Strategies for plant-microbe symbiosis: Mycorrhizal fungi and helper bacteria to improve cold tolerance in rice.

Cold stress is a limiting factor for rice yield. Empirical evidence has demonstrated that arbuscular mycorrhizal fungi (AMF) can bolster the cold resilience of plants. In barren environments, AMF can promote host plant growth and resistance. However, whether the addition of mycorrhizal helper bacteria (MHB) can further enhance AMF's ability to improve cold tolerance in plants remains unclear. In this study, we set up an uninoculated group, a separately inoculated group, and a compound inoculated group and incubated rice at 25 °C until the three-leaf stage, and then each group was equally divided into four portions for treatment at 25 °C, 12 °C, 8 °C, and 4 °C, respectively. The results showed that: (1) Under cold stress conditions, the biomass of rice plants inoculated with AMF was significantly higher than that of the non-inoculated group; (2) AMF and MHB effectively activated the antioxidant enzyme system in rice plants and improved their osmoregulatory capacity under cold stress; (3) The presence of AMF and MHB stimulated and modulated the upregulation of genes related to photosynthesis and cold tolerance in rice plants, thereby enhancing their resilience against cold stress. Our findings corroborate that MHB can further enhance the cold tolerance of rice by promoting the functions of AMF. This study lays the foundation for expanding rice cultivation areas, and ensuring food production security.