Increasing atmospheric CO₂ differentially supports arsenite stress mitigating impact of arbuscular mycorrhizal fungi in wheat and soybean plants

Arbuscular mycorrhizal fungi (AMF) are beneficial for the plant growth under heavy metal stress. Such beneficial effect is improved by elevated CO₂ (eCO₂). However, the mechanisms by which eCO₂ improves AMF symbiotic associations under arsenite (As^III) toxicity are hardly studied. Herein, we compared these regulatory mechanisms in species from two agronomical important plant families – grasses (wheat) and legumes (soybean). As^III decreased plant growth (i.e., 53.75 and 60.29% of wheat and soybean, respectively) and photosynthesis. It also increased photorespiration and oxidative injury in both species, but soybean was more sensitive to oxidative stress as indicated by higher H₂O₂ accumulation and oxidation of protein and lipid. eCO₂ significantly improved AMF colonization by increasing auxin levels, which induced high carotenoid cleavage dioxygenase (CCDs) activity, particularly in soybean roots. The improved sugar metabolism in plant shoots by co-application of eCO₂ and As^III allocated more sugars to roots sequentially. Sugar accumulation in plant roots is further induced by AMF, resulting in more C skeletons to produce organic acids, which are effectively exudated into the soil to reduce As^III uptake. Exposure to eCO₂ reduced oxidative damage and this mitigation was stronger in soybean. This could be attributed to a greater reduction in photorespiration as well as a stronger antioxidant and detoxification defence systems. The grass/legume-specificity was supported by principal component analysis, which revealed that soybean was more affected by As^III stress and more responsive to AMF and eCO₂. This study provided a mechanistic understanding of the impact of AMF, eCO₂ and their interaction on As-stressed grass and legume plants, allowing better practical strategies to mitigate As^III phytotoxicity.