Genome-Wide Analysis of the Caffeoyl Coenzyme A-O-Methyltransferase (CCoAOMT) Gene Family in Platycodon grandiflorus (Jacq.) A. DC. and the Potential Regulatory Mechanism in Response to Copper Stress

In recent years, copper pollution has gradually become one of the major problems of soil environmental pollution. Lignin plays an important role in plant resistance to biotic and abiotic stresses. CCoAOMT is a key enzyme in the lignin biosynthesis process. In this study, the <i>CCoAOMT</i> gene family members of <i>Platycodon grandiflorus</i> were identified by bioinformatics methods, and their basic characteristics and potential functions were analyzed. The results showed that five members of the <i>PgCCoAOMT</i> gene family were identified in <i>P. grandiflorus</i>, with protein lengths ranging from 246 to 635 amino acids, and were evenly distributed on four chromosomes. Phylogenetic analysis indicated that the <i>PgCCoAOMT</i> gene family was divided into two subclades, namely Clade1a, Clade1b, Clade1c, Clade1d, and Clade2. The cis-regulatory element analysis of the promoter revealed that the <i>PgCCoAOMT</i> members contained a large number of cis-regulatory elements responsive to stress, and conjecture <i>PgCCoAOMT2</i>, <i>PgCCoAOMT4</i>, and <i>PgCCoAOMT5</i> were involved in the lignin synthesis. The qRT-PCR results showed that, within 5 days of copper stress treatment, except for the <i>PgCCoAOMT4</i> gene, the other genes exhibited different expression levels. Furthermore, the expression levels of all five <i>PgCCoAOMT</i> genes increased significantly at 7 days of treatment. With the increase in the number of days of treatment, the content of lignin in the seedings of <i>P. grandiflorus</i> showed a trend of increasing first and then decreasing under copper stress. In general, in the copper stress treatment of 1-3 days, the transcriptional inhibition of <i>PgCCoAOMT1</i> and <i>PgCCoAOMT3</i> and the increase in lignin content contradicted each other, suggesting that there was post-translational activation or alternative metabolic pathways compensation. Meanwhile, in the 7-day treatment, the coordinated up-regulation of the genes was accompanied by the failure of lignin synthesis, which pointed to the core bottleneck of metabolic precursors depletion and enzyme activity inactivation caused by root damage. Research objective: This study reveals the expression level of the <i>PgCCoAOMT</i> gene in the seedings of <i>P. grandiflorus</i> under copper stress, providing a theoretical basis for elucidating the mechanism of <i>P. grandiflorus</i> response to copper stress and for subsequent improvement of root resistance in <i>P. grandiflorus</i>.