Mitigating bismuth stress in rye: synergistic effects of arbuscular mycorrhizal fungi and germanium on yield, metabolism, and osmoregulation

Bismuth (Bi) stress significantly challenges plant growth, yield, and metabolism. Thus, this study investigated how arbuscular mycorrhizal fungi (AMF) and Germanium (Ge), and their interactions, could mitigate bismuth (Bi) stress on rye yield and metabolism. Our findings show that AMF and Ge, both individually and combined, enhance plant resilience under Bi stress by improving primary and secondary metabolic pathways. Bi stress significantly reduced seed yield. However, AMF and Ge treatments alleviated this, yielding the highest recovery. This improved yield was linked to enhanced metabolic efficiency, specifically through increased sugar availability. This provided an accessible carbon source, supporting key primary metabolic processes like organic acids, nitrogen assimilation, amino acids, and fatty acids. The study found organic acids, including oxalic, citric, and succinic acids, increased under AMF, Ge, and Bi stress. Bi stress also raised key fatty acid levels, likely a defense response, while AMF and Ge modified these concentrations, suggesting roles in lipid metabolism. Amino acid profiling showed Bi stress caused stress-responsive amino acid accumulation. AMF and Ge treatments modulated these, notably reducing isoleucine under Bi stress. Furthermore, essential amino acids like arginine and glutathione were pivotal in regulating polyamine metabolism, crucial for stress adaptation and cellular stability. Polyamine analysis revealed AMF and Ge treatments caused the highest polyamine accumulation in unstressed plants, with S-adenosyl-L-methionine showing the most enhancement. Under Bi stress, polyamine levels generally increased as part of the defense. However, the combined AMF, Ge, and Bi treatment resulted in a decline, suggesting a regulatory effect preventing excessive accumulation. Overall, these findings highlight the synergistic role of AMF and Ge in improving rye resilience to Bi stress. The observed improvements in sugar-mediated carbon flux, amino acid and polyamine metabolism, and secondary metabolite production collectively contributed to higher yield and stress adaptation. Future research should optimize AMF and Ge applications for stress management and crop improvement.