Differences in the metabolites of brown and milled rice grains of semiwaxy and conventional japonica varieties

In this study, we analyzed the differences in metabolites between semiwaxy japonica rice (Yangnongxiang 28 [YNX28]) and conventional japonica rice (Hongyang 5 [HY5]) before and after brown rice milling. The metabolites of brown and milled rice grains from the two rice varieties were analyzed by LC-MS-based nontargeted metabolomics. A total of 266 differentially abundant metabolites (DMs) were tentatively identified in brown rice grains of YNX28 (YNX28B) compared with milled rice grains of YNX28 (YNX28H), and these included 248 upregulated and 12 downregulated DMs. A total of 273 (234 upregulated and 39 downregulated) DMs were tentatively identified in brown rice grains of HY5 (HY5B) compared with milled rice grains of this variety (HY5H). Kyoto Encyclopedia of Genes and Genomes pathway involved and enrichment analyses revealed that 53 and 7 metabolite pathways were enriched and significantly enriched (p < 0.05), respectively, in the DMs identified in YNX28B compared with YNX28H, and the main enriched pathways were related to starch and sucrose metabolism, glycerol phospholipid metabolism, arginine and proline metabolism, and glycine, serine and threonine metabolism. Forty-six metabolite pathways were enriched in DMs identified in HY5B compared with HY5H, and these included 16 pathways that were significantly enriched (p < 0.05); in addition, the main enriched pathways were related to starch and sucrose metabolism, glycerol phospholipid metabolism, arginine and proline metabolism, and glycine, serine and threonine metabolism. This study provides a theoretical reference for further on the changes in metabolites during rice processing and provides a basis for improving the nutritional quality in rice.