Arbuscular mycorrhizal symbiosis alleviates arsenic phytotoxicity in flooded Iris tectorum Maxim. dependent on arsenic exposure levels

Arsenic (As) pollution in wetlands has emerged as a serious global concern, posing potential threat to the growth of wetland plants. Arbuscular mycorrhizal fungi (AMF) can alleviate As phytotoxicity to host plants, but their ecological functions in wetland plants under flooding conditions remain largely unknown. Thus, a pot experiment was conducted using Rhizophagus irregularis and Iris tectorum Maxim. exposed to light (15 and 30 mg/kg As) and high (75 and 100 mg/kg As) levels of As, to investigate the intrinsic mechanisms underlying the effects of mycorrhizal inoculation on plant As tolerance under flooding conditions. The mycorrhizal colonization rates ranged from 31.47 ± 3.92 % to 60.69 ± 5.58 %, which were higher than the colonization rate (29.55 ± 13.60%) before flooding. AMF significantly increased biomass of I. tectorum under light As levels, together with increased phosphorus (P) and As uptake. Moreover, expression of arsenate reductase gene RiarsC and a trace of dimethylarsenic (1.87 mg/kg in shoots) were detected in mycorrhizal plants, suggesting As transformation and detoxification by AMF exposed to light levels of As. However, under high As levels, AMF inhibited As translocation from roots to shoots, and facilitated the formation of iron plaque. The immobilized As concentrations in iron plaque of mycorrhizal plants were respectively 1133.68 ± 179.17 mg/kg and 869.11 ± 248.90 mg/kg at 75 and 100 mg/kg As addition level, both significantly higher than that in non-inoculated plants. Irrespective of As exposure levels, mycorrhizal symbiosis decreased soil As bioavailability. Overall, the study provides insights into the alleviation of As phytotoxicity in natural wetland plants through mycorrhizal symbiosis, and potentially indicates function diversity of AMF under flooding conditions and As stress, supporting the subsequent phytoremediation and restoration of As-contaminated wetlands.