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

Arbuscular mycorrhizal (AM) fungus, <i>Rhizophagus irregularis</i>, enhances plant drought tolerance by promoting nitrogen (N) and phosphate (Pi) uptake. The <i>RiCARI</i> 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 <i>RiCARI</i> using host-induced gene silencing, targeted N metabolomics, and gene expression analysis. The results revealed that <i>RiCARI</i> silencing suppressed arginine and proline metabolism, lysine degradation, and glycine, serine, and threonine metabolism in roots under drought stress (DS). Furthermore, <i>RiCARI</i> silencing reduced Pi uptake in mycorrhizal plants by downregulating the expression of the vacuolar transporter gene (<i>RiVTC1</i>/2/4), phosphatase genes (<i>RiPPX1</i> and <i>RiPPN1</i>/<i>1-1</i>/<i>1-2</i>), and the phosphate transporter gene (<i>RiPT7</i>) in the mycorrhizal Pi transport pathway, resulting in polyphosphate accumulation in arbuscules. These findings demonstrate that <i>RiCARI</i> is essential for regulating both N metabolism and Pi acquisition in the roots of mycorrhizal plants under DS.