Arbuscular mycorrhizal fungi enhance maize tolerance to combined La-NaCl stress by restructuring the rhizosphere bacterial community

Arbuscular mycorrhizal fungi (AMF) and plant rhizosphere microbes help alleviate the combined abiotic stress of plants. However, the microbial mechanisms by which AMF symbiosis improves plant tolerance to combined rare earth-salt stress remain unclear. We conducted a pot experiment to assess the effects of AMF on maize growth, nutrient contents, toxic ion accumulation, and rhizosphere bacterial community in La or combined La-NaCl stress (La-NaCl). The results revealed that in La and La-NaCl, AMF significantly increased plant biomass, P, Mg, and La contents (35.14 %-468.79 %), significantly decreased soil available P and K concentrations (6.52 %-28.30 %). In La-NaCl, AMF significantly increased Na⁺ content (41.41 %-305.75 %) and decreased the concentrations and translocation rates of La and Na⁺ of plants (13.50 %-56.55 %). Additionally, AMF significantly altered the rhizosphere bacterial community structure and significantly increased the Firmicutes abundance by 15.97 % (La) and 62.51 % (La-NaCl). Moreover, AMF increased the modularity of networks in La and La-NaCl, altered biomarkers and keystones in La (e.g., Noviherbaspirillum) and La-NaCl (e.g., Ramlibacter), thus reshaping the key microorganisms. Molecular ecological networks revealed that AMF strengthened the key microorganisms' positive correlation with plant indices in La-NaCl. The structural equation model further indicated that AMF can affect plant growth by regulating the composition, diversity, and network characteristics of rhizosphere bacterial community in La-NaCl. To summarize, the findings of this study improved our understanding of the underlying microbial mechanisms by which AMF promote plant resistance to combined La-NaCl stress.