Abstract
Some studies have reported the importance of rhizobium in mitigating heavy metal toxicity, however, the regulatory mechanism of the alfalfa-rhizobium symbiosis to resist copper (Cu) stress in the plant-soil system through biochemical reactions is still unclear. This study assessed the effects of rhizobium (Sinorhizobium meliloti CCNWSX0020) inoculation on the growth of alfalfa and soil microbial characteristics under Cu-stress. Further, we determined the regulatory mechanism of rhizobium inoculation to alleviate Cu-stress in alfalfa through plant-soil system. The results showed that rhizobium inoculation markedly alleviated Cu-induced growth inhibition in alfalfa by increasing the chlorophyll content, height, and biomass, in addition to nitrogen and phosphorus contents. Furthermore, rhizobium application alleviated Cu-induced phytotoxicity by increasing the antioxidant enzyme activities and soluble protein content in tissues, and inhibiting the lipid peroxidation levels (i.e., malondialdehyde content). In addition, rhizobium inoculation improved soil nutrient cycling, which increased soil enzyme activities (i.e., β-glucosidase activity and alkaline phosphatase) and microbial biomass nitrogen. Both Pearson correlation coefficient analysis and partial least squares path modeling (PLS-PM) identified that the interactions between soil nutrient content, enzyme activity, microbial biomass, plant antioxidant enzymes, and oxidative damage could jointly regulate plant growth. This study provides comprehensive insights into the mechanism of action of the legume-rhizobium symbiotic system to mitigate Cu stress and provide an efficient strategy for phytoremediation of Cu-contaminated soils.
Highlights
Heavy metal effluence of the soil is a severe issue that directly interrupts environmental resources, ecosystem, and food safety (Sahito et al, 2021; Wang J. et al, 2021)
The total chlorophyll content increased significantly after rhizobium inoculation, and this increase was prominent at a Cu concentration of 800 mg kg−1, which was 1.44 times higher than that of the uninoculated control (Table 1)
Phosphatase in soils is mainly produced by microbial activity and is critical for mobilizing the organic forms of P (Wei et al, 2019; Ma et al, 2021). These results demonstrated that rhizobium inoculation can promote soil nutrient cycling by stimulating enzyme activities in metal contaminated soil while subsequently providing higher C, N, and P requirements for microbial activity and plant growth
Summary
Heavy metal effluence of the soil is a severe issue that directly interrupts environmental resources, ecosystem, and food safety (Sahito et al, 2021; Wang J. et al, 2021). Excessive Cu concentration can inhibit plant growth, lead to nutrient deficiencies, reduce antioxidant enzyme activity, and cause oxidation stress by producing of reactive oxygen species (ROS) (Liu et al, 2016; Abbas et al, 2017). Restoration of soil oxidase and nutrient cycling enzyme activities may be essential for soil health In this case, plant growth can improve the microbial properties of metal-contaminated soils by root activity their or root secretions. To promote plant growth and reduce potential health risks, there is an urgent need to explore effective strategies that can improve plant resistance and enhance soil quality
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