Evolution and expression analysis of the choline monooxygenase gene family in plants

Glycine betaine (GB) is a compatible solute widely found in plants, animals, bacteria, and algae, where it plays a key role in regulating osmotic pressure and maintaining cellular water balance upon accumulation in the cytoplasm. As such, GB is crucial for plant responses to osmotic stress. Choline monooxygenase (CMO) is a rate-limiting enzyme in the biosynthetic pathway of GB in plants. Among 168 genomes analyzed in this study, 131 were found to contain homologous <italic>CMO</italic> genes, with a total of 169 CMO members identified. Selective pressure analysis revealed that most gene pairs have undergone purifying selection, while only seven gene pairs exhibited signs of positive selection. Phylogenetic analysis classified the CMO gene family into six subfamilies, with positive selection sites detected in five subfamilies except subfamily F. Further analysis of the Rieske domain and Ring-hydroxyl A domain showed a significant positive correlation between their dN/dS values, suggesting co-evolution. qRT-PCR analysis demonstrated that the expression of <italic>CMO</italic> genes in maize was upregulated under MgSO₄-induced stress. This study provides a comprehensive analysis of the evolution and expression of the CMO gene family, offering a theoretical foundation for the future application of <italic>CMO</italic> genes in improving crop tolerance to abiotic stress.