Introduction {#Sec1}
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Taxol is a diterpenoid natural product, had been originally isolated and chemically identified from the bark of Pacific yew *Taxus brevifolia*^[@CR1]^. It has been approved by FDA in 1992 as a broad-spectrum drug for treatment of refractory ovarian, breast, melanoma, pancreatic and lung cancers. The anticancer activity of Taxol elaborated from its unique affinity to bind with tubulin of tumor cells, stabilizing microtubules from depolymerization, arresting the cellular division at G2-M phase, causing cell apoptosis^[@CR2]^. With the vast therapeutic applications of Taxol derivatives, their global demand continues to increase by about 6--10% annually, thus, the Taxol sources are the ongoing challenge for sustaining its affordability as commercial drugs^[@CR3]^. Exploring the endophytic fungi from Taxol producing and non-producing plants, with their fast growth, cost effectiveness and ability to produce Taxol independent on plant source, in addition to the feasibility of nutritional and molecular manipulation, raised the hope for using fungi for Taxol industrial production^[@CR3]^. More than 200 fungal endophytes were identified from *T*. *baccata* and about 10% of this population has the potentiality to produce Taxol^[@CR4],[@CR5]^. The biodiversity of endophytic fungi and their Taxol yield have been extensively reviewed^[@CR6],[@CR7]^. The putative Taxol biosynthetic pathway in fungi requires about 19 enzymatic steps starting from the precursor geranylgeranyl diphosphate (GGPP) that cyclized into taxa-4(5),11(12)-diene by taxadiene synthase followed by hydroxylation of taxadiene nucleus by cytochrome P450-monooxygenases (taxadiene 5α-hydroxylase and taxane 10β-hydroxylase)^[@CR3],[@CR8],[@CR9]^. The metabolic pathways for synthesis of Taxol and melanin from acetyl-CoA precursor has been summarized (Fig. [S1](#MOESM1){ref-type="media"}).

Several studies claimed the traditional medicinal plants of ethnopharmacological relevance could be a fertile source for various therapeutic compounds^[@CR10]^. Among these plants, species of Podocarpaceae were nominated for their antibacterial, antifungal and anticancer activities^[@CR7],[@CR11],[@CR12]^. From our previous study, twenty-four endophytic fungal isolates were recovered from *Podocarpus gracilior* and screened for Taxol biosynthetic potency, among them, *Aspergillus terreus* EFB108 was reported as a potent Taxol producer^[@CR7]^. However, the attenuation of Taxol biosynthetic machinery of *A*. *terreus* with subculturing and aging was the technical challenge that limits the ongoing application of this fungal isolate as industrial platform for Taxol production. Similar results were reported the silencing of expression of Taxol encoding genes with multiple subculturing of the endophytic fungi^[@CR13]--[@CR15]^, for examples, *Taxomyces andreanae*^[@CR13]^, *Tubercularia* sp^[@CR16]^ and *Periconia* sp^[@CR17]^. Interestingly, with addition of surface sterilized leaves of *P*. *gracilior*, the biosynthetic potency of Taxol by *A*. *terreus* had been restored completely comparing to the original isolate^[@CR7]^. So, re-triggering of the biosynthetic gene cluster of Taxol by *A*. *terreus* could be due to the intimate physical interaction with the endogenous microbiome of *P*. *gracilior*. Thus, the objective of this work was to evaluate the Taxol yield of *A*. *terreus* "endophyte of *Podocarpus gracilior*" in response to storage, multiple subculturing and potency to restore their Taxol biosynthetic machinery upon addition of *P*. *gracilior* leaves. And to unravel the molecular and metabolic identity of Taxol biosynthetic machineries of *A*. *terreus* associated with the attenuation and restoration of Taxol biosynthesis upon addition of *P*. *gracilior* leaves.

Materials and Methods {#Sec2}
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Aspergillus terreus growth conditions, Taxol extraction and chemical identification {#Sec3}
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*Aspergillus terreus* EFB108 (MF377552), an endophyte of *Podocarpus gracilior*, had the highest Taxol producing potency according to our previous study^[@CR7]^. The fungal isolate was grown on MID media^[@CR18]^ with slight modifications. Briefly, the medium contains (g/l): xylose 27 g, asparagine 6 g, yeast extract 0.5 g, soytone 1.0 g, Ca(NO~3~)~2~ 0.2 g, KNO~3~ 0.08 g, KCl 0.06 g, MgSO~4~ 0.36 g, NaH~2~PO~4.~ H~2~O 0.02 g, FeCl~3~ 0.002 g, MnSO~4~ 0.005, ZnSO~4~. 7H~2~O 0.003 g, H~3~BO~3~ 0.014 g and KI 0.0001 g, dissolved in distilled water of initial pH 8.0. Two plugs of 6-day-old *A*. *terreus* were inoculated to 50 mL medium/250 mL Erlenmeyer flask, incubated for 20 days at 30 °C. The cultures were filtered with sterile cheesecloth, the filtrates were amended with 0.03% sodium bicarbonate to precipitate fatty acids, and Taxol was extracted with double volume of dichloromethane (DCM). The organic phase was collected, evaporated to dryness, and the residues were re-dissolved in 2 mL methanol.

Taxol was identified by thin layer chromatography (TLC) using 1 mm (20 × 20 cm) pre-coated silica gel plates 60 F~254~ (Merck KGaA, Darmstadt, Germany), with the solvent system methylene chloride/methanol/dimethyl formamide (90:9:1, v/v/v)^[@CR19]^. Taxol was detected by UV illumination at 254 nm, then the plates were sprayed with 1% acidic vanillin, gentle heating, a dark gray spot was developed after 24 hours, allocating the Taxol spots^[@CR20]^ comparing to authentic Taxol (Cat. \# T7402).

The putative spots of silica containing Taxol were scraped-off, dissolved in methylene chloride, the purity and concentration of Taxol were analyzed by HPLC (Agilent Technology, G1315D) of C18 reverse phase column (Cat.\# 959963-902) with isocratic mobile phase of methanol/acetonitrile/water (25:35:40, v/v/v) at flow rate 1.0 mL/min for 20 min^[@CR21]^. The fractions were scanned from 200 to 500 nm by photodiode array detector (DAD), their identity and concentration were confirmed from the retention time and peak area at 227 nm comparing to authentic Taxol. The chemical structure of extracted Taxol was confirmed from the ^1^H and ^13^C NMR spectra (JEOL, ECA-500II, 500 MHz NMR) comparing to authentic Taxol^[@CR7]^.

Effect of subculturing and storage time of A. terreus on its Taxol productivity {#Sec4}
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The effect of subculturing of *A*. *terreus*, till the 10^th^ generation, on its Taxol productivity was assessed. First culture, is the originally recovered isolate from *P*. *gracilior*, that subsequently transferred into potato dextrose agar (PDA) plates, incubated at 30 °C for 10 days, and then repeated the subculturing till the 10^th^ subcultural generation. The isolate from each generation was centrally inoculated to PDA plates, incubated at 30 °C, and the conidial and mycelial pigmentation were inspected daily. The fungal isolates from each generation were grown on modified MID media^[@CR7]^, incubated under standard conditions for 20 days, then Taxol was extracted, purified and quantified as described above. The mycelial fungal pellets were washed by sterile saline and kept at −20 °C for further biochemical analyses.

The effect of storage time of *A*. *terreus* on their Taxol biosynthetic stability has been evaluated. The original cultures of *A*. *terreus* were stored as slope cultures on MID media^[@CR18]^ at 4 °C, the Taxol productivities were evaluated after 1, 2, 3, 4, 5 and 6 months of storage. After culture incubation, the Taxol was extracted and quantified by the standard assay as mentioned above. As well as, the mycelial melanin and acetyl-CoA were extracted from each culture in parallel with Taxol estimation, as below.

Estimation of melanin and acetyl-CoA {#Sec5}
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Melanin was extracted from the mycelial culture of *A*. *terreus*^[@CR22]^. Briefly, 0.5 g of mycelial biomass was homogenized in 2 M NaOH (pH 10.5), incubated for 48 h, centrifuged at 5000 rpm for 20 min, the pH of supernatant was acidified to 2.5 with 2 M HCl. The mixture was incubated overnight, centrifuged at 5000 rpm for 20 min, supernatant was decanted, and the precipitate was collected, acid hydrolyzed with 6 M HCl at 100 °C for 2 h. The mixture was treated with ethylacetate, the precipitate was dissolved in 2 M NaOH, centrifuged at 5000 rpm for 15 min, the supernatant was collected in pre-weighed tube, acidified with 6 M HCl, dried at room temperature. The purified melanin was dissolved in 1 mL borate buffer (pH 8.0, 100 mM) and measured at 459 nm^[@CR23]^ regarding to authentic melanin.

The concentrations of acetyl-CoA were assessed for the different cultures of *A*. *terreus*. The fungal biomass was harvested and pulverized (10 g) in liquid nitrogen, dispensed in 10 mL of 100 mM HEPES buffer (pH 7.5) containing 1 mM DTT, thoroughly vortex for 5 min. The homogenate was centrifuged at 10000 rpm for 5 min at 4 °C, and the supernatant was used for acetyl-CoA determination^[@CR24]^. Acetyl- CoA was determined by HPLC (Agilent Technol, G1315D, C18 reverse phase column Cat. \# 959963-902) with mobile phase 100 mM monosodium phosphate, 75 mM sodium acetate and 6% acetonitrile, the pH was adjusted to 4.6 with phosphoric acid. The flow rate of mobile phase was 40 µl, and detection wavelength at λ~259~ nm^[@CR25]^. As well as, the concentration of acetyl-CoA was determined by Acetyl-Coenzyme A assay Kit (Cat. \# MAK039, Sigma-Aldrich, Louis, MO, USA) according to the manufacturer's instructions. The amount of acetyl-CoA was calculated based on the peak area and retention time of the authentic sample (Cat\#. A2056).

Activities of rate-limiting enzymes of Taxol biosynthesis {#Sec6}
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The activities of rate-limiting enzymes of Taxol biosynthesis; taxadiene synthase, 10-deacetyl-baccatin III-*O*-acetyltransferase and baccatin III 13-*O*-(3-amino-3-phenylpropanoyl) transferase were assessed. For extraction of these enzymes, the collected fungal biomass (20 g) were washed, pulverized in liquid nitrogen, and dispensed in 50 mL of 30 mM, pH 8.0 HEPES buffer with 5 mM sodium ascorbate, 5 mM DTT, 5 mM Na~2~S~2~O~5,~ 15 mM MgCl~2,~ 10% (v/v) glycerol and 1% (w/v) polyvinylpolypyrrolidone^[@CR26]^. The mixtures were shacked thoroughly for 15 min, centrifuged at 8000 rpm for 15 min at 4 °C, the supernatant was used as source of crude enzymes. The activities of the putative enzymes for Taxol biosynthesis were estimated as follow;

### Taxadiene synthase (TDS) activity {#Sec7}

The activity of TDS was determined as adopted by^[@CR26]^ with minor modifications. The reaction mixture contains 50 mM GGPP, 5 mM MgCl~2~ in 50 mM Tris-HCl pH 8.0 and 500 µl of the enzyme extract in 2 mL reaction volume. The reaction was incubated at 32 °C for 1 h, terminated by 50 µl of EDTA (0.5 M, pH 8.0). The GGPP level was determined by TLC using silica gel plates 60 F~254~ (Merck KGaA, Darmstadt, Germany) with developing solvent system propan-2-ol, ammonia and water (9:3:1) regarding to the authentic GGPP (Cat\#. G6025) after visualization by vapor iodine^[@CR27]^. The intensity of GGPP spots were determined by ImageJ software^[@CR28]^. One unit of TDS activity was expressed by the amount of enzyme consuming 1 µmol of GGPP per min under the standard assay conditions.

### 10-Deacetylbaccatin III-*O*-acetyltransferase (DBAT) activity {#Sec8}

The activity of DBAT was determined^[@CR29]^. The reaction mixture contains 20 mM 10-deacetylbaccatin (10-DAB), 20 mM acetyl-CoA dissolved in 50 mM Tris-HCl (pH 8.0), and 500 µl of enzyme extract in 2 mL total volume. After incubation at 30 °C for 1 h, the mixture was extracted with equal volume of chloroform, the organic layer was evaporated, and the residue was dissolved in 0.5 mL ethanol. The released baccatin III was quantified by HPLC with solvent system; water: acetonitrile (1:9 v/v) at flow rate 1 mL/min^[@CR30]^. The concentration of baccatin III was determined from the peaks area comparing to authentic sample. One unit of DBAT was expressed by amount of enzyme releasing 1 µmol of baccatin III under standard assay conditions.

### Baccatin III-13-*O*-(3-amino-3-phenylpyropanyoyl) transferase (BAPT) activity {#Sec9}

The BAPT activity was estimated^[@CR31]^ as follows; the mixture contains 10 mM acetyl-CoA, 10 mM baccatin III in Tris-HCl pH 8.0 and 500 µl of enzyme extract in 1 mL reaction volume. The pH was increased to 9.0 with sodium bicarbonate, followed by addition of 10 µM benzoyl chloride, incubated for 1 h to induce *N*-benzoylation^[@CR32]^. The concentration of synthesized 2-deoxytaxol was determined by HPLC. One unit was expressed by the amount of enzyme releasing 1 µmol of 2-deoxytaxol at standard conditions.

RNA Isolation, cDNA Synthesis, real-time PCR analysis {#Sec10}
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The molecular expression of Taxol biosynthesis rate limiting genes such as *tds*, *dbat* and *bapt* by *A*. *terreus* were assessed. The fungal mycelial pellets were pulverized to a fine powder and the total RNA was extracted using IQeasy^TM^ plus Plant Mini Kit (Cat\#. 17491, iNtRON Biotech. Korea). cDNA was synthesized by SuperScript III First Strand Synthesis Kit (Invitrogen, USA) with oligo-dT primes. For qRT-PCR analyses, the reaction mixtures contain cDNA, forward and reverse primers, with Topreal^TM^ One-Step RT qPCR Kit (Cat\#. RT432S) according the manufacturer's instructions using the real-time PCR machine (Agilent Technology, Stratagene Mx3005P). The primers for RT-qPCR molecular expression analysis ion of *tds*, *dbat* and *bapt* were listed in Table [1](#Tab1){ref-type="table"}. The qPCR was programmed to initial denaturation at 95 °C for 3 min, followed by 40 cycles of 95 °C for 15 s, 55 °C for 30 s (annealing), 72 °C for 1 min (extension). Melting curve analyses were performed at 55--95 °C. Triplicates of each sample were conducted. Data were normalized to the constitutively expressed actin *actaA* gene of *A*. *terreus* as endogenous control. The expression folds of the target genes were calculated from standard curve of relative quantification^[@CR33]^. Statistical comparisons were conducted using Student's *t*-test, and the p-value ≤ 0.05 were considered significant. Data are presented as fold of change between the control fungal cultures and cultures amended with leaves of *P*. *gracilior*, comparing to negative controls of *P*. *gracilior*.Table 1Oligonucleotides primers for PCR amplification of fragments of Taxol encoding genes.PrimerGene IDPrimers (F, R) 5′-3′qTDSJQ618974.15′-CCAGGTAGTTTCGTGTCCAA-3′; 5′-GTTGCCTGAACAGGTGAGTA-3′qDBATKR047791.15′-CACCACTTTCGAAGGGATACT-3′; 5′-TCACGATTGTTGACGTGAAATG-3′qBAPTKC959480.15′-TGAGGACCTCCATCTCTTCAT-3′; 5′-TACACATTCGCTCCCACAAC-3′pbcRXM654111.15′-GAATGGCGTAGGACTGTTG-3′; 5′-CGTATCCATAAACTCGGTAATC-3′*acta*AXM749985.15′- GACTGGTTTGGCAATTGATG- 3′; 5′- GCATCAGTGATCTCACGCTT- 3′

Effect of *P*. *gracilior* leaves on restoring the Taxol biosynthesis by *A*. *terreus* {#Sec11}
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The surface sterilized leaves of *P*. *gracilior* had a significant effect on inducing the Taxol yield by *A*. *terreus*^[@CR7]^. The fungal isolate was grown on modified MID media, incubated at 30 °C for 10 days, then the cultures were amended with different concentration (0.5, 1.0, 3.o and 5.0%) of surface sterilized leaves of *P*. *gracilior*, and the cultures were re-incubated for 20 days under standard conditions. Negative control media of *P*. *gracilior* leaves at the same concentrations without fungal spores were used. After incubation, the cultures were filtered, Taxol was estimated, and activity and molecular expression of Taxol encoding genes were analyzed as described above.

Proteomics analyses of *A*. *terreus* in response to addition *P*. *gracilior* leaves {#Sec12}
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The proteomic analyses of *A*. *terreus* in response to supplementation with *P*. *gracilior* leaves were assessed. After incubation of each fungal cultures, the total intracellular proteins were extracted, and quantified^[@CR34]^. Equal concentrations of extracted proteins from each fungal culture were electrophoresed using 12% SDS-PAGE^[@CR35]^. The prominent over-induced protein bands in response to addition of *P*. *gracilior* leaves were identified by MALDI-TOF/TOF analysis (Bruker Daltonics) at the Proteomics Research Lab, Medical Research Center, Alexandria University, Egypt. The target gel bands were excised from the gel, washed with 50 µl washing buffer (10 mM NH~4~HCO~3~ and 50% ACN), followed by addition of 100 µl ACN to shrink the gel and 25 µl 20 mM NH~4~HCO~3~ to rehydrate the gel~,~ then the drying by vacuum centrifuge^[@CR36],[@CR37]^. The gel was reduced by 50 µl reduction solution (10 mM DTT/10 mM NH~4~HCO~3~) for 15 min at 56 °C, then gel alkylation (55 mM IAA/10 mL NH~4~HCO~3~) for 20 min in dark, then gel washing and drying prior to in-gel trypsinization (20 ng/μl of trypsin in 10 mM NH4HCO3)^[@CR37]^. The peptides were extracted with 50% ACN and 1% TFA, analyzed by MALDI-TOF/TOF. A linear gradient of acetonitrile (ACN) (5.0--60%), with flow rate 250 μl min^−1^ was used for eluting peptides from the column to the mass spectrometer. MS/MS data were acquired independently in which the MS1 data were acquired for 250 ms at m/z 400--1250 and at m/z 50--2000 Da. The raw MS/MS data files were extracted, and the peptides were analyzed and identified by Protein Pilot 4.0 (ABSCIEX)^[@CR38]^ normalizing to the proteome of *Aspergillus terreus*^[@CR39]^. The identification criteria included at least five peptide fragment ions per protein with E-values \< 0.05.

Compliance with Ethical Standards {#Sec13}
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This article does not contain any studies with human or animal subjects performed by any of the authors.

Results {#Sec14}
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Metabolic stability of *A*. *terreus* for Taxol production with subculturing and storage {#Sec15}
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The productivity of Taxol by *A*. *terreus* till the 10^th^ subcultural generations have been investigated. The culture of *A*. *terreus* grown on PDA, were incubated at standard conditions for 20 days, and the visual appearance of the fungal mycelial pigmentation was daily inspected and photographed. The cultural mycelial biomass was collected and kept at −20 °C, while, the filtrate was used for Taxol extraction and quantification. From the results (Fig. [1](#Fig1){ref-type="fig"}), Taxol productivity of *A*. *terreus* was sequentially decreased, accompanied with visual fading of the mycelial melanin pigmentation with the multiple subculturing. The Taxol yield for the original culture of *A*. *terreus* grown on modified MID media was 268 µg/l, while their yields for the 5^th^, 7^th^ and 10^th^ generations were 133.4, 94.7 and 78.2 µg/L, respectively, as revealed from the TLC and HPLC chromatograms. Thus, by the 6^th^ subculture, *A*. *terreus* lost about 60% of its metabolic potency for Taxol production comparing to the original isolate. In parallel with the reduction on Taxol productivity by *A*. *terreus*, a visual fading of their mycelial pigmentation was observed with the fungal subculturing. The mycelial melanin was quantitively estimated, intriguingly, the yield of melanin of *A*. *terreus* was reduced sequentially with fungal subculturing, by the 5^th^ generation, the yield of melanin was reduced by about 30%, comparing to the original culture, that was strongly correlated with the total yield of Taxol. Overall, the fungus retains about 50% of its Taxol productivity by the 7^th^ subculture. The metabolic pathways for synthesis of these two metabolites are competing for the same biosynthetic precursors "acetyl-CoA" (Fig. [S1](#MOESM1){ref-type="media"}). So, the decreasing yields of both Taxol and melanin could be ascribed to decreasing on influx of acetyl-CoA to these metabolic pathways, through overall silencing of the different machineries generating pyruvate and acetyl-CoA precursors.Figure 1Taxol productivity and mycelial pigmentation of *A*. *terreus* with the multiple subculturing. (**A**) Morphological features of *A*. *terreus* EFB108 sub-cultured on Czapek's agar (upper panel) and MID medium (lower panel) till the 10^th^ subcultures. (**B**) TLC chromatogram of extracted Taxol. (**C**) HPLC concentration of Taxol. (**D**) Relative concentrations of melanin extracted from the different subcultures of *A*. *terreus*. (**E**) Taxol yield of *A*. *terreus* with the fungal storage as slope cultures at 4 °C for 8 months.

The influence of storage time of the original isolate *A*. *terreus* on its Taxol productivity was assessed. The original isolate was stored as slope culture at 4 °C, and its Taxol productivity was checked monthly along 8 months. From the obtained results (Fig. [1F](#Fig1){ref-type="fig"}), the yield of Taxol was sequentially decreased with the storage time. By the 6^th^ month of storage, the yield of Taxol by *A*. *terreus* was reduced by about 3.5 folds, comparing to the original culture. The mechanism of metabolic changes of *A*. *terreus* for Taxol biosynthetic potency with the storage time remains equivocal, however, it might be attributed to the aging of cells or re-programing of the cellular machinery with the storage time. So, the metabolic attenuation of *A*. *terreus* for Taxol biosynthetic potency could be not only due to the multiple subculturing but also to the storage time negating the carrying-over of Taxol residues from the host plant.

Activity and molecular expression of the rate-limiting enzymes of Taxol biosynthesis {#Sec16}
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To validate the biosynthetic stability of Taxol by *A*. *terreus*, the rate-limiting enzymes of Taxol biosynthesis such as geranylgeranyl-pyrophosphate synthase (GGPPS), taxadiene synthase (TDS), 10-deacetylbaccatin III-*O*-acetyltransferase (DBAT) and baccatin III 13-*O*-(3-amino-3-phenylpropanoyl) transferase (BAPT) were assessed. After cultural incubation, the intracellular proteins were extracted, and the activity of GGPPS, TDS, DBAT and BAPT were estimated. From the results (Fig. [2](#Fig2){ref-type="fig"}), the activity of GGPPS, TDS, DBAT and BAPT were subsequently suppressed with the fungal subculturing comparing to the original culture, correlating with the decreasing on Taxol yield. The activity of these enzymes by *A*. *terreus* were reduced by about 50% by the 5^th^ fungal cultural generation regarding to the original culture. However, these enzymes retain less than 10% of their initial activities by the 10^th^ fungal cultural generation. Interestingly, the activities of Taxol rate-limiting enzymes by *A*. *terreus* being consistent with the Taxol yield for the different cultural generations. In addition to the activity of putative enzymes and quantitative analyses of Taxol, visual inspection of TDS activity on TLC was assessed for the different fungal subcultures (1^st^-10^th^). The TDS activity was assessed, the residual concentration of GGPP substrate was determined by TLC, visualized by vapor iodine solution^[@CR27]^. From the results (Fig. [2E](#Fig2){ref-type="fig"}), the intensity of residual GGPP spots, as determined by ImageJ software, was strongly increased by about 5 folds with the progression of fungal subculturing suggesting the attenuation of TDS activity, in contrary to the original culture.Figure 2Activity and molecular expression of Taxol rate-limiting enzymes (GGPPS, TDS, DBAT and BAPT) by *A*. *terreus* in response to frequent subculturing. After cultural incubation, the intracellular proteins from the fungal biomass were extracted, and the activity of Taxol biosynthetic enzymes such as GGPPS (**A**), TDS (the onset figure shows the relative activity of TDS with the fungal subcultures) (**B**), DBAT (**C**) and BAPT (**D**), were estimated. (**E**) TLC profile of the residual GGPP of TDS reaction (20 µl) of *A*. *terreus* (from 1^st^ till 10^th^ subculture) visualized by vapor iodine solution. Quantitative RT-PCR analyses for expression of TDS, DBAT and BAPT genes with subcultures of *A*. *terreus* (**F**) using one-step real time kit for qPCR analysis as in Materials and Methods.

The molecular expression of Taxol encoding genes were determined by qPCR using cDNA as template, normalizing to actin gene, using the listed primers in Table [1](#Tab1){ref-type="table"}. From the results (Fig. [2F](#Fig2){ref-type="fig"}), the expression of *A*. *terreus* Taxol biosynthetic genes *tds*, *dbat* and *bapt* were sequentially reduced with the fungal subculturing, normalizing to expression of *actinA* as house-keeping gene. Interestingly, the molecular expression of *tds*, *dbat* and *bapt* was conceivably reduced with the fungal subculturing, that correlates with the metabolic yield of Taxol and activities of their rate-limiting enzymes. By the 5^th^ and 10^th^ subculture, the expression of *tds*, *dbat* and *bapt* was reduced by about 15 and 35 folds, respectively, comparing to the original culture. Taken together, the results of Taxol yield by chromatographic approaches, activities of rate-limiting enzymes and molecular expression by qPCR analyses, approve the attenuation of the biosynthetic machinery of Taxol by *A*. *terreus* with the fungal subculturing. These results have been frequently reported for a plethora of secondary metabolites production by fungi with growing as monoaxenic cultures, under standard laboratory conditions^[@CR15]^.

Restoring the Taxol biosynthetic potency of *A*. *terreus* upon addition of *P*. *gracilior* leaves {#Sec17}
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To evaluate the ability of *A*. *terreus* to restore its Taxol biosynthetic machinery, the 7^th^ fungal subcultures were grown on Taxol production medium for 10 days, then amended with sterilized leaves of *P*. *gracilior* and re-incubated under standard conditions. The 7^th^ subculture of *A*. *terreus* was selected as it retains 50% of Taxol biosynthetic potency comparing to original one. Taxol was extracted, and molecular expression of its biosynthetic genes were estimated. Practically, the biosynthetic potency of Taxol by *A*. *terreus* has been gradually restored upon addition of surface sterilized leaves of *P*. *gracilior* comparing to the positive (7^th^ subculture of *A*. *terreus*) and negative control (surface sterilized leaves without fungus) (Fig. [3A](#Fig3){ref-type="fig"}). The yield of Taxol by *A*. *terreus* was significantly increased by about 3.5 folds (395 µg/L) with addition of 3% surface sterilized leaves of *P*. *gracilior* to the fungal culture, comparing to positive controls (7^th^ culture of *A*. *terreus*) (124 µg/L). The Taxol yield by *A*. *terreus* cultures amended with 0.5% and 1.0% sterilized leaves of *P*. *gracilior* was increased by 2 folds (210 µg/L) and 3.2 folds (315 µg/L), respectively, comparing to the positive control fungus as shown from the TLC and HPLC profiles (Fig. [3A,C](#Fig3){ref-type="fig"}). From the HPLC profile, there is no detectable Taxol by the negative control *P*. *gracilior* leaves.Figure 3Effect of surface sterilized *P*. *gracilior* leaves on restoring the Taxol productivity of *A*. *terreus*. The 7^th^ fungal subculture was grown on MID medium, incubated for 10 days, then amended with surface sterilized leaves of *P*. *gracilior* at 0.5, 1, 3 and 5% (w/v), incubated for 20 days, Taxol was extracted and quantified by HPLC (**A**). The activities of Taxol biosynthesis rate-limiting enzymes, TDS, DBAT and BAPT (**B**) were assessed. Molecular expression of TDS, DBAT and BAPT genes by qRT-PCR analysis (**C**).

To validate the chromatographic results, the activity of Taxol rate-limiting biosynthetic enzymes "TDS, DBAT and BAPT" were determined. From the results, the activities of these enzymes by *A*. *terreus* were strongly increased upon addition of *P*. *gracilior* leaves, that highly correlated with the yield of Taxol by HPLC and TLC. The activities of TDS, DBAT and BAPT by *A*. *terreus* were increased by about 2.0--3.0 folds upon addition of 3% sterilized leaves of *P*. *gracilior* comparing to the positive control fungal cultures (Fig. [3B](#Fig3){ref-type="fig"}). The activity of TDS was 4.9 ± 0.1, 5.5 ± 0.1, 6.7 ± 0.2 and 5.9 ± 0.2 µmol/min/mg for the 7^th^ *A*. *terreus* cultures amended with 0.5, 1.0, 3.0 and 5.0% of *P*. *gracilior* leaves, respectively, comparing to positive control cultures of *A*. *terreus* (without *P*. *gracilior*) (2.8 ± 0.1 µmol/min/mg). Consistently, the activities of DBAT and BAPT were compatible with the activity of TDS and Taxol yield in response to addition of *P*. *gracilior* leaves, ensuring the restoration of the whole enzymatic system for Taxol synthesis with addition of *P*. *gracilior* leaves. The activity of TDS for the 7^th^ subculture of *A*. *terreus* amended with *P*. *gracilior* leaves regarding to the residual levels of GGPP as substrate was authenticated from visual inspection of TLC sprayed with iodine vapor (Fig. [3D](#Fig3){ref-type="fig"}). From the TLC chromatogram, the intensity of residual GGPP spots was reduced gradually with addition of *P*. *gracilior* leaves, comparing to control (7^th^ culture of *A*. *terreus* without *P*. *gracilior* leaves), assuming the higher activity of TDS. At 3% *P*. *gracilior* leaves, the intensity of GGPP as results of TDS activity of *A*. *terreus* was reduced by about 3 times, ensuring the higher activity of TDS upon plant leaves addition comparing to control. Practically, the metabolic productivity of *A*. *terreus* for Taxol has been restored completely upon supplementation of the fungal culture with surface sterilized leaves of *P*. *gracilior* as authenticated from the chromatographic assessment of Taxol and activities of their rate-limiting enzymes.

Further molecular expression analyses to the putative Taxol biosynthetic genes have been investigated by qPCR, using the total cDNA as template, and specific primers as listed in Table [1](#Tab1){ref-type="table"}. The expression folds were normalized to the actin *actaA* gene, as house-keeping gene, and 7^th^ subculture of *A*. *terreus* without *P*. *gracilior* leaves as positive control. From the molecular expression results (Fig. [3E](#Fig3){ref-type="fig"}), the expression of Taxol-biosynthetic genes *tds*, *dbat*, *bapt* of *A*. *terreus* was increased sequentially with incorporation of 3% *P*. *gracilior* leaves by about 4--5 folds comparing to the control culture of *A*. *terreus* (Zero plant leaves). While, the expression of these genes was increased by about 2.5--3.0 upon supplementation of 5% *P*. *gracilior* leaves to the cultures of *A*. *terreus*, comparing to control. From the metabolic yield of Taxol and activities of its biosynthetic enzymes, it is clearly shown the supplementation with surface sterilized leaves of *P*. *gracilior* restore the molecular machinery of Taxol production by *A*. *terreus*.

Intracellular levels of *A*. *terreus* acetyl-CoA in response to subculturing and addition of *P*. *gracilior* {#Sec18}
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From the above results, the metabolic correlation of Taxol and melanin biosynthesis by *A*. *terreus* and their attenuation with the multiple fungal subculturing and storage, have been authenticated from the chromatographic, spectroscopic and molecular analyses. Since the two metabolic pathways are acetyl-CoA- dependent as precursors (Fig. [S1](#MOESM1){ref-type="media"}), thus, we hypothesized that this attenuation of Taxol and melanin productivity could be due to diminishing on the influx of acetyl CoA, in addition to silencing of the molecular biosynthetic machinery of polyketides and terpenoids. To validate this hypothesis, the concentration of acetyl-CoA as crucial intermediate of Taxol and melanin biosynthesis by *A*. *terreus* in response to multiple subculturing and addition of sterilized *P*. *gracilior* leaves was determined. From the HPLC profiles (Fig. [4A](#Fig4){ref-type="fig"}), the intracellular levels of acetyl-CoA of *A*. *terreus* was reduced consecutively with the fungal subculturing. At 3^rd^, 5^th^ and 7^th^ subcultural generation of *A*. *terreus*, the concentration of acetyl-CoA was 620.2, 515 and 450.9 ppm, respectively, comparing to the original culture of *A*. *terreus* (870.1 ppm). Thus, by the 7^th^ and 9^th^ subculture of *A*. *terreus*, the intracellular concentration of acetyl-CoA was reduced by 2 and 3 folds, respectively, comparing to the original culture. The gradual decreasing on the acetyl-CoA levels of *A*. *terreus* with the multiple subculturing are highly correlated with the attenuation on their Taxol yield.Figure 4Concentration of *A*. *terreus* Acetyl-CoA in response to subculturing and amendment with various concentrations of *P*. *gracilior* leaves. The cultures of *A*. *terreus* were incubated under standard conditions, then, the fungal biomass was collected, and the intracellular acetyl-CoA were determined. Concentrations of *A*. *terreus* acetyl-CoA in response to subculturing (**A**). The 7^th^ subculture of A. terreus was grown for 10 days on standard conditions, then the cultures were amended with *P*. *gracilior* leaves (zero-5% w/v), continue incubation under standard conditions till 20 days, then Acetyl-CoA was extracted and quantified (**B**).

The impact of addition of *P*. *gracilior* leaves on restoring the biosynthetic potency of acetyl-CoA by *A*. *terreus* has been evaluated. Interestingly, the biosynthetic potency of acetyl-CoA has been resumed completely upon addition of 3% surface sterilized leaves of *P*. *gracilior* (Fig. [4C,D](#Fig4){ref-type="fig"}). Upon addition of 3% *P*. *gracilior* leaves, the intracellular levels of the 7^th^ subculture of *A*. *terreus* acetyl-CoA was increased by about 2.8 and 5.4 folds regarding to the original culture and 7^th^ culture without *P*. *gracilior* leaves, respectively. Negative controls of *P*. *gracilior* leaves without fungal inocula has been used to baseline the calculated concentrations of acetyl-CoA of samples. Practically, the resuming on intracellular levels of acetyl-CoA was typically linked to the restoring of Taxol yield by *A*. *terreus* in response to subculturing and addition of *P*. *gracilior* leaves, suggesting the influx of acetyl-CoA is the profound factor limiting the anabolism of acetyl-CoA dependent metabolites especially Taxol.

Protein profiling of *A*. *terreus* in response to subculturing and addition of *P*. *gracilior* leaves {#Sec19}
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The protein profile of *A*. *terreus* in response to frequent subculturing and addition of *P*. *gracilior* surface sterilized leaves was evaluated using SDS-PAGE. Equal amounts (25 µg/mL) of the isolated intracellular proteins were loaded to gel wells, after gel running, CBB staining, the protein banding patterns were visually inspected, and the intensity of emerged protein bands were analyzed by GelQuant software normalizing to the original culture of *A*. *terreus*. Negative control of the surface sterilized leaves of *P*. *gracilior* without *A*. *terreus* were used, under the same conditions. From the protein banding pattern (Fig. [S2](#MOESM1){ref-type="media"}), substantial changes in the protein profiling were observed with the subsequent culturing of the fungus as well as with the supplementation of *P*. *gracilior* leaves. Visually, the intensity of proteins especially of 74.4, 68.2, 37.9, 24.1 and 22.2 kDa were substantially decreased with the frequent subculturing, connecting to decreasing on Taxol yield, regarding to the original fugal culture (Fig. [S2](#MOESM1){ref-type="media"}). The intensity of proteineous components of 74.4 and 22.2 kDa were reduced by \~ 50% from the 3^rd^ to 10^th^ subculture. While intensities of the protein bands of 68.2, 37.1 and 24 kDa were reduced exponentially with the fungal subculturing, retaining about 15--20% of their original intensities. Since the expression of these proteins were dramatically attenuated with the fungal subculturing, it could be anticipated that, these proteins are associated with the biosynthetic metabolic of Taxol.

Fascinatingly, the intensities of proteins 74.4 kDa, 68.2 kDa and 37.1 kDa were completely restored corresponding to proteins from of the original culture that correlated with the restoring of Taxol biosynthesis, raising the hypothesis that these proteins might be implemented on Taxol biosynthesis. While the protein of 24.1 and 22.2 kDa, that had been exponentially decreased with the fungal subculturing, did not affected by incorporation of *P*. *gracilior* leaves negating the implication of these proteins on Taxol biosynthesis. Thus, correlation of restoring the Taxol biosynthesis with the re-expression of the attenuated proteins (74.4, 68.2 and 37.1 kDa), strongly evidenced the implication of these proteins on Taxol biosynthesis. Particularly, these proteins are generally corresponding to the predicted protein masses implemented in Taxol biosynthesis. For more validation of these results, the restored protein compartments (74.4, 68.2 and 37.1 kDa) were sequence and identified.

Proteomic analysis of *A*. *terreus* in response to subculturing and addition of *P*. *gracilior* leaves {#Sec20}
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From the SDS-PAGE analysis, the expression of proteins of 74.4, 68.2 and 37.1 kDa were the most sensitive proteins for downregulation with fungal subculturing and restoration with addition of *P*. *gracilior* leaves, that practically correlated with the attenuation and resuming of Taxol productivity by *A*. *terreus* (Fig. [5A](#Fig5){ref-type="fig"}). Thus, these proteins were excised, digested with trypsin, sequenced and identified by MALDI-TOF/TOF. For annotation, the recovered peptides were searched in the proteome of *A*. *terreus* and in the NCBI non-redundant protein database. The protein band of 74.4 kDa displayed a higher similarity to ribosome biogenesis protein YTM (Fig. [5B](#Fig5){ref-type="fig"}), with score identity 58% and expected value 0.056 and matches 11. From the conserved domain analysis, this protein had two domains the major one belongs to WD40 superfamily (390 amino acid) and minor one belongs to NLE superfamily (90 amino acids) (Fig. [5C](#Fig5){ref-type="fig"}). The recovered peptide sequences were non-redundantly blast searched on the protein database of NCBI, displaying two biological functions, ribosome biogenesis and microtubule-associated proteins (Fig. [5D](#Fig5){ref-type="fig"}). The ribosome biogenesis peptide sequence from *A*. *terreus* has been deposited into database with UniProt SPIN200017187. However, with the blast searching of recovered peptide sequences within the proteome of *A*. *terreus*, the results displayed a similarity with proteins of multiple biological functions including ribosome biogenesis, chromatin assembly factor, pre-mRNA splicing factor, periodic tryptophan, WD-repeat protein, rRNA processing proteins, in addition to various conserved hypothetical proteins (Fig. [5E](#Fig5){ref-type="fig"}).Figure 5Proteomic analysis of *A*. *terreus* in response to subculturing and addition of *P*. *gracilior* leaves. (**A**) SDS-PAGE profile of *A*. *terreus*, Lane 1 is the Ladder, lane 2 is the original culture, lane 3 is 3^rd^ subculture, lane 4 is the 7^th^ subculture, lane 5 is the 7^th^ subculture with 0.5% P. gracilior leaves, lane 6 is 7^th^ subculture with 1.0% *P*. *gracilior* leaves, 7^th^ subculture with 3% *P*. *gracilior*, lane 8 7^th^ subculture with 5% *P*. *gracilior* leaves. (**B**) Protein sequence of the over-induced protein bands in response to 3% *P*. *gracilior* with molecular mass 74.4 kDa leaves. (**C**) Graphical abstract of the conserved domains of the target peptide from the database. (**D**) BLAST pairwise alignment of the target sequence with non-redundant proteins. (**E**) BLAST pairwise alignment of the target sequence with the proteome of *A*. *terreus* with MEGA7 software portal package.

Discussion {#Sec21}
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Taxol is one of the most clinically valuable worldwide natural product of anticancer activity, since its discovery from the bark of Pacific yew tree, and FDA approval in 1985. Since the discovery of Taxol producing endophytic fungi^[@CR40]^, a plethora of researches reporting the same biosynthetic potency by various fungal endophytes as reviewed by our studies^[@CR37]^. However, the implementation of fungal endophytes for industrial production of Taxol has not seen the light till date, crucially due to the bewildering loss of Taxol productivity with the fungal subculturing and storage^[@CR14],[@CR15]^. *A*. *terreus*, an endophyte of *P*. *gracilior*, has been identified as promising Taxol producer, among the recovered fungal endophyte^[@CR7]^, however, a strong attenuation for Taxol productivity with fungal subculturing and storage was reported. Intriguingly, the biosynthetic potency of Taxol by *A*. *terreus* has been restored completely upon addition of surface sterilized leaves of *P*. *gracilior*, in contrary to the autoclaved parts or methanolic extract of *P*. *gracilior* that have no effect on fungal Taxol biosynthesis^[@CR7]^. So, re-triggering the biosynthetic genes of Taxol by *A*. *terreus* could be due to the intimate physical interaction of endogenous endophytes of *P*. *gracilior* with the fungus. Thus, the objective of this work was to explore the molecular expression of Taxol biosynthetic genes, connected with attenuation and restoring of Taxol yield in response fungal subculturing and storage.

The stability of Taxol productivity by *A*. *terreus* through ten successive cultural generations was investigated. The productivity of Taxol was sequentially decreased with successive subculturing, in parallel to a visual fading to the conidial pigmentation. By the 7^th^ subculture of *A*. *terreus*, the yield of Taxol and mycelial melanin concentration were decreased by \>4--5 folds comparing to the original culture. This attenuation of fungal Taxol yield could be due to downregulation/silencing of Taxol encoding genes in axenic culture of *A*. *terreus* with frequent subculturing, due to lack of elicitors from the host plant or its microbiome^[@CR14],[@CR15]^. Consistently, successive subculturing of *Taxomyces andreanae*^[@CR13]^, *Tubercularia* sp^[@CR16]^ and *Periconia* sp^[@CR41]^ strongly attenuated their Taxol productivity. Plethora of endophytic fungi growing as monoaxenic cultures under standard conditions^[@CR42]^ losses their target metabolites biosynthetic potency, since their encoding genes are clustered on the fungal genome, and due to dilution/absence of inducing signals, their overall biosynthetic machinery were attenuated^[@CR3],[@CR43],[@CR44]^. Similarly, attenuation of secondary metabolites productivity of endophytic fungi with the multiple subculturing and storage were frequently reported^[@CR45],[@CR46]^ that could be due to lack of elicitors carried-over from plant and surrounding microenvironment. Since the fungal endophytes are normally exist within a niche of diverse communities of microbiome that contain a multitude of variegated interspecies crosstalk, so such multiplexed interaction are indispensable for inducing the plethora of natural products^[@CR47],[@CR48]^. As well as, the metabolites by *in vitro* growing fungal endophytes are correlated to the host plant physiology, defense and resistance to biotic and abiotic stresses^[@CR15]^. The yield of Taxol has been correlated with the activity and molecular expression of the rate-limiting enzymes of Taxol biosynthesis "GGPPS, TDS, DBAT and BAPT". The activities of TDS, DBAT and BAPT by *A*. *terreus* were strongly reduced with the fungal subculturing that might be due to the reduction on acetyl-CoA influx and/or autonomous silencing of expression of the genes encoding these enzymes or their transcriptional factors.

Attenuation of Taxol productivity was matched to the fading of melanin pigmentation that could be due to decreasing of acetyl-CoA influx which is the main precursors of Taxol and melanin biosynthetic pathways. Supporting to this hypothesis, production of secondary metabolites by *A*. *flavus*, *A*. *parasiticus* and *A*. *nidulans* "endophytes of maize" are dependent on acetyl-CoA derived from oxidation of fatty acids in the kernel^[@CR49]^ and supplementation of oleic acid increases their productivity to sterigmatocystine and aflatoxin *via* inducing the biogenesis of fungal peroxisome^[@CR50]^. So, the concentration of conidial/mycelial melanin was reduced concomitantly with the fungal subculturing. Taken together, the reduction of both Taxol and melanin yield was strongly correlated, that might be due to reduction on acetyl-CoA influx with the fungal subculturing. This assumption was validated from the intracellular concentration of acetyl-CoA for the fungal subcultures. Intriguingly, under the standard conditions, the yield of intracellular acetyl-CoA was profoundly reduced with fungal subculturing. Thus, the suppression of Taxol and melanin biosynthesis with the fungal subcultures could be due to the loss of influx of acetyl-CoA.

The influence of *P*. *gracilior* leaves on restoring the Taxol biosynthetic machinery of *A*. *terreus* has been evaluated. Remarkably, the machinery of Taxol biosynthesis by the 7^th^ *A*. *terreus* subculture has been completely restored with the implementation of *P*. *gracilior* surface sterilized leaves. Parallel to increasing on Taxol yield, the molecular expression of rate-limiting enzymes of Taxol biosynthesis have been completely restored. Thus, the downregulated gene expression of Taxol biosynthesis of *A*. *terreus* with subculturing, could be induced with signals from plants tissues or from surrounding microbiome. The activation of gene cluster encoding the fungal secondary metabolites could be due to the signals from plants such as homoserine and asparagine, similarly to activation of virulence genes of *Nectria hematococca*^[@CR51]^. Consistently, the expression of gene cluster of lolitrem in *Neotyphodium lolii*, endophyte of Ryegrass, had been dramatically increased *in planta* approving the presence of intrinsic gene inducing signals^[@CR52]^. Coincidently, needle extracts of *Taxus* sp significantly increased the Taxol yield by various endophytes^[@CR53]^. Elicitation of Taxol biosynthesis of *Paraconiothyrium* with the extracts of *Taxus* plant has been reported due to the presence of salicylic and benzoic acids, as well as, *via* cocultivation with *Alternaria* and *Phomopsis*^[@CR54]^. So, fungal metabolism can be greatly affected by the metabolites of other fungi, fungal-fungal interaction, with diffusion of some metabolites from *P*. *gracilior* mycobiome, could be the signals for expression of Taxol biosynthetic gene cluster of *A*. *terreus*. Triggering the expression of Taxol biosynthetic genes of endophytic fungi upon cultivation with non-Taxol endophyte producers have been studied extensively. The yield of Taxol by *F*. *mairei* was reported to be increased by about 38-fold upon cocultivation with *T*. *chinensis*^[@CR41]^. Similar scenario for induction of expression of Taxol encoding genes was reported to *A*. *proliferans* upon cocultivation with *Streptomyces olivaceoviridis*^[@CR55]^. The intimate physical fungal-actinomycetes interaction leads to activation of fungal secondary metabolites encoding genes "polyketide synthase"^[@CR48],[@CR56]^.

From the SDS-PAGE profile, the expression of proteins of 74.4, 68.2 and 37.1 kDa were the most sensitive for downregulation with fungal subculturing and restoration by addition of *P*. *gracilior* leaves, that practically correlated with attenuation and resuming of *A*. *terreus* Taxol productivity. The protein band of 74.4 kDa was excised and their identified by MALDI-TOF/TOF proteomic analysis. This peptide sequence displayed 58% similarity with the ribosome biogenesis protein YTM as the result of searching with proteome of *A*. *terreus*. This protein belongs to WD40 protein superfamily^[@CR57]^. This protein had two domains, the major one belongs to WD40 superfamily (390 amino acid) and minor one belongs to NLE superfamily (90 amino acids). The domain WD40 in eukaryotes have multiple functions as regulatory modules of signal transduction, pre-mRNA processing and cytoskeleton assembly, protein-protein and protein-DNA interaction platform (Xu and Min, 2011). WD40 domain has conserved serine/glycine--histidine (SH) dipeptides at *N*-terminus and tryptophan--aspartate (WD) dipeptides at *C*-terminal motif  ^[@CR58]^. With non-redundant blast search on the protein database, the recovered peptide sequences displaying two biological functions, ribosome biogenesis and microtubule-associated proteins. However, with the blast search of the recovered peptides within the proteome of *A*. *terreus*, the results displayed a similarity with proteins of multiple biological functions including ribosome biogenesis, chromatin assembly factor, pre-mRNA splicing factor, periodic tryptophan, WD-repeat protein, rRNA processing proteins, in addition to conserved hypothetical proteins^[@CR59]^.

In conclusion, attenuation/downregulation of the biosynthetic machinery of Taxol by *A*. *terreus* with the storage and subsequent subculturing are the main limitation for fungal usage as industrial platform. From the metabolic analysis, the attenuation of Taxol yield with the fungal multiple-culturing was assessed to be due to the reduction on main influx of acetyl-CoA, as revealed from the Taxol and melanin yields or due to silencing of ribosome biogenesis proteins that belong to WD40 protein superfamily, that had indirect effect on triggering the Taxol biosynthetic machinery of *A*. *terreus*. It could be deduced, the restoration of Taxol biosynthetic machinery by *A*. *terreus* is associated with resuming of expression of ribosome biogenesis protein YTM, due to the cross communication, interspecies crosstalk, signals production from the microbiome of *P*. *gracilior*. Thus, further ongoing studies to evaluate the overexpression of ribosome biogenesis protein on *A*. *terreus* to obtain a genetically engineered *A*. *terreus* with metabolic stability for sustainable Taxol production, have been raised.

Supplementary information
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 {#Sec22}

Supplementary Data

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Supplementary information
=========================

**Supplementary information** accompanies this paper at 10.1038/s41598-019-47816-y.

We appreciate the financial support from the Academy of Scientific Research and Technology (ASRT), Egypt to ASEA.

A.S.A.E. designed the research plane. N.Z.M., and M.A.Y. performed the experiment. S.S., L.S. and A.A.S. contributing in data analysis. G.S.A., A.S.A.E. and M.Z.S. wrote and edit the manuscript. All authors have read and approved the final manuscript.

The authors declare no competing interests.
