Rhizophagus irregularis RiCARI Enhances Drought Tolerance by Regulating Nitrogen Metabolism and Phosphate Acquisition in Mycorrhizal Plants

Arbuscular mycorrhizal (AM) fungus, Rhizophagus irregularis, enhances plant drought tolerance by promoting nitrogen (N) and phosphate (Pi) uptake. The RiCARI gene in AM fungi has been identified as playing a critical role in mycorrhizal N transport and drought tolerance; however, its regulatory mechanisms remain poorly understood. This study investigated the function of RiCARI using host-induced gene silencing, targeted N metabolomics, and gene expression analysis. The results revealed that RiCARI silencing suppressed arginine and proline metabolism, lysine degradation, and glycine, serine, and threonine metabolism in roots under drought stress (DS). Furthermore, RiCARI silencing reduced Pi uptake in mycorrhizal plants by downregulating the expression of the vacuolar transporter gene (RiVTC1/2/4), phosphatase genes (RiPPX1 and RiPPN1/1-1/1-2), and the phosphate transporter gene (RiPT7) in the mycorrhizal Pi transport pathway, resulting in polyphosphate accumulation in arbuscules. These findings demonstrate that RiCARI is essential for regulating both N metabolism and Pi acquisition in the roots of mycorrhizal plants under DS.