Biosynthesis of pathogenicity hydrolytic enzymes by Botrytis cinerea during infection of bean leaves and in vitro

The basal transcription factor II H subunit Tfb5 is required for stress response and pathogenicity in the tangerine pathotype of Alternaria alternata

The basal transcription factor II H (TFIIH) is a multicomponent complex. In the present study, we characterized a TFIIH subunit Tfb5 by analysing loss- and gain-of-function mutants to gain a better understanding of the molecular mechanisms underlying stress resistance and pathogenicity in the citrus fungal pathogen Alternaria alternata. Tfb5 deficiency mutants (ΔAatfb5) decreased sporulation and pigmentation, and were impaired in the maintenance of colony surface hydrophobicity and cell wall integrity. ΔAatfb5 increased sensitivity to ultraviolet light, DNA-damaging agents, and oxidants. The expression of Aatfb5 was up-regulated in the wild type upon infection in citrus leaves, implicating the requirement of Aatfb5 in fungal pathogenesis. Biochemical and virulence assays revealed that ΔAatfb5 was defective in toxin production and cellwall-degrading enzymes, and failed to induce necrotic lesions on detached citrus leaves. Aatfb5 fused with green fluorescent protein (GFP) was localized in the cytoplasm and nucleus and physically interacted with another subunit, Tfb2, based on yeast two-hybrid and co-immunoprecipitation analyses. Transcriptome and Antibiotics & Secondary Metabolite Analysis Shell (antiSMASH) analyses revealed the positive and negative roles of Aatfb5 in the production of various secondary metabolites and in the regulation of many metabolic and biosynthetic processes in A. alternata. Aatfb5 may play a negative role in oxidative phosphorylation and a positive role in peroxisome biosynthesis. Two cutinase-coding genes (AaCut2 and AaCut15) required for full virulence were down-regulated in ΔAatfb5. Overall, this study expands our understanding of how A. alternata uses the basal transcription factor to deal with stress and achieve successful infection in the plant host.

Cell-Wall-Degrading Enzymes Required for Virulence in the Host Selective Toxin-Producing Necrotroph Alternaria alternata of Citrus

The necrotrophic fungal pathogen Alternaria alternata attacks many citrus species, causing brown spot disease. Its pathogenic capability depends primarily on the production of host-selective ACT toxin. In the current study a Ste12 transcription factor was characterized to be required for conidial formation and the production of cell-wall-degrading enzymes (CWDEs) in the tangerine pathotype of A. alternata. The Ste12 deficiency strain (ΔSte12) retained wild-type growth, ACT toxin production, and sensitivity to oxidative and osmotic stress. However, pathogenicity tests assayed on detached Dancy leaves revealed a marked reduction in virulence of ΔSte12. Transcriptome and quantitative RT-PCR analyses revealed that many genes associated with Carbohydrate-Active Enzymes (CAZymes) were downregulated in ΔSte12. Two cutinase-coding genes (AaCut3 and AaCut7) regulated by Ste12 were individually and simultaneously inactivated. The AaCut3 or AaCut7 deficiency strain unchanged in cutinase activities and incited wild-type lesions on Dancy leaves. However, the strain carrying an AaCut3 AaCut7 double mutation produced and secreted significantly fewer cutinases and incited smaller necrotic lesions than wild type. Not only is the host-selective toxin (HST) produced by A. alternata required for fungal penetration and lesion formation, but so too are CWDEs required for full virulence. Overall, this study expands our understanding of how A. alternata overcomes citrus physical barriers to carry out successful penetration and colonization.

The dual role of oxalic acid on the resistance of tomato against Botrytis cinerea

In order to define the role of oxalic acid (OA) in the invasion of Botrytis cinerea in tomato plants, the OA induction of resistance related to oxalate oxidase (O×O) and germin was examined. In greenhouse experiments, OA at 3 mmol/L significantly induced resistance in tomato plants against B. cinerea strains B05.10 and T4, reducing lesion size of 37.55% and 24.91% by compared with distilled water control, respectively, while 20 mmol/L OA increasing by 36.14% and 41.48%. OA contents were 98 and 46 µg/mL when tomato plants were infected by B. cinerea strains B05.10 and T4, respectively. To define the molecular-genetic mechanisms, we compared the gene expression under four different conditions: 3 mmol/L OA-treated plants, 20 mmol/L OA-treated plants, B. cinerea strain B05.10-infected plants (B05.10 Inf plants) and B. cinerea strain T4-infected plants (T4 Inf plants). In 3 mmol/L OA-treated plants, the expressions of O×O and Germin peaked at 48 h after spraying, with approximate threefold and 18-fold increase compared with the control expression, respectively. In T4 Inf plants, the expression (mRNA accumulation) of O×O and Germin reached the highest levels at 24 h after inoculation, with 3- and 13-times that immediately after inoculation, respectively. In total, these findings suggest that elevated levels of OA correlated with increased fungal invasion and lower OA induced resistance in tomato plants by increasing expressions of O×O and Germin.

Disease severity enhancement by an esterase from non-phytopathogenic yeast Pseudozyma antarctica and its potential as adjuvant for biocontrol agents

The phylloplane yeast Pseudozyma antarctica secretes an esterase, named PaE, and xylanase when cultivated with xylose. We previously observed that the lipophilic layer of Micro-Tom tomato leaves became thinner after the culture filtrate treatment. The leaves developed reduced water-holding ability and became wilted. In this study, the purified enzymes were spotted on Micro-Tom leaves. PaE, but not xylanase, thinned the lipophilic layer of leaves and decreased leaf resistance to the phytopathogenic fungus Botrytis cinerea. Disease severity increased significantly in detached leaves and potted plants treated with the culture filtrate and B. cinerea spores compared with those treated with inactivated enzyme and B. cinerea alone. Spore germination ratios, numbers of penetrating fungal hyphae in the leaves, and fungal DNA contents also increased significantly on the detached leaves. Japanese knotweed (Fallopia japonica), a serious invasive alien weed in Europe and North America, also became susceptible to infection by the rust pathogen Puccinia polygoni-amphibii var. tovariae following the culture filtrate treatment. The culture filtrate treatment increased disease development in plants induced by both phytopathogenic fungi. Our results suggest that P. antarctica culture filtrate could be used as an adjuvant for sustainable biological weed control using phytopathogenic fungi.

Incidence of leaf spot disease on cotton caused by Curvularia verruculosa and role of its hydrolytic enzymes in pathogenesis

The finding described in this study is the first report of leaf spot disease of cotton caused by Curvularia verruculosa surveyed in the state of Maharashtra (India). The isolated phytopathogenic fungal strain was identified using morphological characteristics and molecular identification of ITS gene sequence (MF784436) and D1D2 region of LSU gene (KY978073). The ability of fungal strain to secrete hydrolytic enzymes viz., pectinase, xylanase, protease, cellulase and lipase was detected. The secretion profile of hydrolytic enzymes by C. verruculosa was also examined in planta and in vitro. The secretion of cellulase, xylanase and protease was found to be inducible on cotton-stalk powder containing media; while secretion of pectinase and lipase was constitutive in glucose containing medium. The hydrolytic enzymes secretion during etiological progression of disease was detected on cotton leaves at regular interval of 24 h up to 10 days. A significant correlation (P < 0.05) was observed between hydrolytic enzymes secretion and disease severity index. The increased level of hydrolytic enzymes in infected plant sample indicates their role in disease progression. The newly documented fungal phytopathogen Curvularia verruculosa was deposited at National Fungal Culture Collection of India, Pune with accession number of NFCCI-4119.

Control efficiency and expressions of resistance genes in tomato plants treated with ε-poly-l-lysine against Botrytis cinerea

The antifungal properties and the induction of resistance by ε-poly-l-lysine (ε-PL) were examined to reveal its potential in protecting tomato plants against Botrytis cinerea. As presented herein, ε-PL at 1200mg/L was found to have optimal in vitro antifungal activities, achieving an inhibition rate of 94.96%. In first-year field tests, ε-PL (1200mg/L) had a control effect of up to 79.07% against tomato grey mould. Similar results were obtained in the second year. In greenhouse experiments, ε-PL was observed to effectively reduce leaf infection, with an observed control rate at 89.22%. To define the molecular-genetic mechanisms, we compared the gene expression under four different conditions: sterile water sprayed plants (Control), Botrytis-infected plants (Inf), ε-PL-treated plants (ε-PL) and ε-PL-treated+infected plants (ε-PL+Inf). Quantitative PCR analysis at 36h after inoculation revealed that ε-PL+Inf plants exhibited significant expression and priming of several key Botrytis-induced genes in tomato. The results indicate that ε-PL promoted plant capacity of tomato to activate defense mechanisms upon pathogen attack. In total, these findings revealed that ε-PL should be an excellent biocontrol agent candidate that combined direct antifungal activity against B. cinerea and plant resistance capacity.

Modelling of different enzyme productions by solid-state fermentation on several agro-industrial residues

A simple kinetic model, with only three fitting parameters, for several enzyme productions in Petri dishes by solid-state fermentation is proposed in this paper, which may be a valuable tool for simulation of this type of processes. Basically, the model is able to predict temporal fungal enzyme production by solid-state fermentation on complex substrates, maximum enzyme activity expected and time at which these maxima are reached. In this work, several fermentations in solid state were performed in Petri dishes, using four filamentous fungi grown on different agro-industrial residues, measuring xylanase, exo-polygalacturonase, cellulose and laccase activities over time. Regression coefficients after fitting experimental data to the proposed model turned out to be quite high in all cases. In fact, these results are very interesting considering, on the one hand, the simplicity of the model and, on the other hand, that enzyme activities correspond to different enzymes, produced by different fungi on different substrates.

Analysis of Clonostachys rosea-induced resistance to tomato gray mold disease in tomato leaves

Tomato gray mold disease, caused by Botrytis cinerea, is a serious disease in tomato. Clonostachys rosea is an antagonistic microorganism to B. cinerea. To investigate the induced resistance mechanism of C. rosea, we examined the effects of these microorganisms on tomato leaves, along with changes in the activities of three defense enzymes (PAL, PPO, GST), second messengers (NO, H2O2, O2(-)) and phytohormones (IAA, ABA, GA3, ZT, MeJA, SA and C2H4). Compared to the control, all treatments induced higher levels of PAL, PPO and GST activity in tomato leaves and increased NO, SA and GA3 levels. The expression of WRKY and MAPK, two important transcription factors in plant disease resistance, was upregulated in C. rosea- and C. rosea plus B. cinerea-treated samples. Two-dimensional gel electrophoresis analysis showed that two abundant proteins were present in the C. rosea plus B. cinerea-treated samples but not in the other samples. These proteins were determined (by mass spectrum analysis) to be LEXYL2 (β-xylosidase) and ATP synthase CF1 alpha subunit. Therefore, C. rosea plus B. cinerea treatment induces gray mold resistance in tomato. This study provides a basis for elucidating the mechanism of C. rosea as a biocontrol agent.

Pectinolytic enzyme production by Colletotrichum truncatum, causal agent of soybean anthracnose

BACKGROUND

Colletotrichum truncatum is the most common pathogenic fungus associated with soybean anthracnose, a prevalent disease in Argentina. Pectinolytic enzymes are involved in the pathogenicity of a wide range of plant pathogenic fungi.

OBJECTIVES

To explore pectinolytic enzyme production in Argentinian Colletotrichum strains isolated from diseased soybean plants from different geographic locations, as a preliminary step to establish the biological role of the pectinolytic enzymes in the Colletotrichum spp.-soybean system, yet unknown.

METHODS

Ten strains were screened for in vitro pectinolytic enzyme production on a defined medium based on pectin as carbon source.

RESULTS

All isolates were able to grow in this medium and polymethylgalacturonase (PMG), polygalacturonase (PG) and pectin lyase (PL) activities were detected. On the whole, the peak of polygalacturonases activities preceded the day of maximum growth, while PL activity reached its highest level afterwards. Strain BAFC 3097 (from Santa Fe province) yielded high titles of the three enzymes (1.08U/ml PG, 1.05U/ml PMG, 156U/ml PL), after a short incubation period (7-10 days). Low synthesis of polygalacturonases in cultures containing glucose as unique carbon source suggests that these enzymes are constitutive in contrast with PL, which was not detected.

CONCLUSIONS

The disparity observed in enzyme production among strains cannot be related to fungal growth, since no major differences in mycelial yield were found; it was not connected with their geographic origin, but might be associated with differences in virulence among strains not yet evaluated.

Wheat domestication accelerated evolution and triggered positive selection in the beta-xylosidase enzyme of Mycosphaerella graminicola

Plant cell wall degrading enzymes (PCWDEs) of plant pathogens are receiving increasing interest for their potential to trigger plant defense reactions. In an antagonistic co-evolutionary arms race between host and pathogen, PCWDEs could be under strong selection. Here, we tested the hypothesis that PCWDEs in the fungal wheat pathogen Mycosphaerella graminicola have been positively selected by analyzing ratios of non-synonymous and synonymous nucleotide changes in the genes encoding these enzymes. Analyses of five PCWDEs demonstrated that one (beta-xylosidase) has been under strong positive selection and experienced an accelerated rate of evolution. In contrast, PCWDEs in the closest relatives of M. graminicola collected from wild grasses did not show evidence for selection or deviation from a molecular clock. Since the genealogical divergence of M. graminicola from these latter species coincided with the onset of agriculture, we hypothesize that the recent domestication of the host plant and/or agricultural practices triggered positive selection in beta-xylosidase and that this enzyme played a key role in the emergence of a host-specialized pathogen.

A proteomic study of pectin-degrading enzymes secreted by Botrytis cinerea grown in liquid culture

Botrytis cinerea is a pathogenic filamentous fungus, which infects more than 200 plant species. The enzymes secreted by B. cinerea play an important role in the successful colonization of a host plant. Some of the secreted enzymes are involved in the degradation of pectin, a major component of the plant cell wall. A total of 126 proteins secreted by B. cinerea were identified by growing the fungus on highly or partially esterified pectin, or on sucrose in liquid culture. Sixty-seven common proteins were identified in each of the growth conditions, of which 50 proteins exhibited a SignalP motif. Thirteen B. cinerea proteins with functions related to pectin degradation were identified in both pectin growth conditions, while only four were identified in sucrose. Our results indicate it is unlikely that the activation of B. cinerea from the dormant state to active infection is solely dependent on changes in the degree of esterification of the pectin component of the plant cell wall. Further, these results suggest that future studies of the B. cinerea secretome in infections of ripe and unripe fruits will provide important information that will describe the mechanisms that the fungus employs to access nutrients and decompose tissues.

Disruption of Botrytis cinerea pectin methylesterase gene Bcpme1 reduces virulence on several host plants

The pectinolytic enzyme pectin methylesterase (PME) hydrolyses pectin in methanol and polygalacturonic acid. In the expressed sequence tag library of Botrytis cinerea T4, we identified a 1,041 bp Bcpme1 cDNA potentially encoding a 346-amino acid protein of 37 kDa showing 46.8% identity with Aspergillus sp. PMEs. Bcpme1 is a single copy gene and is similarly expressed in glucose and pectin containing media. To evaluate the role of Bcpme1 in Botrytis cinerea virulence, a mutant in Bcpme1 was generated by gene disruption. The Bcpme1 mutant showed similar growth on rich medium but reduced growth on pectin medium. Two isozymes of pI 7.4 and 7.1 were detected in pectin liquid-culture supernatants of wild-type strain Bd90 analyzed by isoelectric focusing-polyacrylamide gel electrophoresis, while those of Bcpme1 mutant possessed only the pI 7.1 isozyme. BCPME1, the pI 7.4 isozyme, is the major PME activity, as PME activity is 75% reduced in Bcpme1 mutant. Moreover, the Bcpme1 mutant was less virulent on apple fruits, grapevine, and Arabidopsis thaliana leaves. Those phenotypes were complemented by reintroducing a Bcpme1 copy in the Bcpme1 mutant. These results showed that B. cinerea possessed more than one PME-encoding gene and that BCPME1 is an important determinant of B. cinerea virulence.