Differential Potential of Phytophthora capsici Resistance Mechanisms to the Fungicide Metalaxyl in Peppers

Metalaxyl is one of the main fungicides used to control pepper blight caused by Phytophthora capsici. Metalaxyl resistance of P. capsici, caused by the long-term intense use of this fungicide, has become one of the most serious challenges facing pest management. To reveal the potential resistance mechanism of P. capsici to fungicide metalaxyl, a metalaxyl-resistant mutant strain SD1-9 was obtained under laboratory conditions. The pathogenicity test showed that mutant strain SD1-9 had different pathogenicity to different host plants with or without the treatment of metalaxyl compared with that of the wild type SD1. Comparative transcriptome sequencing of mutant strain SD1-9 and wild type SD1 led to the identification of 3845 differentially expressed genes, among them, 517 genes were upregulated, while 3328 genes were down-regulated in SD1-9 compared to that in the SD1. The expression levels of 10 genes were further verified by real-time RT-PCR. KEGG analysis showed that the differentially expressed genes were enriched in the peroxisome, endocytosis, alanine and tyrosine metabolism. The expression of the candidate gene XLOC_020226 during 10 life history stages was further studied, the results showed that expression level reached a maximum at the zoospores stage and basically showed a gradually increasing trend with increasing infection time in pepper leaves in SD1-9 strain, while its expression gradually increased in the SD1 strain throughout the 10 stages, indicated that XLOC_020226 may be related to the growth and pathogenicity of P. capsici. In summary, transcriptome analysis of plant pathogen P. capsici strains with different metalaxyl resistance not only provided database of the genes involved in the metalaxyl resistance of P. capsici, but also allowed us to gain novel insights into the potential resistance mechanism of P. capsici to metalaxyl in peppers.


Introduction
Phytophthora capsici, an important plant-pathogenic oomycete of Phytophthora, damages various crops including members of Solanaceae (such as pepper and tomato) and Cucurbitaceae (such as cucumber and pumpkin) [1]. Pepper blight caused by this pathogen was first discovered in the United States in 1918, and the disease can be catastrophic in a short time, endangering the entire growth period of pepper [2]. At present, metalaxyl is the main fungicide used for preventing and controlling plant diseases such as pepper blight. However, because this type of agent is a site-specific inhibitor and has a single site of action on pathogens, Phytophthora can develop resistance as they are susceptible to mutations [3][4][5]. Metalaxyl resistance is mainly found in Phytophthora species such as P. capsici, P. infestans, and P. parasitica. A metalaxyl-resistant strain of P. infestans was discovered in the Netherlands in 1980 [6]. Subsequently, European and American countries reported metalaxyl-resistant strains of P. capsici [7][8][9]. China also reported resistant P. capsici in the 1960s [10]. To date, metalaxyl-resistant strains of P. capsici have been successively found in Anhui, Gansu, and Yunnan Provinces [10][11][12][13].
There are some studies on the resistance mechanism of Phytophthora to metalaxyl. Chen et al. [14] revealed two evolutionary pathways of resistance involving the RPA190 gene. The results of their research indicate that changes in the activity of Phytophthora RNA polymerase are important resistance mechanisms. Similar results were also confirmed in P. infestans. The diversity of the RNA polymerase I large subunit sequence of P. infestans plays a key role in its resistance to metalaxyl [15]. The biological degradation of metalaxyl by Phytophthora is another resistance mechanism. The RNA polymerase activity of sensitive strains was shown to be significantly inhibited by metalaxyl, while resistant strains showed only a slight inhibitory effect and had a certain degradation effect on metalaxyl [14].
Although crop losses caused by P. capsici have increased in recent years, we know very little about the molecular basis of its pathogenicity in peppers. Therefore, excavation of metalaxyl-resistance genes of P. capsici at the molecular level has become an important means of developing resistance to pepper blight. With the decrease in the cost of high-throughput sequencing, transcriptome sequencing has been widely used in molecular biology research and has become one of the most commonly used high-throughput sequencing technologies [16,17]. It has a number of advantages, such as a wide range of applications, good repeatability, good sensitivity, and high sequencing throughput. It can be used to discover new genes, optimize structural genes, and analyze differential expression of different transcripts, making it very convenient for differentially expressed gene (DEG) screening. The genome of P. capsici LT1534 was sequenced in 2012 and its size is 64 MB [18]. Transcriptome information was compared against the known genome sequence of P. capsici, and then the sequencing depth and expression of DEGs were analyzed to predict new genes and identify alternative splicing and gene fusion. This is of great significance for elucidating the resistance mechanism of P. capsici to pesticides. Parada-Rojas et al. [19] identified and characterized microsatellites in the P. capsici transcriptome, and then assayed a subset of 50 microsatellites in a diverse set of P. capsici isolates to find polymorphism. Their findings revealed that 12 microsatellites were useful to characterize the population structure of P. capsici and were potentially transferable to closely-related Phytophthora spp. Root rot caused by P. capsici is the most serious disease in black pepper. Researchers performed transcriptome analysis to identify candidate genes for field tolerance to black pepper root rot [20]. Sequence analysis revealed a series of proteins involved in black pepper tolerance to root rot, including signal proteins and defense enzymes such as premnaspirodiene oxidase, a phosphatase 2C-like domain protein, a mature protein of the nitrous oxide reductase family, disease resistance protein RGA3, asparaginase, β-glucosidase, a cytochrome P450 signal protein, serine/threonine protein kinase WAG1, and nucleoredoxin 1-1 enzyme. Chen et al. [21] applied RNA-Seq technology to reveal a large number of genes related to pathogenicity at three stages of mycelia (MY), zoospores (ZO), and germinating cysts with germ tubes (GC) were identified, including 98 predicted effector genes. Therefore, transcriptome sequencing can be used to study the gene functions of pathogenic bacteria and analyze the expression differences between different strains to screen genes related to target traits.
In this study, the sensitive strain SD1 and the resistant mutant strain SD1-9 were used as test materials, and their transcriptomes were sequenced and analyzed. Referring to the published P. capsici LT1534 genome, we performed data splicing, gene expression analysis, and function prediction. Our results will help to understand the molecular mechanism of metalaxyl resistance in P. capsici, provide a theoretical basis for the monitoring and treatment of metalaxyl resistance of P. capsici, and provide an important reference for studying the molecular mechanism of metalaxyl resistance in other Phytophthora species.

Obtainment of the Resistant P. capsici Mutant Strain
The test strain SD1 was cultured on a 10% V 8 plate containing 10 µg·mL −1 metalaxyl for two weeks. The resulting rapidly growing sector (Figure 1) was transferred onto a fresh 10% V 8 plate containing 10 µg·mL −1 metalaxyl for normal growth. This strain was identified as the metalaxyl resistant mutant SD1-9. The results suggest that P. capsici was susceptible to high metalaxyl application induced metalaxyl resistance.

Determination of Pathogenicity
P. capsici strains SD1 and SD1-9 were inoculated onto green bell pepper, squash, cucumber, red pepper, and purple eggplant fruits. Disease spots appeared after 3 days (Figure 2), indicating that P. capsici could infect these five fruits. Moreover, the pathogenicity of the strain SD1 and the metalaxyl-resistant mutant strain SD1-9 differed on each vegetable, and obvious white colonies were produced on green bell pepper, cucumber, and purple eggplant. Table 1 shows the average diameter of lesions was 3.72-6.60 cm. The pathogen had the strongest pathogenicity on purple eggplant. Red pepper produced a small number of white colonies after being infected by the pathogen, showing a soft rot, and the parent strain SD1 had higher pathogenicity than the mutant strain. The pathogen showed the lowest pathogenicity on squash. Analysis of the data with the SPSS statistical software showed that each strain had different pathogenicity on different vegetables.  3.72 ± 0.25b 1.20 ± 0.05b 6.92 ± 0.38b 4.43 ± 0.27b 6.60 ± 0.26b Note: In the same column, the same lowercase letter indicates that the difference is not significant, while different lowercase letters indicate that the difference is significant (p < 0.05).
P. capsici strains SD1 and SD1-9 were treated with metalaxyl at concentrations of 0, 5, and 100 µg·mL −1 , and their pathogenicity on green peppers and pepper leaves was consistent ( Figure 3). It can be seen from Table 2 that the pathogenicity of strain SD1 was stronger than that of SD1-9 on green peppers and pepper leaves. With increasing metalaxyl concentration, the pathogenicity of P. capsici gradually decreased, indicating that metalaxyl can effectively inhibit P. capsici.  3.72 ± 0.28b 2.9 ± 0.23b 1.28 ± 0.06a 2.50 ± 0.05b 1.90 ± 0.05b 0.57 ± 0.03b Note: In the same column, the same lowercase letter indicates that the difference is not significant, while different lowercase letters indicate that the difference is significant (p < 0.05).

Transcriptome Sequencing Quality Analysis
To study the molecular mechanism of P. capsici resistance to metalaxyl, six transcriptomes were analyzed for resistance genes. SD1 (CK) and SD1-9 libraries were created in triplicate, and the transcriptome statistics of the six processed samples were analyzed by the Illumina sequencing method (as shown in Table 3). After low-quality reads were filtered out, the clean reads of the SD1 sequencing libraries were in the range of 38,634,410-73,337,800, and the clean reads of the SD1-9 sequencing libraries were in the range of 56,374,338-109,537,004. The clean reads were then further filtered to obtain high-quality reads (High Quality Clean Reads, referred to as HQ Clean Reads). The HQ Clean Reads of the SD1 and SD1-9 sequencing libraries were in the ranges of 37,789,918-72,092,776 and 55,305,346-107,700,064, respectively, accounting for more than 97% of the clean reads. The GC content was between 56.00% and 57.31%. In addition, the Q30 value was greater than 95%, indicating that RNA-Seq sequencing data was of good quality and could be used for bioinformatics analysis. However, depending on sample quality and species, ribosomal RNA may not be completely removed during transcriptome analysis. To avoid contamination from ribosomal RNA affecting subsequent analysis, the reads comparison tool bowtie2 (2.2.8) was used to compare the HQ Clean Reads to the ribosomal RNA of P. capsici (mismatch number: 0) to remove ribosomal RNA reads; the remaining data were used for subsequent analysis. The SD1 reads matching ribosomal sequences ranged from 2,118,438 to 5,264,802, which accounted for 5.05% to 7.30%; the unmatched reads ranged from 35,671,480 to 66,827,974, which accounted for 92.70% to 94.95%. The SD1-9 reads matching ribosome sequences ranged from 2,101,888 to 3,900,268, accounting for 3.62% to 4.13%; the unmatched reads ranged from 53,203,458 to 103,799,796, accounting for 95.87% to 96.38%.
Using the comparison software Tophat2 (2.1.1), the reads not aligned to ribosomal RNA were compared to the P. capsici LT1534 genome ( Table 4). The numbers matched for SD1 were in the range of 23,943,117-44,456,782, with a comparison rate of 67.12%-67.26%; the numbers matched for SD1-9 were in the range of 37,397,410-73,232,460, with a comparison rate of 70.18%-70.55%. The reason for the low comparison rate may be that the V8 medium was contaminated during the sampling process or other impurities were introduced during the washing of the hyphae. Note: SD1-1 = control 1, SD1-2 = control 2, and SD1-3 = control 3; SD1-9-1, SD1-9-2, and SD1-9-3 are three biological repeats of the SD1-9 group. Clean Reads: the number of reads after initial filtering; HQ Clean Reads: the number of reads obtained by further filtering of the clean reads. Mapped Reads: the number of reads mapped to ribosomal sequences; Unmapped Reads: the number of reads not mapped to ribosomal sequences. Note: SD1-1 = control 1, SD1-2 = control 2, and SD1-3 = control 3; SD1-9-1, SD1-9-2, and SD1-9-3 are three biological repeats of the SD1-9 group. Total Reads: the number of reads after excluding ribosomal RNA; Mapped Reads: the number of reads uniquely mapped to the reference genome; Mapping rate (%): comparison rate.

Analysis of Principal Components
To investigate the replication of the transcriptome samples, we performed principal component analysis (PCA), and the results are shown in Figure 4. The PCA results clearly divided the transcriptome samples into two groups: the control group (SD1-1, SD1-2, SD1-3) and the SD1-9 sample group (SD1-9-1, SD1-9-2, SD1-9-3). The three replicate samples within each group were brought together to form an independent population. According to the numerical values of the sample gene expression in the first principal component (PC1) and the second principal component (PC2), a two-dimensional coordinate map of the principal components was drawn. PC1 (84.2%) and PC2 (8.3%) revealed a change in gene expression of the six samples of 92.5% and showed good agreement between sample biological replicates. samples; SD1-9-1, SD1-9-2, and SD1-9-3 are the three biological repeats.

Effect of Metalaxyl on Gene Expression of P. capsici
In the metalaxyl-treated mutant strain, the expression of metalaxyl-resistance genes of P. capsici changed significantly. In the SD1-9 resistant mutant strain, 517 differentially expressed genes (DEGs) were upregulated, while 3328 DEGs were downregulated ( Figure 5). After analysis of the difference in gene expression, FDR < 0.05 and |log2FC| > 1 were selected as screening criteria for comparison. The top 20 gene IDs in the samples that were significantly upregulated and downregulated are listed in Tables 5 and 6, respectively. In Table 5, the gene descriptions for genes with significantly downregulated expression include a hypothetical protein, a conserved hypothetical protein, 5-methyltetrahydropteroyltriglutamate-homocysteine S-methyltransferase, a potential polyprotein, quinone oxidoreductase 2, serine/threonine-protein kinase drkB, acyl-coenzyme A oxidase, and a cyst germination-specific acidic repeat protein. In Table 6, the gene descriptions for genes with significantly upregulated expression include a hypothetical protein, Nef-associated protein 1, exportin-5, ABC transporter G family member 2, a C-factor, pol polyprotein fruit fly (Drosophila melanogaster) transposon, and NPP1 protein. These gene descriptions indicate the resistance of P. capsici to metalaxyl is a complicated process.

Gene Ontology (GO) Analysis
To elucidate the stress response of P. capsici to metalaxyl, we performed a GO functional enrichment analysis on the DEGs. GO is a classification system that describes the functions of genes and the relationships between genes. GO includes three ontologies: molecular function, cellular component, and biological process [22,23]. In the SD1-9 samples, 874 upregulated and 4536 downregulated DEGs were annotated, for a total of 5410 DEGs (Table S2). Of these, 2464 DEGs (397 up, 2067 down) belonged to the biological process category, 1207 DEGs (229 up, 978 down) belonged to the cellular component category, and 1739 DEGs (248 up, 1491 down) belonged to the molecular function category ( Figure 6 and Table S2). In the biological process ontology, the functions of the genes were related to biological adhesion, biological regulation, cellular component organization or biogenesis, cellular processes, developmental processes, localization, locomotion, metabolic processes, multi-organism processes, multicellular organismal processes, responses to stimulus, signaling, and single-organism processes. The largest proportions of genes were in the cellular process, metabolic process, and single-organism process categories. Among them, 98 genes were significantly upregulated and 573 genes were significantly downregulated in the cellular process category; 132 genes were significantly upregulated and 613 genes were significantly downregulated in the metabolic process category; and 94 genes were significantly upregulated and 488 genes were significantly downregulated in the single-organism process category. In the cellular component ontology, the gene functions were related to the cell, cell part, macromolecular complex, membrane, membrane part, membrane-enclosed lumen, organelle, organelle part, virion, and virion part. The largest proportions of genes were in the cell and membrane categories; 42 genes were significantly upregulated and 164 genes were significantly downregulated in cell and cell part, 42 genes were significantly upregulated and 214 genes were significantly downregulated in membrane, and 41 genes were significantly upregulated and 200 genes were significantly downregulated in membrane part. In the molecular function ontology, the functions appearing most frequently were antioxidant activity, binding, catalytic activity, molecular function regulator, molecular transducer activity, nuclear acid binding transcription factor activity, signal transducer activity, structural molecule activity, and transporter activity. Among them, the main functions were binding and catalytic activity; 88 genes were significantly upregulated and 567 genes were significantly downregulated in binding, and 129 genes were significantly upregulated and 789 genes were significantly downregulated in catalytic activity. Genes are often associated with multiple different functions, such as the significantly downregulated gene XLOC_020226, which was associated with single-organism process, localization, cellular process, and transporter activity functions. GO analysis showed that genes were significantly upregulated and downregulated in the five categories single-organism process, cellular process, metabolic process, catalytic activity, and binding.

Validation of RNA-Seq Data
To verify the reliability of the RNA-Seq data, we randomly selected 10 genes in the SD1-9 group for Quantitative Real-Time PCR (qRT-PCR) differential expression verification. Fold changes calculated from qRT-PCR were compared with the RNA-Seq expression analysis data, and the results are shown in Figure 8. The qRT-PCR results were in agreement with the RNA-Seq high-throughput sequencing data, indicating similar expression patterns of up-and downregulated genes in the RNA-Seq and qRT-PCR tests. XLOC_001584, XLOC_005234, XLOC_018749, fgenesh1_pg.C_scaffold_14000251 (14000251), gw1.17.333.1 all showed increased expression, while XLOC_018738, XLOC_020738, XLOC_011516, e_gw1.14.281.1, estExt_fgenesh1_kg.C_180404 (180404) all showed decreased expression (180404). However, the gene expression levels were different, which may have been caused by errors in the experimental process or differences in the quantitative instruments.

Expression of Candidate Gene XLOC_020226 During the Life History of P. capsici
The expression of the candidate gene XLOC_020226 in different life history stages is shown in Figure 9. The expression of XLOC_020226 gradually increased in the SD1 strain throughout the 10 stages. In the SD1-9 strain, the expression level reached a maximum at the ZO stage and basically showed a gradually increasing trend with increasing infection time in pepper leaves. We speculate that the XLOC_020226 gene may be related to the growth and pathogenesis of P. capsici; it may also be involved in regulating the zoospore release of P. capsici.

Discussion
Pathogenicity refers to the strength of the pathogenic infection of the host plant. There are differences in pathogenicity among different strains of Phytophthora species, which is manifested by different pathogenicities of strains in the same Phytophthora species to the same host plant and different pathogenicity of the same strain to different host plants. In traditional biology, the pathogenic inoculation methods for Phytophthora are root inoculation, stem inoculation, and leaf spraying. These methods are relatively simple and fast, but are susceptible to variability due to factors such as host plants, inoculation methods and environmental conditions, which can make their results differ [24]. Therefore, when measuring the pathogenicity of Phytophthora on host plants under laboratory conditions, the experimental conditions must be consistent to make the results accurate and reliable. Tian et al. [25] determined the virulence of P. capsici isolates on different pumpkin varieties, and reported that P. capsici isolates were more virulent towards jack-o-lantern pumpkins than processing pumpkins. Similarly, the pathogenicity test results of five P. capsici strains from different regions of Guangdong on eight hot (sweet) pepper materials indicated that the pathogenicities of different strains were significantly different, with differences in pathogenicity in the same strain in different pepper materials [26]. In this study, the results of the pathogenicity test showed that each strain had different pathogenicity on different host materials. The phenotypes of strains SD1 and SD1-9 on green peppers and pepper leaves showed that their pathogenicity gradually weakened as the metalaxyl concentration increased.
Previous genetic studies have shown that insensitivity to metalaxyl is regulated by one or two major MEX loci, while other genes are less affected [27][28][29][30][31]. Mionor sites that cause chemical insensitivity may include non-specific efflux pumps and detoxification. These functions can be performed by proteins such as the ATP-binding cassette (ABC) transporter and the cytochrome P450 protein, respectively [32,33]. However, P. infestans isolates that have essentially non-specific insensitivity to chemicals have not shown a corresponding increase in transcriptional abundance from genes encoding ABC transporters [29]. Transcriptome sequencing was used to analyze the gene functions and expression of the sensitive strain SD1 and metalaxyl-resistant mutant strain SD1-9. The results showed that compared with the sensitive strain SD1, the mutant strain SD1-9 treated with metalaxyl had 517 significantly upregulated genes and 3328 significantly downregulated genes. The functions of these DEGs varied. Researchers [34][35][36] have used 3 H uridine incorporation biochemical analysis to show that metalaxyl has specific effects on the synthesis of RNA, especially ribosomal RNA (rRNA), while messenger (mRNA) and transport RNA (tRNA) synthesis is less affected. These effects are related to RNA polymerase I (RNApolI) because it transcribes rRNA. In addition, another study showed that metalaxyl exerts its activity when the RNA polymerase complex binds to DNA [34]. RNA polymerases that rely on eukaryotic DNA are multi-subunit complexes, and up to seven subunits can be shared among the three major RNA polymerases [37,38]. Other proteins such as topoisomerases and transcription factors may also affect RNA polymerase activity [39]. These factors complicate the identification of metalaxyl-targeted RNA polymerase subunits and sequence variations that lead to insensitivity.

Tested Strain
P. capsici strain SD1 was isolated from diseased plants with the typical symptoms of pepper blight. The plants were collected from a metalaxyl-free pepper field in Taian, Shandong Province. According to the metalaxyl sensitivity test, the EC 50 value was 0.4 µg·mL −1 , indicating that the strain was sensitive to metalaxyl. The strain has been deposited to the Fungal Laboratory of Anhui Agricultural University.
Generation of the metalaxyl-resistant mutant strains: The metalaxyl-sensitive P. capsici strain SD1 was used as the wild type strain. After the strain was cultured on 10% V 8 (V 8 juice 10 mL, H 2 O 90 mL, CaCO 3 0.02 g, agar 3 g) plates for 5-7 d, the mycelium discs with a diameter of 6-10 mm were transferred onto 10% V 8 solid medium containing 10 µg·mL −1 metalaxyl [40], and incubated in a 25 • C incubator. After 7 d of incubation, the growth of the colonies was observed. After 10-14 d of incubation, if a rapidly growing sector (mycelial growth rate > 6 mm·d −1 ) appeared, the colonies in that sector were transferred onto 10% V 8 solid medium containing 10 µg·mL −1 metalaxyl. A normal mycelial growth rate (3-6 mm·d −1 ) was considered to be a sign of acquisition of metalaxyl-resistance and such strains were chosen for further study.

Pathogenicity Determination
Pathogenicity on different vegetables: Green bell peppers, squash, red peppers, purple eggplants, and cucumbers that were fresh, healthy, and basically the same size were purchased from a local market in Hefei. Strains SD1 and SD1-9 were transferred to 10% V 8 medium and cultured in the dark at 25 • C for 5-7 days. Mycelium discs with a diameter of 6-10 mm were cut out. Three small insect needles were used to puncture the surface of each vegetable, and the discs were applied with the mycelium side facing the wound. Absorbent cotton was dipped in water and placed over the wound for 24 h, and each treatment was repeated four times, with strain SD1 as a control. The samples were then cultured in a 25 • C light incubator, and lesion size was measured after 72 h.
Pathogenicity under different metalaxyl concentrations: Pepper leaves (variety Xinsujiao 5) and green peppers that were fresh, healthy and consistent in size were selected. Strains SD1 and SD1-9 were transferred to 10% V 8 medium and cultured in the dark at 25 • C for 5-7 days. Mycelium discs with a diameter of 6-10 mm were cut out. Three small insect needles were used to puncture the surfaces of the pepper leaves and green peppers, and the discs were applied with the mycelium side facing the wound. Each wound was sprayed with 1 mL of metalaxyl medicinal solution, at concentrations of 0, 5, and 100 µg·mL −1 . After this treatment, absorbent cotton was put on the wound and moistened for 24 h. Each treatment was repeated four times. A blank 10% V 8 medium block was inoculated and sprayed with sterile water as a control. After inoculation, the samples were cultured in a 25 • C light incubator. Lesion size was measured after 72 h and the test was repeated twice.

RNA Extraction, cDNA Library Preparation and Transcriptome Sequencing
The P. capsici strains SD1 and SD1-9 were transferred to 10% V 8 medium and cultured in the dark at 25 • C for 5-7 days. Mycelium discs with a diameter of 5 mm were transferred to 10% V 8 liquid medium (V 8 juice 10 mL, H 2 O 90 mL, CaCO 3 0.02 g). Ten mycelium discs per dish and 10 dishes per strain were placed in the dark at 25 • C for 3 days. After 3 days, the mycelium was filtered with gauze, rinsed with 25% ethanol, and repeatedly washed with deionized water three to four times. The excess water was squeezed out to collect mycelia. The collected mycelia were quickly frozen and ground into powder with liquid nitrogen for RNA extraction. Each strain was repeated three times. RNA extraction was performed using the Takara RNA extraction kit, following the method provided. The concentration and purity of the RNA samples were quantified using a spectrophotometer (NanoDrop ND-1000; Thermo Fisher Scientific, Waltham, MA, USA), and the RNA degradation of the six samples was assessed in 1% agarose gels. The RNA integrity was assessed using the RNA Nano 6000 Assay Kit with the Agilent Bioanalyzer 2100 system (Agilent Technologies, CA, USA). Sequencing libraries were generated using a NEBNext Ultra™ RNA Library Prep Kit for Illumina (NEB, Ipswich, MA, USA), following the manufacturer's recommendations. The effective concentration of each library was accurately quantified using qPCR to ensure library quality, and then cDNA library sequencing was conducted with an Illumina high-throughput sequencing platform (HiSeq™ 2500) by Genedenovo Biotechnology Co., Ltd. (Guangzhou, China).

RNA-Seq Data Analysis
The constructed libraries were sequenced with an Illumina HiSeq™ 2500. After filtering the sequencing data to obtain clean data, the reads were compared to the P. capsici LT1534 genome and Cufflinks was used to splice the reads to obtain transcript data. Then, the obtained genes were statistically analyzed, and differential expression and functional enrichment analyses were performed. The edgeR software was used to analyze the differences in gene expression between groups. FDR (corrected p-value, indicating significance) and log 2 FC (FC is the fold change multiple) were used to screen for DEGs. The screening criteria were FDR < 0.05 and |log 2 FC| > 1. The library corresponding to the SD1 sample was used as a control.

Gene Annotation
GO annotation is based on the significant enrichment of GO functions to analyze DEGs and related gene modules for bioinformatics analysis. Referring to the annotation information in the NCBI non-redundant (Nr) database, the Blast2 GO software (version 3.0, https://www.blast2go.com/, BioBam Bioinformatics S.L., Valencia, Spain) was used to perform GO annotation on the core DEGs and the co-expressed gene modules, and the WEGO software was used to annotate and statistically analyze the GO functional classifications of all genes. GO covers three aspects of biology: cellular components, molecular functions, and biological processes. The KEGG database (https://www.kegg.jp/) can systematically classify and annotate the metabolic pathways of genes and can be used to study genes and their expression information at a general level. The KO-BAS software (version 2.0, KOBAS, Surrey, UK) was used to detect the enrichment of DEGs in KEGG pathways, and the biological functions of specific genes of P. capsici were considered and evaluated at a macro level.

qRT-PCR
The remaining P. capsici SD1 and SD1-9 samples were taken for RNA extraction and purification treatment experiments and then reverse transcribed into cDNA. Ten differentially expressed genes (including genes that were upregulated and downregulated) were randomly selected, and their relative expression levels were verified using a quantitative real-time PCR method. Primer 3.0 was used to design qRT-PCR primers online. The gene names and quantitative primers are shown in Table S1. Quantification was performed following the operation method of the CFX96 Real-time PCR Detection System and the TB Green™ Premix Ex Taq™ II kit. Each analysis consisted of three biological replicates and three technical replicates per biological replicate. Quantitative fluorescence analysis was performed using the β-actin gene of P. capsici as an internal reference. The mRNA levels were normalized with the relative mRNA level of the P. capsici β-actin gene using the 2 −∆∆Ct method [41].

RNA Extraction at Various Growth Stages of P. capsici
Total RNA was extracted from P. capsici SD1 and SD1-9 at 10 stages, including the mycelia (MY), zoosporangia (SP), zoospores (ZO), cysts (CYST), germinating cysts (GC), and infected pepper leaves at 1.5, 3, 6, 12, and 24 h. The RNA extraction method in each period followed Zhang [42]. The cDNA was synthesized using the TaKaRa PrimeScript™ RT reagent Kit with gDNA Eraser and used as the template for qRT-PCR. The qRT-PCR primers are listed in Table S1.