Abstract
BACKGROUND: Arbuscular mycorrhizal (AM) fungi can promote plant growth and improve plant nutrition. However, how AM fungi affect plant nitrogen uptake and assimilation through their influence on related gene regulation and enzyme systems remain poorly understood. This study investigated the effects of AM fungi on biomass, nitrogen uptake, nitrogen-assimilating enzyme activities, and the expression of genes related to nitrogen assimilation and nitrogen transport in Camellia oleifera under varying nitrogen levels, from deficit to excessive. RESULTS: Plant growth initially increased but then decreased with increasing nitrogen application level. The addition of nitrogen increased the concentrations of NH(4)(+) and NO(3)(-), as well as the activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT). This suggests that under excessive nitrogen levels, the increased enzyme activity was insufficient to counteract the rapid accumulation of inorganic nitrogen, leading to a nitrogen metabolism imbalance and inhibiting plant growth. Inoculation with AM fungi increased the biomass. While AM fungi had no significant effect on the nitrogen concentrations in roots, stems, or leaves, they significantly increased total plant nitrogen content and nitrogen use efficiency (NUE) by promoting biomass accumulation. Inoculation with AM fungi increased NR, GS, and GOGAT activities and up-regulated the expression of CoNRT, CoAMT1.1b, CoAMT1.2, and CoAMT3.1a in leaves. The NO(3)(-) concentration in roots and stems decreased in response to AM fungi under excessive nitrogen. CONCLUSIONS: The results demonstrated that plant nitrogen assimilation and metabolism were enhanced by AM fungi. This enhancement promoted plant growth under optimal nitrogen concentrations, and under excessive nitrogen levels, it led to reduced excessive accumulation of inorganic nitrogen while mitigating high-nitrogen-induced stress. This study establishes a foundational understanding of the role of AM fungi in the nitrogen uptake and metabolism of plants.