Multi-omics insights into rhizobium-induced cadmium tolerance in Robinia pseudoacacia: antioxidant-metabolic coordination and transporter activation

Background and aims

Rhizobia can enhance legume growth in the presence of heavy metals, making the legume-rhizobium symbiosis promising for heavy metal phytoremediation. This study aimed to elucidate the molecular mechanisms by which rhizobia enhance cadmium (Cd) tolerance of Robinia pseudoacacia.

Methods

We investigated two R. pseudoacacia genotypes with contrasting Cd accumulation capacity: SX (high-accumulating) and HB (low-accumulating). The study examined oxidative stress responses, reactive oxygen species (ROS) scavenging mechanisms, and changes in transcriptome and metabolome profiles following Cd exposure and rhizobium inoculation.

Results

Rhizobium inoculation effectively reduced H₂O₂ and MDA accumulation in both genotypes, enhancing their antioxidative substances, despite increased shoot Cd accumulation. Furthermore, rhizobium symbiosis promoted the accumulation of amino acids and carbohydrates, providing additional carbon resources and osmo-protective compounds to support Cd stress responses. Transcriptome analysis revealed elevated expression of metal transporter genes (ABC, OPT, HIP, DTX families) in SX shoots, leading to differential Cd accumulation between the genotypes. Under Cd exposure, SX plants exhibited significantly higher levels of flavonoid-related transcripts and metabolites, also antioxidant enzymes and substances compared to HB plants.

Conclusion

Rhizobium inoculation alleviates cadmium toxicity to R. pseudoacacia by mitigating oxidative stress and activating amino acid and carbohydrate metabolism, enhancing the antioxidant defense system and Cd tolerance of R. pseudoacacia. These findings provide new insights into the Cd tolerance mechanisms of the Robinia-rhizobium symbiosis and its utilization for phytoremediation and heavy metal pollution management.