Hydrogen sulfide is a crucial element of the antioxidant defense system in Glycine max–Sinorhizobium fredii symbiotic root nodules

Aim: H₂S is emerging as a signaling molecule involved in the regulation of many physiological processes in plants. Here, we investigated the potential antioxidant role of H₂S in soybean (Glycine max)-rhizobia (Sinorhizobium fredii) symbiotic root nodules. 

Method: An endogenous H₂S production deficit rhizobia mutant ∆CSE was constructed to study the effect of decreased content of H₂S in soybean nodules. Fluorescent probes and confocal microscope were used to observe the production and accumulation of H₂S and reactive oxygen species. Transmission electronic microscopy was conducted to study the structural changes in ∆CSE soybean nodules. Finally, qRT-PCR, enzymatic activity, and oxidative damage parameters were measured. 

Result: The results demonstrated that abundant H₂S was generated in the nitrogen-fixing zone of soybean nodules. The deletion of the cystathionine γ-lyase (CSE) gene in S. fredii (∆CSE) caused a sharp decrease in H₂S production in both free-living rhizobia and soybean nodules. We found that decrease in the H₂S level in nodule cells inhibited nitrogenase activity. In addition, to elevated H₂O₂ and malondialdehyde accumulation, increased protein carbonyl content and decreased O₂ ⁻ scavenging ability was observed in ∆CSE root nodules. Transmission electron microscopy revealed that an H₂S deficit caused the deformation of bacteroids and damage of peribacteroid membranes in nodule cells. Moreover, the expression of some rhizobial and soybean genes related to antioxidant defense was up-regulated in ∆CSE nodules. 

Conclusion: H₂S is crucial for the nitrogen-fixation ability of soybean nodules by acting as an antioxidant element that protects nodule cells and bacteroids from oxidative damage.