Figure 1. The disease symptom (A) and disease severity index (B) of ginseng plants treated with different concentrations of ginsenoside Rg1: 0.5 mM (A2), 1 mM (A3), 2 mM (A4), and no Rg1 control (A1). (A) Images acquired on day 30 after inoculation. The average severity index of the cylindrical root rot of (B) ginseng (%).
Characterization of I. robusta cultured in different concentrations of ginsenoside Rg1
The characteristics of I. robusta cultured with different concentrations of ginsenoside Rg1 to observe the phenotypic changes of I. robusta. Compared to Control, the density, diameter, and color of the colony increased with the increase in concentration of ginsenoside Rg1 (Fig. 2D). Also, I. robusta grown in different treatment concentration of Rg1 showed significant differences in the sporulation yield (Fig. 2A), cell density under logistic increment (Fig. 2B), and The mycelium growth of I. robusta growing in Rg1 increased with increase Rg1 concentration (Fig. 2C). The results showed that the growth of I. robusta was enhanced in Rg1 treatment. These observations point to a mechanism by which Rg1 concentrations in the soil increase to a certain level to cause more serious ginseng rust rot disease.
Figure 2. Effects of ginsenoside Rg1 on the growth of I. robusta cultured in vitro. (A) Sporulation production at DAI 3 (3 days after inoculation); (B) Fungal cell densities (OD600) at logistic increment; (C) Fungal mycelial growth at increment; (D) Colonies of I. robusta observed at DAI 3 in no ginsenoside Rg1 control (D1, top view; D2, bottom view), 0.5 mM ginsenoside Rg1 (D3, top view; D4, bottom view), 1 mM ginsenoside Rg1 (D5, top view; D6, bottom view), and 2mM ginsenoside Rg1 (D7, top view; D8, bottom view). The different letters in Fig. 2A and Fig. 2B indicate a highly significant difference (P<0.01). The error bar represents the standard deviation of at least three repeats.
Illumina sequencing, de novo assembly, and gene annotation
The transcripts of Illumina sequencing, ab initio assembly and gene annotation, and Rg1 treatment of independent biological samples were sequenced to produce a total of 352959960 original RNA sequences. (Table 1). All original sequence reads are saved in the NCBI sequence read file (BioProject ID: PRJNA575915). In addition, 26006870987 and 25343652159, Q20% (base mass >20%) clean-paired end sequence reads were produced in the control group and Rg1 treatment group, respectively (Table 2). Based on the GO horizontal annotation, 32,625 single genes were divided into three categories: cellular component (cc), molecular functional (mf), and biological process (bp) (Fig. 3A). In the KEGG annotation analysis, 48,256 unigenes were identified and divided into seven metabolic pathways (metabolism, genetic information processing, environmental information processing, cellular processes, organismal systems, human diseases, and drug development).
Figure 3.Transcriptome functional annotation analysis. (A) GO classification of the unigenes. Based on the functional annotation, these unigenes were grouped into three categories: biological process, cellular component, and molecular function. (B) KEGG classified these metabolic pathways into seven categories: metabolism, genetic information processing, environmental information processing, cellular processes, organismal systems, human diseases, drug development).
Table 1. Summary of I. robusta mycelium transcriptome data obtained in this study
Sample
|
Raw reads
|
Raw bases
|
Clean reads
|
Clean bases
|
Q20 (%)
|
Q30 (%)
|
CK_Third1
|
49702570
|
7505088070
|
48391680
|
7207952662
|
98.79
|
96.2
|
CK_Third2
|
65590538
|
9904171238
|
64156582
|
9556773169
|
98.88
|
96.43
|
CK_Third3
|
63762930
|
9628202430
|
62116156
|
9242145156
|
98.89
|
96.47
|
total
|
179056038
|
27037461738
|
174664418
|
26006870987
|
|
|
G_Third1
|
52424732
|
7916134532
|
51073008
|
7614818082
|
98.88
|
96.41
|
G_Third2
|
63197160
|
9542771160
|
61819738
|
9217635830
|
98.88
|
96.41
|
G_Third3
|
58282030
|
8800586530
|
57040238
|
8511198247
|
98.9
|
96.48
|
total
|
173903922
|
26259492222
|
169932984
|
25343652159
|
|
|
Elucidation of biological functions altered in response to Rg1 treatment
Analyze the results. Among 213131 unigenes, 7735 showed differential expression between the two groups (Fig. 2). In order to study the changes in the I. robusta transcriptional group caused by Rg1 treatment, a GO/KEGG enrichment analysis was performed in the control and Rg1 treatment groups (p<0.05). Taken together, KEGG terminology annotation analysis of Rg1 induced the expression of related genes in multiple pathways, with pyruvate (KO00620), propionate (KO00640), alanine, aspartic acid, and glutamic acid (KO00250) at the top of the pathway enrichment analysis (Table S1) related to carbohydrate or amino acid metabolism[8]. It is speculated that ammonia can be used as a nitrogen source of fungi, which maximizes the growth of the total population, and the digestibility of carbohydrates is the main factor to control the growth and reproduction of fungi[9].
Figure 4. Differential gene enrichment analysis. (A) GO enrichment analysis and (B) KEGG enrichment analysis. The ratio of the number of unigenes commented to the GO/KEGG term in the gene set to the number of unigenes commented to the GO/KEGG term. The larger the ratio, the greater the degree of enrichment. The size of the point represents the number of unigenes in the GO/KEGG term, and the color of the point corresponds to a different FDR (pvaule_corrected) range.
Rg1 treatment affects the pathogen-host interaction.
In order to determine the potential virulence and pathogenicity-related genes of destructive I. robusta, the function of the whole single gene in the pathogen-host interaction (Phi) gene database (http://www.phi-base.org) was analyzed by BLAST (cutoff E value ≤10-5). This database is a collection of genes that indicate the effect of the results of pathogen-host interactions from fungi, oomycetes, and bacteria[10]. After selecting the DEGs with log2 multiplier≥2 in different Rg1 treatments, 26 hypothetical pathogen-host interaction related genes were identified (Table S2). A number of Fusarium orthologs were found in the cereal pathogenic fungi I. robusta In Fusarium graminearum (3 genes), Fusarium oxysporum (2 genes), pseudoBurkholderia (2 genes), and Candida albicans (2 genes), a large number of Fusarium protoplasts were found in Campylobacter jejuni (2 genes) and Mycobacterium tuberculosis (3 genes). Salmonella enteritis (2 genes) and others (9 genes)[11]. This analysis revealed CK vs Rg1 DEGs in the Phi gene (Fig. 6A). Phi database demonstrated that 13 (50%) genes did not have any effect on the fungal pathogenicity and they were considered to be necessary for the cell communication/signal transduction, metabolism, and transcription between pathogen and host[8]. Other genes were associated with decreased virulence (10 genes; 38.5%), loss of virulence (2 genes; 7.7%), and increased virulence (1 gene; 3.8%) (Fig. 5A).
Among the 26 genes, 10 were upregulated and 16 were downregulated after Rg1 treatment, of which, the increased_virulence gene was upregulated (Fig. 5B). The overall changes in the expression of Phi-related genes showed that their expression was greatly affected by Rg1. This phenomenon explicated the effect of Rg1 on the severity of destructive rust rot disease in P. ginseng.
Figure 5. (A) DEGs related to Phi and the phenotype of pathogen. (B)Bar graph presents the expression estimates for genes differentially regulated according to the Rg1 treatment.
Figure 6. (A) Cluster analysis of gene expression patterns selected in Phi. (B) Cluster analysis of cell growth-related gene expression patterns.
Figure 7. qRT-PCR analysis of selected genes.
Cell growth is affected by Rg1 treatment
All nitrogen uptake and transformation processes are mediated by enzyme systems, which require carbon, nitrogen, and energy for synthesis and expression. The current analysis identified the differential expression of carbon, nitrogen, and energy-related genes (Table S2). In addition, the up- or downregulation responses after Rg1 treatment differed significantly (Fig. 6B). Cluster analysis of cell growth-related gene expression patterns revealed that the expression of genes for nicotinamide adenine dinucleotide phosphate (NADP) or nicotinamide adenine dinucleotide (NAD)-dependent glutamate dehydrogenase (NADP/NAD-GDH) was highly variable (FDR-corrected P-value <0.01) when the pathogen was treated in Rg1. The expression of these genes was also validated by qRT-PCR (Fig. 7). These results suggested that glutamate plays a key role in cell growth, which is consistent with the results of previous studies[9].
The expression of aminomethyltransferase was significantly increased in Rg1 treatment, which possible promoted the release of ammonia (ammonia) and the transfer of methylene carbon unit to tetrahydrofolate moiety (4.06-fold change in Rg1 versus control). Consecutively, the expression of carbon-nitrogen ligase, transaminase, and L-glutamate transmembrane transporter showed 3.75, 33.78, and 2.23-fold higher than that of the control after Rg1 treatment, respectively. The results also showed significant differences in the expression of transporters, (FDR-corrected P<0.01). These results of great significance for the effective utilization of carbon and nitrogen in Rg1-treatements cells.