Arbuscular Mycorrhizal Fungi Alleviate Nitrogen Stress in Tea Seedlings Through Integrated Physiological and Molecular Responses

Nitrogen (N) availability strongly influences tea plant (Camellia sinensis (L.) O. Ktze. (C. sinensis)) productivity through its regulation of N metabolism, yet the role of arbuscular mycorrhizal fungi (AMF) in mediating N metabolism in tea plants under N-deficient conditions remains poorly understood. We examined the effects of inoculating C. sinensis cv. Fuding Dabaicha seedlings with Claroideoglomus etunicatum (C. etunicatum) on growth, tissue N status and N-metabolism traits under N-sufficient (1.43 mM NH4NO3) and N-deficient (0 mM NH4NO3) conditions. After 12 weeks, N deficiency suppressed mycorrhizal colonization and shoot growth, reduced the foliar accumulation of amino acids and carbohydrates, and lowered N-metabolism enzyme activities in leaves and roots, while stimulating root development and increasing leaf phenolic content. Inoculation with C. etunicatum increased seedling growth and biomass accumulation, improved root system architecture and enhanced key N-metabolism enzyme activities under both N levels, and altered several leaf metabolites. Under N deficiency, AMF inoculation upregulated the root expression of CsAMT1.1, CsAMT1.2, CsAMT1.3, CsAMT1.4 and CsNRT1.2 and downregulated CsNRT3.1, whereas CsAMT3.1 and CsNRT2.4 responded to N deficiency itself rather than to AMF inoculation. These findings indicate that AMF inoculation was associated with changes in N metabolism and improved N status in tea seedlings under N deficiency; direct effects on N uptake and assimilation rates remain to be quantified.