Arbuscular mycorrhizal symbiosis enhances the drought tolerance of Populus cathayana by modulating glutamate and lysine metabolism

Arbuscular mycorrhizal fungi (AMF) can effectively increase plant drought tolerance by promoting nitrogen (N) uptake. However, the regulatory pathways through which AMF-induced additional N uptake affects plant drought tolerance remain poorly understood. In this study, we investigated the regulation of amino acid metabolic pathways in Populus cathayana under drought stress (DS) by the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis. We found that AM symbiosis improved chlorophyll fluorescence parameters and root system characteristics, decreased electrolyte leakage, leaf H₂O₂ content and root malondialdehyde content in P. cathayana under DS, increased the contents of key intermediates in the 5-aminolevulinic acid (ALA) pathway (leaf ALA, Proto IX, chlorophyll a and heme), proline synthesis pathway (root glutamate, proline, pipecolic acid, and α-aminoadipate), and associated regulatory genes (leaf PcGluTR, PcPBGD, PcFECH1, PcFECH2 and PcHO1; root PcLKR, PcSDH, PcP5CS1 and PcP5CS2) expression. These results indicated that AM symbiosis improved photosynthesis and alleviated the oxidative damage caused by DS, thereby increasing drought tolerance in plants. These effects are attributed to AMF inoculation promoting chlorophyll and heme synthesis by regulating the ALA pathway in glutamate metabolism in leaves and promoting the synthesis of proline, pipecolic acid, and α-aminoadipate by regulating the proline synthesis pathway in glutamate metabolism and the saccharopine pathway in lysine metabolism in the roots. These findings reveal the potential regulatory mechanisms by which AMF inoculation aids the leaves and roots of plants in coping with DS through enhanced N uptake, providing a theoretical basis for applying AMF to improve plant growth under DS conditions.