The effects of cadmium-copper stress on the accumulation of antibiotic-resistance genes in soil and pakchoi leaves

Antibiotic resistance and heavy metal contamination are two worldwide environmental concerns. Especially in soil, the interaction between these two pollutants may exacerbate the problem of antibiotic resistance genes (ARGs) entering the food chain, which can have severe consequences on the environment and the wellness of humankind. This study investigated the influences of cadmium (Cd) and copper (Cu) pollution on ARG profiles in the soil rhizosphere and endogenous microbiome of roots and leaves in pakchoi. We further determined the impact of these heavy metals and various environmental factors on the accumulation of ARGs in leaves and discussed the possible transfer pathways of ARGs in the soil-plant system. Our results showed that Cd–Cu stress increased the relative abundance of most target genes in the samples, especially the sulfonamide resistance genes sul1 and sul3, the quinolone resistance gene qepA, and the Class 1 integron integrase gene intI1. Variance partitioning analysis indicated that the Cd–Cu contents primarily contributed (45.7%) to ARG variations in leaves. Cadmium and Cu led to a pollution-induced community tolerance phenomenon in microbial communities, enriching Cd- and Cu-tolerant bacteria. ARGs were highly correlated with mobile genetic elements (e.g., intI1) and metal resistance genes, increasing the co-selection of metals on ARGs. Structural equation modeling showed that metal resistance genes and intI1 in leaves had significant direct positive effects on the relative abundances of ARGs (p < 0.05). Higher Cu concentrations facilitated the migration of most target ARGs from soil to leaves.