Potential function of plant-growth-promoting endophytic Serratia fonticola CPSE11 from Codonopsis pilosula in phytoremediation of Cadmium ion (Cd).

Plant growth promoting rhizobacteria (PGPR) have potential application value in reducing metal accumulation in medicinal plants. The objective of this study was to isolate, characterize and evaluate the effects of endophyte on the growth and metal resistance of Codonopsis pilosula under Cadmium ion (Cd) stress. Five endophytic strains were isolated from the root of C. pilosula. Serratia (CPSE11, CPSE12), Enterobacter (CPSE22), Bacillus subtilis (CPSE32), and Microbacterium (CPSE8). Serratia fonticola CPSE11 showed high tolerance to Cd. The adsorption of Cd was consistent with the first-order kinetic model, and had a strong correlation with the Langmuir model. The maximum single-layer adsorption capacity (q^m) was 58.47 mg/g (R > 0.9). In hydroponic experiments, 10 cfu/mL CPSE11 could effectively alleviate the toxic effect of Cd (0, 5, 10 and 15 mg/L) on C. pilosula. Genomic analysis showed that CPSE11 has genes involved in extracellular polysaccharide (EPS) synthesis, transcription, transport, and metal resistance. These include czcB, cusA/czcA, cusB, and cusC, and genes encoding copper (Cu), silver (Ag), cadmium (Cd), zinc (Zn), and cobalt (Co) efflux pumps. CPSE11 enhances the resistance of C. pilosula to Cd stress by producing siderophores, IAA, ACC, fixing nitrogen and regulating the antioxidant system. CPSE11 alleviates the toxic effect of Cd stress on C. pilosula through EPS fixation, RND-type efflux system and specific translocation of metal tolerance gene family. EPS depend on ABC transporter and Wzx/Wzy pathway to produce. The research will promote the sustainable development of medicinal plants and the application of PGPR in metal stress.