ITRAQ-based proteomic analysis provides insights into the biological mechanism of ammonium metabolism in tea plant (Camellia sinensis L.)

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Tea plant prefers ammonium as the dominant inorganic nitrogen source. However, the mechanism behind this preference has not been fully understood. To obtain a better understanding of the preferential assimilation of ammonium in tea, iTRAQ-based proteomics was conducted between 0 and 12 h after ammonium treatment of tea leaves. The quality-related components were positively affected after 12 h, with the highest contents of the four main amino acids and with the primary catechins beginning to decrease. Changes of catechins and differentially expressed proteins suggested that contents of catechins in tea could affect the availability of carbon skeletons needed in photosynthesis and energy metabolism. The biosynthesis of flavonoid was related to ammonium through the regulation of phenylalanine ammonia-lyase (PAL) gene expression and the enzyme activity. After the application of ammonium, the expression of PAL protein and its enzyme activity were both inhibited to decrease the release of ammonium during the reaction converting l-phenylalanine to ammonia and trans-cinnamic acid. Furthermore, to accelerate the metabolism of ammonium, the expression of glutamine synthetase and its isogenes was enhanced, as well as glutamine synthetase activity. Hence, during the ammonium assimilation process, tea plants likely increase the activity of glutamine synthetase and inhibit PAL to balance the ammonium content. These results provide insights into the metabolism of ammonium nutrition orchestrated by primary and secondary metabolic pathways via glutamine synthetase and PAL in tea plant.