Rice seeds of different varieties hold different metabolic profiles [15]. In the study, we would reveal the differences among the six varieties via comparing the predominant metabolites (e.g., amino acids, sugars, and organic acids), especially the predominant DMs, and the main targeted metabolic pathways.
4.1 Metabolome-based global responses in rice seeds of different varieties
Our study revealed that amino acids, sugars and fatty acids played important roles in distinguishing rice seeds of CH and other varieties. In the study, metabolite profiles in CH could by discriminated from others based on PCA and PLS-DA (Figures 3 and 4), amino acids, sugars, and fatty acids were contained in the predominated DMs (Figures 6 and 7), and the main potential target metabolic pathways were amino acid metabolisms, and sugar metabolisms (Figure 7; Table 1). Similarly, Sun [16] found that there were significant differences in metabolite profiles between rice seeds of different varieties (landrace and cultivated rice seeds). While differing from our research results, Feng [17] found that GC-MS-based metabolite profiles in rice seeds of different varieties did not show significant differences in the same area (Chahayang Area, Heilongjiang Province). The differences might be caused by that varieties of the seed samples in these different studies were different.
Protein, starch and lipids were the main nutritious substances of rice seeds, the synthesis of which were closely related to amino acids, sugars, and fatty acids, respectively [18]. The variety-based different in contents of amino acids, sugars, and fatty acids suggested that contents of proteins, starch and lipids were different between CH and others (Figure 6). Rice of different varieties presented different photosynthetic characteristics and leaf nitrogen contents [19]. As the small molecule substances (e.g., amino acids, sugars, and fatty acids) required for the synthesis of proteins, starch and lipids in seeds mostly came from leaves, these might result in the significant differences of contents of amino acids, sugars, and fatty acids in rice seeds, leading to the differences in the accumulation of protein, starch and lipids (Figures 6 and 7) [19-21].
4.2 Amino acids played vital roles in distinguishing CH and others
The present study revealed that amino acids acted vital roles in discriminating rice seeds of different varieties, especially between CH and NX, and between CH and YX. Amino acids and amino acid derivatives were contained in the predominated DMs, especially phenylalanine and glycine (Figure 6), and most potential target metabolic pathways, including phenylalanine metabolism, glycine, serine and threonine metabolism, and alanine, aspartate and glutamate metabolism, were amino acid metabolisms of NX and YX (Figure 7; Table S3). Phenylalanine was essential amino acid of human, which must be obtained through dietary protein and participated in carbohydrate metabolism and fat metabolisms [22]; glycine, the simplest amino acid, was the primitive nutritional form in organisms, and participated in the synthesis of purines, porphyrins, creatine, and glyoxylate, acting as an important inhibitory neurotransmitter in the central nervous system [23, 24]. In the study, the content of phenylalanine in NX was more than twice that in CH, and glycine content of CH was more than 93 times that of YX. These suggested that, compared to CH, rice seeds of NX were more conducive to metabolism of carbohydrate and fat, and healthy growth maintenance of the human body, and compared to YX, rice seeds of CH was more suitable as potential glycine supplement.
Besides, the main storage protein glutenin and gliadin in CH might be lower than those both in YX and NX. Glutamic acid and aspartic acid hold high proportions in the synthesis of glutenin and gliadin, respectively, and were the foundation of the two main storage protein [25-27]. In the present study, glutamate and aspartate upregulated in rice seeds both of NX and YX (Figure 6; Table S1). These suggested that content of glutenin and gliadin in rice seeds of NX and YX were higher than those of CH. Similarly, Shi [28] sampled rice seeds of different varieties, and found that rice seeds of different varieties presented different content of storage proteins.
4.3 Sugars played vital roles in distinguishing different varieties
The study identified that sugars acted vital roles in discriminating rice seeds of different varieties, especially between CH and HM, between CH and HY, between CH and NX, and between CH and MX. The proportion of sugars and sugar derivatives were high in the predominated DMs, especially lactulose, and 1,5-anhydroglucitol (Figure 6; Table S1). Lactulose was a functional oligosaccharide, and an effective proliferation factor for bifidobacteria. It had special physiological functions such as bacterial proliferation, lowering cholesterol in the blood, improving blood lipids, and promoting calcium absorption [29-30]. In the study, lactulose contents of HM, HY, and MX were up-regulated. This suggested that rice seeds of HM, HY, and MX were more suitable as potential lactulose supplement, compared to those of CH.
Besides, rice seeds of CH might be more suitable for the patients with diabetes, compared to those of NX. 1,5-anhydroglucitol content in seeds of NX was more than 13 times that of CH (Figure 6; Table S1). 1,5-anhydroglucitol, one of the main polyol sugars in the human body, was mainly derived from food, and could be affected by dietary habit [31]. 99.9% of 1,5-anhydroglucitol in the body was reabsorbed by the kidneys [32]. When the blood sugar content was higher than the renal threshold in patients with diabetes, it would competitively inhibit the reabsorption of 1,5-AG in kidney, resulting in the increased excretion of 1,5-AG in urine and decreased content in serum [33]. The content of 1,5-anhydroglucitol in serum could be used as indicators for short-term blood glucose monitoring [34]. Eating food with a higher percentage of 1,5-anhydroglucitol, patients with diabetes might enhance the kidney reabsorption of 1,5-anhydroglucitol, and interfere with blood sugar monitoring [33].