Data quality control and qualitative and quantitative metabolite analysis
The overlapping display analysis for the total ion flow which was detected and analyzed by mass spectrometry for the quality of the control samples (QC), showed that the overlap was high in both the positive and negative ion modes (Fig. S1a and b). This provides an important guarantee for the authenticity and reliability of the data. According to the PCA score diagram (Fig. 1a) and the sample correlation diagram (Fig. S1c), the Pearson correlation coefficients in the four sample groups are R > 0.8, and the overall repeatability is good. Among them, the contribution values of PC1 and PC2 are 26.34% and 22.1%, respectively. There are obvious differences among the groups, which is of research significance. The results showed that 1078 metabolites were detected, including 95 amino acids and their derivatives, 178 phenolic acids, 67 nucleotides and their derivatives, 234 flavonoids, 30 lignans and coumarins, 3 tannins, 111 alkaloids, 22 terpenoids, 84 organic acids, 154 lipids, and 100 others. A heat map showing all the metabolites classified by substance is shown in Fig. 1b. The substances with many metabolites that may vary in each group are the flavonoids, organic acids, lipids, and alkaloids. Before the difference analysis, the samples of each group were analyzed by opls-da. The opls-da score diagram and opls-da verification diagram (Fig. S2) showed that there were significant differences among the groups, especially N-G vs P-G and P-K vs P-G, Q2, which were greater than 0.85. Most of the P values for the established models were < 0.05, and their prediction ability was better.
Analysis of metabolite differences among the combinations
The obtained multivariable analysis of the variable importance in projection (VIP) from the opls-da model was combined with the fold change to further screen out the differential metabolites. Metabolites with fold changes ≥ 2 or ≤ 0.5 and VIP ≥ 1 were regarded as differential metabolites. The quantities of the differential metabolites between the different combinations are shown in Table S1.
Taking N-G vs P-G, N-K vs P-K, and P-K vs P-G as the key research objects, a metabolite-differences diagram (Fig. 2) shows the relative content difference and statistical significance of the metabolites in different combinations, and the differences in multiple histograms (Fig. S3) show the top ten metabolites with up-down differences in each combination. The results show that there are 200 difference metabolites in N-G vs P-G (up 133; down 67). There are 34 alkaloids in the up-regulated substances, and phenolic acids and flavonoids were the most down-regulated substances, and these included vitexin-7-O-glucoside, N-p-coumaroyl-N'-feruloylputrescine, p-coumaroylferuloylputrescine, N-feruloylserotonin, and LysoPC 20:0, while down-regulated substances included DIMBOA, neochlorogenic acid, 5-O-caffeoylquinic acid*, 3-O-p-coumaroylquinic acid*, and 4-acetoxy-3-ethoxybenzaldehyde. There are 223 differential metabolites (up 140; down 83) in P-K vs P-G. Lipids and alkaloids account for more up-regulation, and include L-cysteine, kaempferol, heptadecanoic acid, 2'-O-methyladenosine, and cis-moschamine. There are more phenolic acids and flavonoids that were down-regulated. The first 10 down-regulated phenolic acids are mostly the same as N-G vs P-G. There were other down-regulations, such as for 13s-hydroperoxy-6z, 9z, 11e-octadecaterinoic acid 6-methoxy-2-4h-chrome-4-one, lysoPC 19:2 (2n isomer), and lysoPC 19:2. There were 138 differential metabolites in N-K vs P-K (up 60; down 78). The most up-regulated substances were phenolic acids, such as 1-O-p-hydroxycinnamoyl-3-O-caffeoylglycerol, vitexin-7-O-glucoside, and 3-indolepropionic acid. The most down-regulated substances were phenolic acids and flavonoids, such as L-cysteine, acetate, and benzoylmalic acid. There are 18 differential metabolites (up 8; down 10) in N-K vs N-G. Up-regulated substances included pimaric acid, 3-hydroxydecanoic acid, and 3-indolepropionic acid, and down-regulated substances include epicatechin gallate*, palmitoleic acid, and the glutathione (reduced form). In conclusion, phenolic acids, alkaloids, and flavonoids were found to be closely related to the resistance of iron shell wheat to powdery mildew.
K-means analysis
The relative levels of all the differential metabolites identified according to the screening criteria in all grouping comparisons were standardized by Z-score, and then the changes in the trends of the nine differential metabolites were obtained after K-means cluster analysis (Fig. 3a). The results showed that there were 76 differential metabolites in sub-class 2, 59 in sub-class 5, and 23 in sub-class 7. In sub-class 4, the metabolite content of P-G was significantly lower than that of P-K and n-k, and this included L-alanine, N-propionylglycine, and chlorogenic acid. In sub-class 2, sub-class 7, and sub-class 8, the metabolite content in P-G was significantly higher than that in P-K and n-k, and this included L-tryptophan, 2-piperidone, indole, agmatine, and scutellarin.
The statistics for the different metabolites among the groups and the relationships between the different metabolites in each group are shown in a Venn diagram (Fig. 3b). The results show that there are 389 significantly different metabolites in total, and 64 specifically different metabolites (up 39; down 25) in N-G vs P-G. Alkaloids and flavonoids are more abundant, such as indole, spermidine, and some benzoxazine metabolites such as 6-hydroxy-2'-methoxyflavone, and syringen-7-O-glucoside. There were 98 specific metabolites (up 48; down 59) in P-K vs P-G, and the most abundant were flavonoids, such as kaempferol and eriodictyol. There were 48 specific differential metabolites (up 22; down 26) in N-K vs P-K, and most substances were phenolic acids; N-K vs N-G had 6 specific differential metabolites (up 4; down 2). A common differential metabolite among the four combinations was lysophosphatidylcholine 20:0 (2n isomer).There are 10 specific metabolites in the N-G vs P-G, P-K vs P-G, and N-K vs P-K groups. Such as 7-O-methylnaringenin, L-dopa, neochlorogenic acid, and 3-methyl-1-pentanol.
Differential metabolite enrichment analysis using the KEGG pathway
KEGG pathway enrichment analysis was performed according to the results of the differential metabolites, and a P value ≤ 0.05 indicated that the pathway was enriched and annotated with a significant difference. The top 20 metabolic pathways with enrichment degree in different combinations are shown in Fig. 4. The results show that 64 metabolic pathways were enriched and annotated in N-G vs P-G, and three of them were significantly different, namely tryptophan metabolism, pyrimidine metabolism, and isoquinoline alkaloid biosynthesis. There were 7 significantly different metabolites in tryptophan metabolism and all of them were up-regulated. Indole, 3-indoleacetonitrile, tryptamine, 2-benzoic acid, N-acetylisatin, and L-tryptophan, methoxyindoleacetic acid, in pyrimidine metabolism; there were 7 significantly different metabolites, and only 2-deoxyribose-1-phosphate was down-regulated. A total of 57 metabolic pathways were enriched and annotated in P-K vs P-G, and the three significant differences were pyrimidine metabolism, folate biosynthesis, and alpha-linolenic acid metabolism. There were 8 significantly different metabolites in the pyrimidine metabolism. Compared with the pyrimidine metabolism of N-G vs P-G, one more metabolite was cytidine 5'-monophosphate. The other 7 metabolites were identical with the same expression trends, such as cytosine, orotic acid, thymidine, and cytidine. There were 52 metabolic pathways enriched and annotated in N-K vs P-K, of which 11 were significantly different. The top 3 pathways were the sulfur relay system, biosynthesis of amino acids, and lysine degradation, respectively. In the sulfur relay system, three different metabolites were annotated and down-regulated, including L-cysteine, O-acetylserine, and S-adenosyl-l-methionine. In the biosynthesis of amino acids, 9 differential metabolites (up 4; down 5) were annotated. The up-regulated were L-proline, L-ornithine, phosphoenolpyruvate, and D-erythrose-4-phosphate, while the down-regulated included L-cysteine, anthranilic acid, and α -ketoglutaric acid. A total of 25 metabolic pathways were enriched and annotated in N-K vs N-G, and 6 were significantly different, including acridone alkaloid biosynthesis, nitrogen metabolism and biotin metabolism.