Rhizobia and arbuscular mycorrhiza fungi (AMF) are important symbiotic microbes that are advantageous to plants growing in metal-contaminated soil. However, it remains unclear how inoculated microbes affect rhizosphere microbial communities or whether subsequent changes in rhizosphere microbiomes contribute to improving plant resistance under metal stress. This study investigated the effects of rhizobia and AMF inoculation on alfalfa resistance to Cd stress. The response of rhizosphere microbial communities to inoculation and its role in increasing alfalfa’ ability to cope with stress were further analyzed using high-throughput sequencing of 16S and ITS rRNA genes. Results showed that single rhizobia or AMF inoculation significantly improved alfalfa resistance to Cd stress, while their co-inoculation resulted in the greatest overall improvement. Improved resistance was reflected by the significant mitigation of Cd-induced lipid peroxidation and reactive oxygen species (ROS) stress caused by increases in antioxidant enzyme activities along with co-inoculation. Furthermore, co-inoculation significantly altered the rhizosphere microbial community structure by decreasing fungal community diversity and increasing bacterial community diversity. Results of partial least squares path modeling (PLS-PM) and variation partitioning analysis (VPA) showed that the rhizosphere bacterial community predominated over the fungal community with respected to improvements in resistance to Cd stress under the co-inoculation treatments. This improvement was specifically seen in the enrichment of certain key bacterial taxa (including Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi) induced by the rhizobia and AMF co-inoculation, enhancing alfalfa’ ability to uptake rhizosphere nutrients and reduce its release of photosynthetically-derived carbon (C) into soil. Our findings revealed that the co-inoculation of multiple symbiotic microbes can assist plants to effectively cope with Cd stress, providing a greater understanding of rhizosphere bacterial taxa in the microbe-induced phytomanagement.

根瘤菌和丛枝菌根真菌（AMF）是重要的共生微生物，对在金属污染土壤中生长的植物有利。然而，目前尚不清楚接种的微生物如何影响根际微生物群落，或者根际微生物群的后续变化是否有助于提高金属胁迫下的植物抗性。这项研究调查了根瘤菌和AMF接种对苜蓿对Cd胁迫的抵抗力。使用16S和ITS rRNA基因的高通量测序进一步分析了根际微生物群落对接种的反应及其在增加苜蓿应对压力的能力中的作用。结果表明，单根瘤菌或AMF接种显着提高了苜蓿对Cd胁迫的抵抗力，而它们的联合接种则带来了最大的总体改善。抵抗力的改善反映在Cd诱导的脂质过氧化和活性氧（ROS）胁迫的显着缓解，这是由抗氧化酶活性的增加以及共同接种引起的。此外，共接种可通过减少真菌群落多样性和增加细菌群落多样性来显着改变根际微生物群落结构。偏最小二乘路径建模（PLS-PM）和变异分区分析（VPA）的结果表明，在共接种处理下，对提高Cd胁迫的抵抗力方面，根际细菌群落高于真菌群落。在某些关键细菌类群（包括根瘤菌和AMF共同接种引起的变形杆菌，放线菌，酸性细菌和绿藻（Cloroflexi）增强了苜蓿吸收根际养分的能力，并减少了光合作用碳（C）向土壤中的释放。我们的研究结果表明，多种共生微生物的共同接种可以帮助植物有效应对Cd胁迫，从而在微生物诱导的植物管理中更深入地了解根际细菌类群。