Identification and Expression Analysis of the MLO Gene Family Under Salt Stress in Cotton (Gossypium hirsutum L.)

MLO (Mildew Resistance Locus O) genes encode seven-transmembrane proteins that function as critical regulators of powdery mildew resistance and abiotic stress responses. Despite their established importance, the <i>MLO</i> gene family in <i>Gossypium hirsutum</i> L. has not been systematically investigated under salt stress conditions. Here, we performed genome-wide identification of 46 <i>GhMLO</i> members using Hidden Markov Model and BLAST searches based on the latest cotton genome assembly. Phylogenetic analysis classified these genes into four distinct subfamilies. Transmembrane topology and conserved domain analyses revealed that all GhMLO proteins contain typical MLO domains and transmembrane structures, maintaining high structural similarity with dicotyledonous model plants. Synteny analysis demonstrated that the expansion of the <i>GhMLO</i> family was primarily driven by segmental and tandem duplications. Integration of transcriptomic data from the COTTONOMICS database revealed tissue-specific expression patterns, with higher transcript abundance in receptacles, stems, and roots, but lower levels in stamens and petals. Salt, drought, and cold stress treatments induced upregulation of <i>GhMLO</i> family members, with most genes showing increased expression over time. RT-qPCR analysis validated that five candidate <i>GhMLO</i> genes were significantly upregulated under salt stress. In summary, this study provides a comprehensive genome-wide characterization of the <i>GhMLO</i> gene family, elucidating their phylogenetic relationships and expression dynamics, which establishes a theoretical basis for identifying key regulatory genes involved in abiotic stress responses and offers novel genetic resources for improving stress tolerance in cotton molecular breeding.