Genome-Wide Identification and Analysis of Plant Cysteine Oxidase (PCO) Family Genes and Expression Pattern Under Abiotic Stresses in Medicago sativa

Plant cysteine oxidase (PCO) catalyzes the oxidation of cysteine residues in the N-degron pathway, thereby regulating the stability and activity of the seventh group of ethylene response factors (ERF-VII), which play a crucial role in reactive oxygen species (ROS)-mediated signal transduction. By regulating the degradation of ERF-VII, the <i>PCO</i> family genes control hormone signaling, which is highly valuable for plant growth and abiotic stress responses. However, systematic studies on <i>PCO</i> genes in <i>Medicago sativa</i>, a key forage legume, remain lacking. Herein, 35 <i>MsPCO</i> genes were identified from the alfalfa (<i>Medicago sativa</i>) genome, and their biological characteristics were comprehensively analyzed via bioinformatics approaches. The results showed that <i>MsPCO</i> genes are asymmetrically distributed across 18 chromosomes and clustered into 5 subgroups phylogenetically. Most MsPCO proteins are hydrophilic and localized in the cytoplasm. A total of 56 duplication events were detected, with most duplicated pairs undergoing purifying selection (Ka/Ks analysis). Collinearity analysis revealed close evolutionary relationships between <i>Medicago sativa</i> and <i>Medicago truncatula</i>, <i>Arabidopsis thaliana</i> or <i>Glycine max</i>. Promoter cis-acting elements in <i>MsPCO</i> genes are involved in light response, stress adaptation, hormone signaling, and growth regulation. Transcriptomic data indicated differential expression of <i>MsPCO</i> genes under abiotic stresses. <i>MsPCO20</i> is dispersed throughout the cell membrane and nucleus, whereas <i>MsPCO19</i> is localized to the nucleus, according to subcellular localization experiments. These findings provide candidate genes and a theoretical basis for further functional characterization of <i>PCO</i> genes in alfalfa.