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Arbuscular mycorrhizal fungi (AMF) are present in paddy fields, where they suffer from periodic soil flooding and sometimes shading stress, but their interaction with rice plants in these environments is not yet fully explained. Based on two greenhouse experiments, we examined rice-growth response to AMF under different flooding and/or shading regimes to survey the regulatory effects of flooding on the mycorrhizal responses of rice plants under different light conditions. AMF had positive or neutral effects on the growth and yields of both tested rice varieties under non-flooding conditions but suppressed them under all flooding and/or shading regimes, emphasizing the high importance of flooding and shading conditions in determining the mycorrhizal effects. Further analyses indicated that flooding and shading both reduced the AMF colonization and extraradical hyphal density (EHD), implying a possible reduction of carbon investment from rice to AMF. The expression profiles of mycorrhizal P pathway marker genes (GintPT and OsPT11) suggested the P delivery from AMF to rice roots under all flooding and shading conditions. Nevertheless, flooding and shading both decreased the mycorrhizal P benefit of rice plants, as indicated by the significant decrease of mycorrhizal P responses (MPRs), contributing to the negative mycorrhizal effects on rice production. The expression profiles of rice defense marker genes OsPR1 and OsPBZ1 suggested that regardless of mycorrhizal growth responses (MGRs), AMF colonization triggered the basal defense response, especially under shading conditions, implying the multifaceted functions of AMF symbiosis and their effects on rice performance. In conclusion, this study found that flooding and shading both modulated the outcome of AMF symbiosis for rice plants, partially by influencing the mycorrhizal P benefit. This finding has important implications for AMF application in rice production.

Rice (Oryza sativa L.) is the most widely consumed staple crop for approximately half of the world’s population. Many interactions take place in paddy soil, particularly in the rice mycorrhizosphere region. Arbuscular mycorrhizal fungi (AMF) and soil microbe interactions are among the most important and influential processes that occur, as they significantly influence the plant growth and soil structure properties. Their interactions may be of crucial importance to the sustainable, low-input productivity of paddy ecosystems. In this study, we summarize the major groups of microbial communities interacting with arbuscular mycorrhizal fungi in the rice mycorrhizosphere, and discuss the mechanisms involved in these arbuscular mycorrhizal fungi and microbe interactions. We further highlight the potential application of arbuscular mycorrhizal mutualism in paddy fields, which will be helpful for the production of bioinoculants in the future.