Sucrose and proline metabolisms are often associated with drought tolerance of plants. This study was conducted to investigate the effects of two arbuscular mycorrhizal fungi (AMF) species (Funneliformis mosseae and Paraglomus occultum) on root biomass, lateral root number, root sucrose and proline metabolisms in trifoliate orange (Poncirus trifoliata) seedlings under well-watered (WW) or drought stress (DS). All the AMF treatments significantly increased root dry weight, taproot length, and the number of lateral roots in 1st, 2nd, and 3rd class under WW and DS. Mycorrhizal seedlings conferred considerably higher fructose and glucose concentrations but lower sucrose accumulation, regardless of soil water status. Under DS, F. mosseae treatment significantly increased root sucrose synthase (SS, degradative direction) and sucrose phosphate synthase (SPS) activity but deceased root acid invertase (AI) and neutral invertase (NI) activity, and P. occultum inoculation markedly increased root AI, NI, SS, and SPS activities. AMF treatments led to a lower proline accumulation in roots, in company with lower activities of Δ1-pyrroline-5-carboxylate synthetase (P5CS), δ-ornithine aminotransferase (OAT), Δ1-pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (ProDH) in roots. It appears that the AM symbiosis induced greater root development and sucrose and proline metabolisms to adapt DS.

The development and quality of tea plants (Camellia sinensis (L.) O. Ktze.) are greatly hampered by drought stress (DS), which affects them in a number of ways, including by interfering with their metabolism of nitrogen (N). Arbuscular mycorrhizal fungi (AMF) are known to enhance water and nutrient absorption in plants, but their specific effects on tea plant N metabolism under DS and the associated regulatory mechanisms remain unclear. This study aimed to evaluate the impact of Claroideoglomus etunicatum inoculation on N assimilation in tea plants (C. sinensis cv. Fuding Dabaicha) under well-watered (WW) and DS conditions, and to explore potential molecular mechanisms. After 8 weeks of DS treatment, root mycorrhizal colonization was significantly inhibited, and the biomass of tea shoots and roots, as well as the contents of various amino acids (AAs) were reduced. However, AMF inoculation significantly increased the contents of tea polyphenols and catechins in leaves by 13.74%-36.90% under both WW and DS conditions. Additionally, mycorrhizal colonization notably increased N content by 12.65%-35.70%, various AAs by 11.88%-325.42%, and enzymatic activities associated with N metabolism by 3.80%-147.62% in both leaves and roots. Gene expression analysis revealed a universal upregulation of N assimilation-related genes (CsAMT1;2, CsAMT3;1, CsGS1, CsNADH-GOGAT, CsTS2, CsGGT1, and CsADC) in AMF-colonized tea roots, regardless of water status. Under DS condition, AMF inoculation significantly upregulated the expressions of CsNRT1;2, CsNRT1;5, CsNRT2;5, CsNR, CsGS1, CsGDH1, CsGDH2, CsTS2, CsGGT1, CsGGT3, and CsSAMDC in tea leaves. These findings suggest that AMF improved tea plant adaptability to DS by enhancing N absorption and assimilation, accompanied by the synthesis and accumulation of various AAs, such as Glu, Gln, Asp, Lys, Arg, GABA and Pro. This is achieved through the upregulation of N metabolism-related genes and the activation of related enzymes in tea plants under DS condition. These findings provide valuable insights into the role of AMF in regulating tea plant N metabolism and enhancing stress tolerance.