Metabolomics combined with physiology and transcriptomics reveal the regulation of key nitrogen metabolic pathways in alfalfa by foliar spraying with nano-selenium

Selenium promotes plant growth and improves nutritional quality, and the role of nano-selenium in alfalfa in regulating nutritional quality is unknown. In this study, using the ¹⁵N labeling method, it was found that nano-selenium could promote plant nitrogen metabolism and photosynthesis by increasing the light energy capture capacity and the activities of key enzymes of the nitrogen metabolism process, leading to an increase in alfalfa nitrogen accumulation and dry matter content. The transcriptome and metabolome revealed that nano-selenium mainly affected the pathways of 'biosynthesis of amino acids', 'starch and sucrose metabolism', 'pentose and glucuronate interconversions', 'pentose phosphate pathway', and 'flavonoid biosynthesis'. At the early stage of nano-selenium treatment, the nitrogen metabolism, sugar metabolism, and flavonoid metabolism pathways were regulated by modulating the expression of genes such as NR, Nir, GS, GOGAT, E3.1.1.11, adh, CHS, FLS, etc., which increased the amount of L-glutamic, L-histidine, glycerone-P, coniferin, naringenin chalcone, and other beneficial substances, thus promoting the acceleration of nitrogen accumulation by plants. In summary, this study provides a better understanding of the mechanisms by which nano-selenium regulates key nitrogen metabolic pathways in alfalfa.