As described in our previous study, B. cinerea is highly sensitive to BT (Cui et al., 2017). An isolate, SX-9, was selected for the following study to explore the effect of BT on B. cinerea. The B. cinerea SX-9 isolate used in this study was collected from a pepper greenhouse in Shandong Province, China, and it was obtained by the single-spore method and maintained on potato dextrose agar (PDA; Difco^TM BD Diagnostics, Franklin Lakes, NJ, United States) plates. After a 24-h incubation period at 25°C in the dark (considered 0 h), B. cinerea isolate SX-9 was fumigated with 0.5 μL of BT (purity of 99%; Beijing Lark Technology Co., Ltd., Beijing, China) in 200 mL-volume fumigation plates, to create a final concentration of 2.5 μL/L (the concentration that caused 50% inhibition given 24 h of preincubation). For comparison, PDA plates without BT were used as controls. The mycelia were collected at three specific time points (12, 24, and 48 h) and quickly frozen in liquid nitrogen. Thus, six groups were subjected to RNA-seq, including the control (CON) at 12, 24, and 48 h and BT treatment at 12, 24, and 48 h. There were three biological replicates for each treatment, and mycelia collected from 10 PDA plates were pooled to form one biological replicate.

The culture of B. odoriphaga was previously described (Zhao et al., 2016a, b). The population of B. odoriphaga has been cultured in the laboratory since 2013. Fresh chive rhizomes in 1-cm pieces were used to rear the insects, which were maintained in Petri dishes at 25 ± 1°C with 70 ± 5% relative humidity and a 14:10 (light:dark) photoperiod. The newly emerged 4th instar larvae of B. odoriphaga were transferred from the Petri dishes and fumigated with the LC₃₀ of BT (0.47 μL/L) in fumigation aquariums (10 L, 20 cm × 20 cm × 25 cm) at 25°C. After fumigation with BT for 6 h and 24 h, the live larvae in the BT and control groups were collected. Three biological replicates of 20 larvae each were frozen in liquid nitrogen.

Transmission electron microscopy (TEM) was carried out to investigate the mechanism of action of BT on the ultrastructure of B. cinerea. Hyphae samples were collected from the edges of control and BT-treated B. cinerea colonies. The hyphae were fixed with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4) for 2 h at 4°C. After rinsing in the same buffer several times, the samples were postfixed in 1% OsO₄ in the same buffer for 2 h at 4°C, dehydrated through an ethanol series, and embedded in Epon-Araldite resin. Thick sections were prepared with an LKB ultramicrotome, stained with uranyl acetate and lead citrate, and viewed under a JEOL 1200EX transmission electron microscope (JEOL, Tokyo, Japan). The thickness of the cell wall was also measured and recorded. Due to the complexity of the larval body and the lack of a predicted specific site of action of BT, we did not observe B. odoriphaga by TEM.

Total RNA was extracted from frozen B. cinerea mycelial samples using an E.Z.N.A. Fungal RNA Kit (Omega, Norcross, GA, United States). Total RNA was extracted from B. odoriphaga samples using TRIzol reagent (Invitrogen, Grand Island, NY, United States) and treated with DNase I. The RNA integrity value was confirmed with an Agilent 2100 Bioanalyzer (Agilent, Santa Clara, CA, United States). The concentration and purity of the RNA were determined with a NanoDrop 2000 (Thermo Fisher Scientific, Waltham, MA, United States). According to the manufacturer’s (Illumina) protocol, poly-(A) messenger RNA (mRNA) was isolated using oligo-(dT) magnetic beads and fragmented into short pieces using fragmentation buffer. Random hexamer primers were used to synthesize cDNA. After purification for terminal repair and adaptor ligation, the products were amplified through PCR to create cDNA libraries. Then, the B. cinerea and B. odoriphaga libraries were sequenced on a BGISEQ-500 platform and an Illumina HiSeq^TM 4000 platform, respectively, at the Beijing Genomics Institution (BGI, ShenZhen, China).

For the B. cinerea transcriptome, the raw reads from the sequencing machine were cleaned by filtering out the adaptor reads, low-quality reads (bases with quality value ≤5 accounting for more than 50% of the read), and reads with unknown bases accounting for more than 10% of the read. The clean reads were saved in FASTQ format and then mapped to the B. cinerea B05.10 genome database using HISAT (Kim et al., 2015) with no more than two base mismatches. The gene expression level was normalized and estimated by FPKM (fragments per kilobase of transcript per million mapped reads) (Sims et al., 2014). Differentially expressed genes (DEGs) were screened between the CON and BT treatments at each time point using the NOISeq method (Tarazona et al., 2011). Genes with a combination of probability ≥ 0.8, FDR ≤ 0.001 and absolute value of log₂ ratio ≥ 1 were considered DEGs.

For the B. odoriphaga transcriptome, the raw reads from the sequencing machine were cleaned by filtering out the adaptor reads, low-quality reads (bases with quality value ≤15 accounting for more than 20% of the read), and reads with unknown bases accounting for greater than 5% of the read. Trinity software (Grabherr et al., 2011) was used to perform de novo assembly with the clean reads, and the TGICL package (Pertea et al., 2003) was used to cluster the transcripts into unigenes. To obtain a comprehensive functional annotation, unigenes were determined using the BLAST program (Altschul et al., 1990) by querying against public functional databases as follows: National Center for Biotechnology Information (NCBI) non-redundant nucleotide sequences (NT), NCBI non-redundant protein sequences (NR), Clusters of Orthologous Groups of proteins (COG), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Swiss-Prot. Blast2GO (Conesa et al., 2005) with NR annotation was used to obtain the Gene Ontology (GO) annotation and InterProScan 5 (Quevillon et al., 2005) was used to obtain the InterPro annotations. Bowtie2 (Langmead and Salzberg, 2012) was used to map the clean reads to the assembled unigenes, and RSEM (Li and Dewey, 2011) was used to obtain the read count for each unigene in each sample. The gene expression level was normalized and estimated by FPKM. Differentially expressed genes were screened between the CON and BT treatments at each time point using DESeq2 (Love et al., 2014). Genes with both an absolute value of log₂ ratio ≥ 1 and an adjusted P-value ≤ 0.05 were considered DEGs.

The Blast2GO program was used to assign all the DEGs to GO categories, and WEGO software (Ye et al., 2006) was used to perform functional classification of GO terms. In addition, GO (Ashburner et al., 2000) and KEGG (Kanehisa and Goto, 2000) pathway enrichment analyses were performed on the DEGs. Gene Ontology terms with a threshold-corrected P-value ≤ 0.05 were considered significantly enriched in DEGs, and KEGG pathways that fulfilled the criterion of Q-value ≤ 0.05 were defined as significantly enriched.

In the fumigation assay, we found, by accident, that BT may accelerate the melanization of B. cinerea. Therefore, we performed a series of tests to verify this phenomenon and investigated whether this effect could affect mycelial growth and virulence. Mycelial plugs (5 mm) from the edges of 3-day-old colonies were inoculated on the centers of PDA plates with or without tricyclazole, an inhibitor of fungal dihydroxynaphthalene (DHN) melanin biosynthesis (Zhang et al., 2015). We set three concentrations of tricyclazole at 0, 10 and 50 μg/mL. Beyond that, 0.5 or 1 μL of BT (final concentration of 2.5 or 5 μL/L) was added to each plate to build a fumigation environment. Plates without BT and tricyclazole were used as controls. Every treatment consisted of six parallel samples. The diameter of each mycelium colony was measured after two days, and a photograph of B. cinerea colony melanization was observed and recorded. Then, mycelium plugs from the edge of each B. cinerea colony in each treatment were inoculated on detached cucumber leaves (one plug per leaf) to verify the role of melanin in the virulence of B. cinerea. The infected leaves were maintained in a container with 85% relative humidity at 25°C with a photoperiod of 12 h. The lesion diameter minus 5 mm (the diameter of the mycelium plug) was considered the lesion growth diameter. There were six cucumber leaves for each treatment. In addition, we identified the expression levels of seven melanin biosynthesis-related genes using reverse transcription quantitative PCR (RT-qPCR).

Gene-specific primers for RT-qPCR were designed using the Beacon Designer 7.0 program, and their sequence information is listed in Supplementary Table S1. Total RNA was extracted from frozen B. cinerea mycelia and B. odoriphaga larvae samples using an E.Z.N.A. Fungal RNA Kit and TRIzol reagent (Invitrogen, Grand Island, NY, United States), respectively. Then, 1 μg of RNA for each sample was used to synthesize first-strand cDNA using the TransScript All-in-One First-Strand cDNA Synthesis SuperMix for qPCR Kit (Transgen Biotech, Beijing, China). Reverse transcription quantitative PCR analysis was performed using TransStart^® Top Green qPCR SuperMix (TransGen Biotech, Beijing, China) on a Light Cycler 96 system (Roche, Basel, Switzerland). Each 20 μL reaction mixture contained 10 μL of 2 × TransStart^® Top Green qPCR SuperMix, 0.4 μL of each forward and reverse primer, 8.2 μL of double distilled water and 1 μL of cDNA. The cycling program was 94°C for 30 s, followed by 45 cycles at 94°C for 5 s, 56°C for 15 s and 72°C for 10 s. There were three biological replicates and three technical replicates for each gene of each sample. β-tubulin and ribosomal protein S15 (RPS15; Shi et al., 2016) were used as the internal control genes for the RT-qPCR analyses of B. cinerea and B. odoriphaga, respectively. The relative gene expression level in each comparison between a BT-treated sample and its control was calculated using the 2^{– ΔΔCt} method (Livak and Schmittgen, 2001). The statistical analysis was performed with one-way ANOVA followed by Tukey’s HSD method (P < 0.05) via SPSS 18.0 software (SPSS Inc., Chicago, IL, United States).

Untreated B. cinerea showed normal organelles, including regularly shaped mitochondria and rough endoplasmic reticulum (Figures 1A–C). When B. cinerea was treated with 2.5 μL/L BT for 12 h, some large, anomalous vacuoles were observed (Figure 1D). As the fumigation time was prolonged, the vacuolization increased, and the organelles became indistinguishable in the cytoplasm (Figures 1E,F). Most obviously, greatly increased cell wall thickness was observed in the BT-treated samples, and there was a positive correlation between fumigation time and cell wall thickness (Figure 2).

Six libraries with three biological replicates per treatment were constructed. The RNA-seq analysis data were deposited in the NCBI Sequence Read Archive (SRA) (SRP174462). Approximately 23 million clean reads were generated per sample, with >86% of reads mapping to the B. cinerea B05.10 genome, and an average of 81% of these reads were matched uniquely (Supplementary Table S2). The expression levels of 15,451 genes were calculated using the FPKM method.

As shown in Figure 6A, BT could stimulate B. cinerea to produce black pigment, and the DHN melanin synthesis inhibitor tricyclazole induced orange pigment production in the fungi, indicating that the black pigment was melanin. Melanin in fungi is considered protective and plays a role in fungal pathogenesis; however, we found that even with the increase in melanin, B. cinerea was less virulent after BT treatment (Figure 6B). Moreover, the addition of tricyclazole decreased the lesion growth of B. cinerea (Figures 6B,C). Based on the RT-qPCR results, we verified that the expression of melanin synthesis-related genes was increased (Figure 6D).

“Pentose and glucuronate interconversions” were significantly enriched in both the B. cinerea and B. odoriphaga transcriptomes (Figure 7D).

After BT treatment, 36 and 18 genes (Supplementary Table S1) for B. cinerea and B. odoriphaga, respectively, were chosen for RT-qPCR validation. Although the exact fold change ratio for the expression of each gene between the RT-qPCR and RNA-seq data was different, they showed consistent trends (Figures 8, 9). This result suggested that the RNA-seq and RT-qPCR results presented high consistency.

At the ultrastructural level, increased cell wall thickness was the most significant change caused by BT, and this change presented a concentration-responsive profile. The cell wall is the first protective barrier of B. cinerea, and this increase in cell wall thickness may be a defense response. DHN melanin is a crucial component of the extracellular matrix of B. cinerea (Zhang et al., 2015), and melanin granules are localized to the cell wall (Eisenman and Casadevall, 2012). Benzothiazole increased melanin biosynthesis, and it can reasonably be speculated that this increase in melanin synthesis may play a role in the thickening of the cell wall. However, this hypothesis requires further in-depth verification of whether there is an inevitable connection between these two processes. We have shown that inhibition of melanin attenuates B. cinerea infection, proving the importance of melanin in B. cinerea infection. Interestingly, the accumulation of melanin did not necessarily increase virulence. It has been reported that genes related to melanin synthesis, such as Bcpks13, Bcbrn1 and Bcscd1, participate in the melanization of B. cinerea but negatively regulate its virulence (Zhang et al., 2015). Therefore, we investigated seven melanin-related genes (including Bcpks13, Bcbrn1 and Bcscd1) to verify their expression levels, and the RT-qPCR results showed that these genes showed a trend toward increased expression. In addition, copper plays an important role in melanin production, likely because copper works as a required cofactor for melanin biosynthetic enzymes (Eisenman and Casadevall, 2012). Consistent with this role of copper, in the RNA-seq analysis of B. cinerea, we found that the copper ion transporter process was significantly different in the 48 h fumigation group. All the results showed that BT led to melanin accumulation at both the phenotype and transcript levels but negatively regulated the virulence of B. cinerea. Melanin is multifunctional and can improve fungal survival ability under a wide range of environmental stresses (Toledo et al., 2017). Considering that melanin accumulation in the fungal cell wall can prevent solute efflux (Liu et al., 2016), it is reasonable to speculate that melanin could avoid or reduce nutrient leakage in B. cinerea. In that case, however, why did the increase in melanin inhibit the growth and virulence of B. cinerea? We speculate that the production of melanin, a secondary metabolite, consumes many metabolic resources, thereby interfering with other normal cellular growth and development processes.

The eight commonly downregulated genes were all hypothetical or predicted genes, which made understanding their functions challenging. Nevertheless, among them, two genes (GeneID: 5,427,346 and 5,435,024) had descriptions in the KEGG database and conserved domain descriptions on NCBI, which helped us to predict their functions in response to the chemical compound BT. The hypothetical protein BC1G_14644 (5,427,346) has a conserved domain of the SIR2 superfamily of proteins, which includes silent information regulator 2 (Sir2) enzymes¹. Sir2 proteins (sirtuins) act as NAD⁺-dependent deacetylases in all eukaryotes; with NAD⁺ as a cofactor, they catalyze the conversion of acetylated lysine to deacetylated lysine, nicotinamide and 2’-O-acetyl-ADP-ribose (OAADPr) (Smith and Denu, 2006). Silent information regulator 2 NAD⁺-dependent protein deacetylases display a variety of physiological functions, such as transcriptional silencing (Gasser and Cockell, 2001; Rusche et al., 2003), apoptosis (Luo et al., 2001), mitosis (Dryden et al., 2003), fatty acid metabolism (Starai et al., 2002), and lifespan extension (Tissenbaum and Guarente, 2001; Rogina and Helfand, 2004). Thus, the observed decrease in BC1G_14644 transcription suggested a disruption of the above cellular processes and a shortened lifespan in B. cinerea. Another hypothetical protein, BC1G_07058 (5,435,024), contains a conserved domain of glycosyl hydrolase family 71, which is a family of α-1,3-glucanases. Fungal α-1,3-glucanases attack the plant cell wall by hydrolyzing α-1,3-glucan, and the observed decrease in α-1,3-glucanase suggested weakened virulence after BT treatment, consistent with the result seen in Figure 6B.

Among the commonly increased genes, the gene 5,434,344 (ATP-binding cassette, subfamily G, member 2, SNQ2) showed the highest expression during exposure to BT, and it was the only ABC transporter gene detected in our study. It is well known that ATP-binding cassette subfamily G member 2 (ABCG2) proteins can act on a broad array of xenobiotic compounds as substrates (Kusuhara and Sugiyama, 2007), and the ABC transporter plays an important role in the energy-dependent efflux of fungicides in fungi (Andrade et al., 2000; Hayashi et al., 2001). The rapid and high increase in the expression of this ABC transporter could decrease the accumulation of BT in B. cinerea. We also identified an increase in cytochrome P450 (gene 5,437,017), which plays an important role in the detoxification and metabolism of xenobiotics and drugs (Mcquarters et al., 2014). The increase in cytochrome P450 transcript level suggested a positive response to 2.5 μL/L BT. Overall, ABCG2 and cytochrome P450 were vital for the survival of B. cinerea under BT stress. Bcbrn1 (1,3,8-naphthalenetriol reductase), which is associated with melanin biosynthesis (Schumacher, 2016), was also increased throughout the experiment, suggesting possible melanization of the mycelia, and the current study has proven this hypothesis. In summary, BT activated the drug efflux, detoxification and secondary metabolite biosynthesis pathways of B. cinerea.

Phosphatidylserine decarboxylase (PSD) is responsible for the synthesis of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). Phosphatidylserine decarboxylase plays a central role in the phospholipid metabolism of prokaryotes and eukaryotes, and its catalytic product, PtdEtn, is a crucial phospholipid in mitochondria (Voelker, 1997) and plays an important role in full mitochondrial respiratory function (Birner et al., 2001; Gohil et al., 2005). Especially in the absence of fermentable carbon sources (e.g., glucose), the PtdEtn produced by PSD becomes essential (Birner et al., 2001). Thus, the increase in PSD transcription in the current study may suggest that the baseline glucose content of B. cinerea is not enough to support a defense against BT. Similarly, one gene (5,427,613) encoding a sugar transporter showed upregulation at all the exposure time points, suggesting a requirement for sugars as an energy source for B. cinerea. Overall, these results indicated that a larger energy supply was needed for B. cinerea in response to BT.

The current study indicated that BT mainly affected the metabolism of B. cinerea, especially carbohydrate metabolism. “Pentose and glucuronate interconversions” and “starch and sucrose metabolism” were potential targeted pathways in B. cinerea in response to BT, suggesting that carbohydrate metabolism was required. The most significant differences were the inhibition of pectin degradation and activation of starch decomposition. In the pectin degradation process, four pectinases, including two PMEs (Bcpme1 and Bcpme2) and two polygalacturonases (Bcpgx1 and Bcpg1), which are components of cell-wall-degrading enzymes and important virulence factors (Valette-Collet et al., 2003; Manfredini et al., 2006), were significantly downregulated in B. cinerea. It is known that pectin demethylation by PMEs is helpful for depolymerization by endopolygalacturonases (endoPGs) (Lionetti et al., 2007); thus, it is coincidental that the expression of Bcpme1, Bcpme2, and Bcpgx1 showed the same trend. The inhibition of pectin degradation suggested that BT had a major effect on the utilization of carbohydrate sources in B. cinerea.

AMPK is a metabolic sensor and is usually activated under conditions of reduced intracellular ATP production (low energy) (Mihaylova and Shaw, 2011). It has been reported that mitochondrial inhibitors (e.g., biguanides and resveratrol) and starvation could increase AMP and ADP levels, thereby activating the AMPK signaling pathway (Mihaylova and Shaw, 2011; Garcia and Shaw, 2017). The AMPK signaling pathway plays an important role in increasing glucose uptake and fatty acid oxidation (Garcia and Shaw, 2017) and regulating gene transcription (Mihaylova and Shaw, 2011). It is possible that starvation caused by apastia (Zhao et al., 2016a) activated the AMPK signaling pathway, which indicated a lower energy condition and metabolic dysfunction.

The insulin signaling pathway was the second most enriched signaling pathway affected by BT; insulin is a vital metabolic hormone regulating carbohydrate and lipid metabolism in organisms. The majority of DEGs in this pathway were fatty acid synthases (FASs), indicating a dysregulation of fatty acid biosynthesis and metabolism. Other genes, such as insulin receptor (INSR), insulin receptor substrate 1 (IRS1), 3-phosphoinositide dependent protein kinase-1 (PDPK1), RAC serine/threonine-protein kinase (AKT) and PEPCK, were upregulated to regulate glucose homeostasis. Insulin signaling can influence feeding and locomotion behavior in insects such as Drosophila (Erion and Sehgal, 2013). Our previous study found that B. odoriphaga showed no ingestion or activity when treated with BT for 6 h (Zhao et al., 2016a). However, motivation of foraging is crucial for the survival of the insect, and insulin signaling could probably initiate starvation-induced food acquisition behavior (Zhao and Campos, 2012).

The adipocytokine signaling pathway includes the signaling cascades caused by adipocytokines (TNF-α, leptin and adiponectin), which are pivotal signaling molecules associated with insulin resistance (Borst, 2004; Zhao et al., 2013). In our study, we identified 17 upregulated genes (out of 20 total in this pathway), including IRS1 and tyrosine-protein phosphatase non-receptor type 11 (SHP2), suggesting increased glucose uptake and decreased insulin resistance (Rung et al., 2009; Ogunyemi et al., 2013). Similarly, in the KEGG enrichment analysis, insulin resistance was significantly changed. Upregulation of PEPCK could promote glucose production, and upregulation of INSR and IRS could stimulate glucose uptake. The GO enrichment analysis also showed that carbohydrate transport, such as glucose transport, was significantly activated when B. odoriphaga was exposed to BT for 6 h. In summary, BT improved glucose uptake and transport in B. odoriphaga, consistent with a previous study of BT derivatives (Pasternak et al., 2014).

FoxO transcription factors are important in regulating responses to many stimuli, such as insect diapause, stress resistance, nutritional adversity and energy homeostasis (Kramer et al., 2003; Cyclase et al., 2009; Poelchau et al., 2011). The activation of the FoxO signaling pathway suggested a positive response by B. odoriphaga involving the regulation of physiological processes. The fifth most changed pathway affected by BT was the PPAR signaling pathway; PPAR acts as a fat sensor to regulate the transcription of fat-metabolizing enzymes. Notably, the AMPK signaling pathway can influence the other four signaling pathways, and FoxO and PPAR can be regulated by the insulin, AMPK and adipocytokine pathways (Suzuki et al., 2007). Overall, these five signaling pathways interact with each other and mainly regulate glucose and lipid metabolism. The activation of these signaling pathways suggested that the energy and nutrient levels in B. odoriphaga fluctuated severely under BT treatment. Previous biochemical results have shown that BT decreases the levels of three nutrient types, proteins, glucose and lipids, in B. odoriphaga (Zhao et al., 2016b). Meanwhile, the Venn diagram shown in Supplementary Figure S3 suggested an important role of PEPCK in the signal transduction of B. odoriphaga in response to BT.

Due to self-protection, B. odoriphaga refused to consume rhizomes exposed to the noxious and odorous compound BT. The above and previous results proved that low energy and glucose levels were present in B. odoriphaga after BT treatment for 6 h (Zhao et al., 2016b). In a state of starvation, insects adapt through risky behaviors, including searching for and consuming food under unfavorable conditions. We found that B. odoriphaga began to ingest the rhizomes at 24 h, thereby inducing alterations in the digestive system, including the “protein digestion and absorption,” “pancreatic secretion” and “salivary secretion” pathways. The associated digestive enzymes, especially trypsin, were greatly affected by BT, possibly leading to nutrient uptake impairment. As proteins are difficult to absorb through the lining of the small intestine, tryptic digestion is an essential component of protein absorption. Therefore, we can conclude that BT functions as a trypsin disrupter, interfering with protein absorption in B. odoriphaga.

In the glutathione metabolism pathway, we also found inhibition of 8 APN genes. In the small intestine, peptides from the hydrolysis of pancreatic and gastric proteases are finally digested with APN; therefore, the inhibition of APN again proved that BT affected protein absorption in B. odoriphaga. In addition, it has been proven that APN is a receptor for the Bacillus thuringiensis Cry1Ac toxin in Spodoptera litura and Helicoverpa armigera (Rajagopal et al., 2002; Sivakumar et al., 2007). Because BT is one of the secondary metabolites of B. subtilis (Liu et al., 2009a), we speculate that APN may be a potential target of BT. However, in the current study, the insect B. odoriphaga was treated with fumigation and not by feeding. Two reasons may explain these results: little BT residue was deposited on the rhizomes that were ingested by B. odoriphaga; there were still several upstream genes that regulated APN. Therefore, further studies are needed to verify this hypothesis, and we will investigate further in the future.

Furthermore, inhibition of tyrosine metabolism and melanogenesis were detected in our study, suggesting an inhibition of melanization in B. odoriphaga. Downregulation of 7 tyrosinase (also known as phenoloxidase, a rate-limiting enzyme for melanin biosynthesis) genes was responsible for the inhibition of the production of melanins, such as eumelanin. Moreover, tyrosinase is important for both developmental and defensive functions in insects, including melanization, sclerotization, wound healing, and parasite encapsulation; thus, tyrosinase inhibitors are being developed as alternative insecticides for controlling insect pests (Kim and Uyama, 2005). Given the vital role of tyrosinase in the immune defense mechanism of insects (Barnes and Siva-Jothy, 2000; Lee et al., 2008), its inhibition may cause decreased immunocompetence. Whether BT functions as a tyrosinase inhibitor still requires investigation.

The “pentose and glucuronate interconversions” pathway was altered in both the fungus B. cinerea and the insect B. odoriphaga, showing that carbohydrate metabolism was connected with the mode of action of BT in both B. cinerea and B. odoriphaga. However, the genes involved were different in the two species. In the insect B. odoriphaga, the UDP-glucuronosyltransferase (UGT; EC 2.4.1.17) and aldehyde reductase (AR; EC 1.1.1.21) genes accounted for a large percent of the DEGs participating in this pathway. UGTs are a group of phase II detoxification enzymes, and ARs are important in the accessory pathway of glucose metabolism, the polyol pathway. In contrast, in the fungus B. cinerea, the expression of pectin degradation enzyme genes such as Bcpme1 and Bcpgx1 was obviously decreased. Possible reasons for this difference are that the dose of the treatment was different (EC₅₀ for B. cinerea; LC₃₀ for B. odoriphaga), and the defense mechanisms of different species in response to the same exogenous compound are not necessarily the same. In summary, the most obvious commonality was that BT caused significant alterations in nutrient metabolism in both fungi and insects, and the “pentose and glucuronate interconversions” pathway was changed in both species.

One difference was that BT may act as a melanogenesis disrupter for the insect but as an activator for the fungus. In fungi, the increased melanin biosynthesis may have been based on compensation for other normal growth processes, such as carbohydrate metabolism. This may reflect the fact that an organism exposed to an exogenous compound shifts the balance among its potential physiological processes when energy conflict arises, supporting one process at the expense of another. However, melanogenesis is an immune defense mechanism in insects, and its decrease suggests an unfavorable growth condition in insects, which negatively affects the subsequent developmental process. Overall, when encountering adversity, the principles of resource allocation differ in different species.