Peanut (Arachis hypogaea), as an important food (oilseed) crop, is widely cultivated in the world. However, with the irrational application of chemical fertilizers, sewage irrigation, etc. has caused serious heavy metal pollution of farmland soils. Cd (Cadmium) has the highest rate of exceedance in the soil, causing irreversible harm to peanut quality, in addition to the accumulation in the food chain also threatens human health. Recent studies have shown that in situ immobilization remediation techniques that impose multiple barriers to the migration of heavy metals from soil, inter-root, and plant bodies and other links are more suitable for the safe application of contaminated agricultural soils, in which heavy metal-tolerant plant inter-root promoting bacteria play an important role. Microbial cells can complex metal ions through extracellular precipitation, surface adsorption, and bioaccumulation to limit their transfer in the soil. Bacillus spp. as a typical representative of PGPR can secrete growth hormone, iron-producing carriers to help plant growth, and in addition extracellular polysaccharide-producing properties give them potential remediation potential for contaminated soil.
Associate Professor He Wei from College of Life Sciences, Nanjing Normal University, China, funded by the National Natural Science Foundation of China, simulated the effect of Bacillus megaterium A14 inoculation to alleviate cadmium stress in peanut under high cadmium contamination and explored the systemic changes at soil and plant levels after A14 inoculation. It was found that Bacillus megaterium A14 had good cadmium tolerance and secreted extracellular polysaccharides containing various functional groups (-NH2, -OH, -COOH) on the surface, which provided cadmium ion binding sites; after A14 inoculation, the form of cadmium assembly in peanut inter-root soil changed, with the proportion of exchangeable cadmium decreasing and the proportion of organic-bound and residual cadmium increasing. Peanut was subjected to the effect of A14, the number of root nodules in the roots increased and the dry matter mass accumulation increased. Meanwhile, antioxidant enzyme activities in peanut were elevated to help reduce oxidative stress in peanut; synergistic expression of cadmium transporter genes (ABCC3, ABCC4, ZIP4) inhibited the transfer of cadmium from nutrient organs to seeds. The results reveal that Bacillus megaterium A14 plays a cadmium fixation function in the inter-rhizosphere soil by its good cadmium adsorption property, and reduces the bioavailabilities of cadmium to peanut by regulating the expression of cadmium transporter-related proteins, and finally reduces the accumulation of cadmium in seeds. The related results have been published in Journal of applied microbiology.
In conclusion, the use of heavy metal-tolerant microorganisms to manage contaminated agricultural soils and mitigate plant toxicity has become a popular research nowadays, but less attention has been paid to legumes under contaminated areas. In this study, we used Bacillus megaterium A14 inoculation to block the uptake of cadmium by legume peanut under potted conditions and alleviated peanut cadmium toxicity, and its use as an innovative method and remediation strategy to manage contaminants in a cost-effective and environmentally friendly manner provides a promising new technology for green and sustainable remediation of contaminated soil and safe production of food crops.