Arbuscular mycorrhizal fungi alter carbohydrate distribution and amino acid accumulation in Medicago truncatula under lead stress

This study aims to understand the effect of arbuscular mycorrhizal (AM) inoculation on the regulation of carbon and nitrogen metabolites in Medicago truncatula plants under lead (Pb) stress. Root morphology, nutrient, photosynthesis, sucrose cleaving enzymes, the transcription levels of sucrose transporters, and metabolite profiling were evaluated via a two-factor (AM inoculation and Pb stress) pot experiment. AM inoculation alleviated the damage caused by Pb to photosynthesis and increased phosphorus uptake. AM inoculation decreased the shoot sucrose content but increased root sucrose content. The transcription of MtSUT1-1 and MtSUT4-1 in roots was upregulated by AM inoculation, which indicates that AM plant has a higher ability of sucrose transport. Both Pb stress and AM inoculation stimulated the activity of acid invertase and sucrose synthetase in roots. In shoots, minor amino acids such as cysteine were increased by AM inoculation. Sucrose content was negatively related to asparagine content. Therefore, AM inoculation may alleviate Pb toxicity by promoting the transport of sucrose from shoot to root, promoting the cleaving of sucrose in roots, and increasing minor amino acids accumulation.