2.2 1H NMR metabolites profile of leaf tissue from parental oil palms
1H NMR analysis was performed on the leaf tissue of palm oil samples (Table 1) to assess the differences in their metabolite compositions. In characterizing 1H NMR spectra, the ASICS package was utilized to obtain metabolite data. Twenty-nine (29) types of metabolites were tentatively identified from 7 categories of disease resistance against Ganoderma. Each category has a different number of metabolites identified, there were 25 metabolites identified from susceptible P and moderate P, 24 metabolites from moderate D, 23 metabolites from resistant D, 21 metabolites from resistant T/P, and 19 metabolites from moderately resistant DxP and susceptible D (Supplementary Table 2). The number of types of metabolites discovered in this study was similar to Isha et al. (2019), which identified 22 metabolites from oil palm leaf tissue.
The metabolites relative concentrations were obtained through enrichment analysis and then classifying the metabolites in this study into 11 main metabolite classes of organic compounds: benzamides, fatty alcohols, organic dicarboxylic acids, alcohols, polyols, sulfonic acids, cholines, furanones, alkanolamines, fatty acids and conjugates, amino acids and peptides, and monosaccharides. The results showed monosaccharides, amino acid, and fatty acid compound classes were dominated in all categories of disease resistance (Fig. 2). Resistant T/P has the highest relative percentage of monosaccharides (37%) compared to other categories, while moderately resistant DxP has the lowest relative percentage of monosaccharides (19%). Susceptible P has the highest relative percentage of amino acids (23%) compared to other categories, while Resistant T/P has the lowest relative percentage (16%). Moderately resistant DxP. Resistant T/P has the highest relative percentage of fatty alcohols (16%) compared to other categories, both susceptible D and moderately resistant DxP have a relative percentage of fatty alcohols 13%, while other categories have a relatively lower percentage of fatty alcohols (5% for moderate D, resistant D, susceptible P, and moderate P). On the other hand, moderate D, resistant D, susceptible P, and moderate P equally have the highest relative percentage of fatty acids (19%) compared to moderately resistant DxP (6%) and susceptible D and resistant T/P were not present. Moderately resistant DxP did not have the highest relative percentage of each compound, but it has the most complete class of compounds.
Compared with previous research on 1H NMR analysis of oil palm resistance to Ganoderma, this research revealed different relative metabolites. The results of 1H NMR analysis of leaf tissue of oil palm seedlings infected with Ganoderma (14 dpi and 30 dpi) obtained 10 main metabolite classes of organic compounds: carboxylic acids and derivatives, organooxygen compounds, benzene and substituted derivatives, biotin and derivatives, organonitrogen compounds, hydroxy acids and derivatives, flavonoids, indoles and derivatives, prenol lipids, and steroids and steroid derivatives (Isha et al. 2019; Isha et al. 2020). Meanwhile, the results of 1H NMR analysis of stem tissue of oil palm infected with Ganoderma in the fields obtained 10 main metabolite classes of organic compounds: polyketide, nucleic acids, organooxygen compounds, fatty acyls, organonitrogen compounds, organoheterocyclic compounds, benzenoids, carbohydrates, and organic acids (Pancoro et al. 2022).
Different developmental stages and different specific tissues will influence metabolite accumulation (Shen et al. 2023). Therefore, different tissues sampled in this study might affect the metabolite variation observed compared to previous research published. This phenomenon is caused by the spatiotemporal expression of metabolites, as occurs in some plants, such as barley, blueberries, poplar, and ginseng (Gupta et al. 2019; Günther et al. 2020; Balasubramanian et al. 2023; Sun et al. 2021). Other than that, the variances in the metabolites relative concentrations may be due to the solvent used. Only methanol was used in this study, while the previous research used a mixture of methanol and water. Methanol is effective in dissolving phenolics, flavonoids, saponins, and polar organic compounds. On the other hand, water is effective in dissolving salts, ions, carbohydrates, amino acids, and organic acids (Oh et al. 2021). This could be the reason why there were fewer organic acid compounds in this study compared to previous research.
2.3 Classification and identification of metabolites from parental palms and moderately resistant DxP
Multivariate data analysis was used to determine the compound variation of each parental palm and their resistance towards Ganoderma. The pairwise 3D and 2D PCA results of Fig. 3 and Supplementary Fig. 3 were able to classify visually between parental palms and their resistance level with principal components (PC1, PC2, and PC3) value of 63.5%. The paired PCA analysis successfully separated between resistant and susceptible parental palms. Resistant D, resistant T/P, moderate P, and moderately resistant DxP relatively in one group. The metabolite content of moderately resistant DxP showed similarities to the resistant parental palms group. The results were in line with Wening et al. (2020), in the construction of Ganoderma resistant oil palm material, the selected crosses were derived from a resistant female parent (Dura) and a resistant male parent (Pisifera). As supporting data, pairwise 2D PCA between susceptible D and moderately resistant DxP, a distinction was observed, although there was still an overlap between the two groups, the cumulative total of the principal components was 82.5%. Meanwhile, pairwise 2D PCA between susceptible P and moderately resistant DxP also observed a distinction with the cumulative total of the principal components was 69.4% (Supplementary Fig. 4).
Heatmap was used to picture the different concentrations of each metabolite from different types of parental palms and their resistance levels against Ganoderma. The heatmap results showed the same pattern as PCA, moderately resistant DxP grouped with resistant parental palms (Fig. 4), although the individual heatmap results showed that there were a few outliers found, such as one replication of resistant T/P which was grouped with moderate and susceptible groups (Supplementary Fig. 3). There were upregulated metabolites such as taurine, glycerol, D-mannose, D-glucose, guanidino acetic acid, xylitol, and ascorbic acid in the resistant palm group, while in the susceptible palms, the metabolites were down-regulated. Consistent with Pancoro et al. (2022), taurine, D-mannose, xylitol, guanidino acetic acid, and ascorbic acid were up-regulated in healthy palms compared to severe palms caused by Ganoderma. On the contrary, threonic acid, L-arabitol, and L-serine were down-regulated in resistant palms compared to susceptible palms.
PLS-DA analysis was utilized to differentiate significant metabolites in resistant, susceptible parental palms, and moderately resistant DxP. A total of 11 metabolites were identified as significant with VIP scores > 1 (Fig. 5). The following metabolites were identified: taurine, betaine, TMAO, glycerol, threonic acid, and L-serine, which belong to the class of monosaccharide and amino acid compounds. Three significant metabolites (taurine, D-mannose, and threonic acid) were also detected in the PLS-DA results of Pancoro et al. (2022) in the response of oil palm to Ganoderma. Taurine plays a role as an anti-stress agent in several plants (Lee et al. 2015; Hafeez et al. 2022). D-mannose belongs to the lectin family, responsible for the action of resistance when infected with pathogens (Hwang and Hwang 2011). Threonic acid and myo-inositol were reported to play a role in the plant response to the endophytic fungus Cladosporium tenuissimum (Zhou et al. 2022). In this study, two categories of resistance were also compared using OPLS-DA. The aim was to identify significant metabolites for both categories (Supplementary Fig. 5). Based on the analysis, 8, 12, 10, 8, 11, and 12 significant metabolites were identified for categories D sus – D mod, D sus – D res, D sus – DxP modres, P sus – P mod, P sus – P res, and P sus – DxP modres respectively (VIP score > 1.0).
2.4 Biomarkers and pathway analysis for oil palm resistance to Ganoderma
The use of biomarkers is becoming more critical in the early detection of diseases (Wang and Huang 2019). This study used OPLS-DA and ROC to analyze the oil palm metabolites that can differentiate oil palm resistance against Ganoderma. The OPLS-DA results and the ROC curve (AUC) = 1 can be utilized as biomarkers. An AUC score of 1 indicates that the classifier can effectively distinguish between positive and negative classes, without allowing for false positives (Pancoro et al. 2022). Several metabolites were indicated as significant biomarkers (Table 2). Interestingly, glycerol and ascorbic acid were found to have a significant on susceptible D and susceptible P. They can be used as potential biomarkers for screening moderately resistant DxP. Furthermore, the box plot visualization was used to highlight a significant difference in concentrations of ascorbic acid and glycerol (Fig. 6).
Glycerol is known to be involved in the plant defense system, mainly in the form of glycerol-3-phosphate (G3P). G3P acts as a regulator that moves freely in the systemic acquired resistance (SAR), which will give broader spectrum immunity against pathogen infection in soybeans (Shine et al. 2019). In Arabidopsis increased G3P levels are associated with enhanced resistance to Colletotrichum higgisianum fungi (Chanda et al. 2008). Even the application of glycerol in wheat could increase their resistance to powdery mildew (Li et al. 2020). The same role is also played by ascorbic acids. Ascorbic acid is known to have a role in protecting plant cells from pathogen attacks that induce oxidative stress (Boubakri 2017). In vitro studies, showed the growth of colonies of the soil-borne hemibiotrophic fungus Macrophomina on media containing ascorbic acid decreased significantly compared to media that did not contain ascorbic acid (Noor and Little 2022).
Different biomarker results were obtained when using LC-MS, which identified pyridine, chelidonic acid, and asparagine as metabolites that determined the resistance of oil palm to Ganoderma (Nusaibah et al. 2016; Dzulkafli et al. 2019). Metabolomic studies using GC-MS also discovered different metabolites that determine oil palm resistance against Ganoderma, namely steroidal compounds and fatty acids with their derivatives and mannose with xylose by (Isha et al. 2020; Rozali et al. 2017). These differences can be caused by measuring instruments or differences in the tissue being analyzed.
Table 2
List of metabolite compounds from OPLS-DA analysis of oil palm leaves samples of Ganoderma resistant, moderate, and susceptible parental palms and moderately resistant DxP. The asterisk (*) indicates that the metabolite has AUC = 1 and has the potential as a candidate biomarker.
No. | Metabolite | VIP scores between Dsus with | VIP scores between Psus with |
Dmod | Dres | DxP modres | Pmod | T/Pres | DxP modres |
1 | 2-Oxoisovalerate | 1.70* | 1.16* | - | 1.50 | 1.46 | 1.50* |
2 | 2-Propanol | 0.93 | 1.36* | 1.35* | 1.16 | 0.52 | 1.08 |
3 | alpha-Hydroxyisobutyric Acid | 0.46 | 0.65 | 0.95 | 0.81 | 1.57 | 1.18 |
4 | Ascorbic Acid | 0.52 | 1.56* | 1.44* | 1.32 | 0.86 | 1.67* |
5 | Betaine | 0.32 | 0.56 | 0.68 | 1.38 | 1.13 | 0.99 |
6 | Choline Chloride | 0.92 | 0.05 | 0.74 | 0.02 | 0.02 | 0.04 |
7 | D-Sorbitol | 0.22 | 1.11 | 1.04 | 0.84 | 0.19 | 1.40* |
8 | Ethanolamine | 0.35 | 0.53 | 0.14 | 0.83 | 0.95 | 0.13 |
9 | Galactitol | 0.60 | 0.29 | 0.94 | 0.56 | 0.59 | 0.38 |
10 | Glycerol | 1.22 | 0.57 | 1.51* | 0.87 | 0.35 | 1.69* |
11 | Guanidinoacetic Acid | 0.22 | 0.32 | 0.62 | 0.23 | 0.67 | 1.12 |
12 | Isovaleric Acid | - | 1.16* | - | 1.17 | 0.52 | 0.29 |
13 | L-Alanine | - | - | - | 0.42 | 0.52 | 0.39 |
14 | L-Arabitol | 0.27 | 1.41* | 1.10 | 1.03 | 1.03 | 1.56* |
15 | L-Serine | 0.54 | 0.57 | 1.30* | 1.34 | 1.50 | 1.24 |
16 | Malonate | 0.64 | 0.56 | 0.41 | 0.91 | 0.54 | 0.80 |
17 | Methylmalonic Acid | 0.98 | 0.90 | 0.34 | 1.29 | 1.02 | 0.01 |
18 | Myo-Inositol | 0.77 | 0.25 | 0.79 | 0.31 | 0.87 | 0.11 |
19 | N-Acetylglycine | 1.57* | 1.16* | - | 0.27 | 0.52 | 0.29 |
20 | Propylene Glycol | 1.70* | 1.29* | 1.36* | 0.54 | 1.34 | 0.61 |
21 | Taurine | 1.78* | 1.42* | 1.10* | 0.84 | 1.70 | 1.33 |
22 | Threitol | 1.47 | 1.18 | 0.95 | 0.27 | 1.05 | 0.93 |
23 | Threonic Acid | 1.03 | 1.69* | 1.20* | 0.88 | 0.62 | 0.53 |
24 | TMAO | 0.42 | 0.49 | 0.12 | 1.40 | 1.07 | 0.18 |
25 | Xylitol | 0.01 | 1.11 | 1.12 | 0.63 | 0.29 | 1.39* |
Pathway analysis was used to determine the metabolite pathways involved in oil palm resistance to Ganoderma. Based on pathway analysis using the KEGG database (p-value < 0.05 and pathway impact > 0.1), BSR disease may affect two metabolite pathways: glycine, serine, and threonine metabolism and the taurine and hypotaurine metabolism (Fig. 7). Pathway visualization using KEGG of the metabolites involved was presented in (Supplementary Fig. 6). The discovery of the glycine, serine, and threonine metabolism pathways in this study was in line with the findings of Pancoro et al. (2022). However, L-serine and threonic acid as stated earlier were lower in resistant parental palms. In contrast, taurine was higher in the resistant parent plants. This may be in pathway analysis that integrates all metabolites produced without considering the categories of resistant and susceptible plants.
The glycine, serine, and threonine metabolism function as the basis of protein synthesis and plays an important role in plant responses against environmental stress (Trovato et al. 2021). This was also observed in the metabolomic analysis of mandarin fruit preventively applied with cyclic lipopeptides (CLP) from Bacillus subtilis and showed that the glycine, serine, and threonine metabolism pathway had the highest score and indeed had an important role in the mandarin fruit response towards post-harvest stresses (Tunsagool et al. 2019). Several proteins that fell into the glycine-rich protein (GRP) superfamily were involved in the signaling and plant response towards stress cellularly (Czolpinska and Rurek 2018). Meanwhile, the taurine and hypotaurine metabolism pathways were the most notably enriched in the resistant interaction during Heterodera glycines infection in soybeans (Song et al. 2019). The taurine and hypotaurine metabolism were also the top pathways in resistant and susceptible tomatoes infected by Fusarium oxysporum (Zhao et al. 2017). The amino acid derivatives taurine and hypotaurine induce an efficient detoxifying enzymatic action and scavenge singlet oxygen in many eukaryotes (Pitari et al. 2022). The metabolite pathways that were identified in this study can be used as a reference for further research on oil palm defense mechanisms against Ganoderma boninense.