes were assigned to three separate clades based on phylogenetic analysis. Members possessing close phylogenetic relationships have similar conserved protein motifs and gene structures. All DBB proteins were predicted to be nuclear-localized, consistent with their roles as transcription factors. Furthermore, the presence of multiple cis-acting elements related to light, hormone, and stress responses in the promoters implies that <i>GhDBBs</i> are integral to cotton's environmental stress adaptation. Expression pattern analysis indicated that the expression of <i>GhDBB</i> genes was associated with the plant's response to multiple abiotic stresses, such as salt, drought, heat (37 °C), and cold (4 °C). The reliability of the expression data was confirmed by qPCR analysis of eight selected <i>GhDBBs</i>. Under 200 mM NaCl, Arabidopsis plants overexpressing <i>GhDBB22</i> displayed longer roots and healthier true leaves than the wild-type controls. Conversely, VIGS-mediated silencing of <i>GhDBB22</i> in <i>G. hirsutum</i> led to significantly reduced salt tolerance, accompanied by exacerbated oxidative damage. Taken together, the findings from our integrated genomic and functional analyses provide a foundational understanding of the molecular mechanisms through which proteins encoded by <i>DBB</i> genes are involved in the plant's response to salt stress.