Biological Management of Banana Fusarium Wilt Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 Using Antagonistic Fungal Isolate CSR-T-3 (Trichoderma reesei)

Fusarium wilt in bananas is one of the most devastating diseases that poses a serious threat to the banana industry globally. With no effective control measures available to date, biological control has been explored to restrict the spread and manage the outbreak. We studied the effective biological control potential of different Trichoderma spp. in the management of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4). Expression of the defense related genes and metabolites in banana plants inoculated with Foc TR4 and treated with effective Trichoderma sp interactions were also studied. The in vitro growth inhibition of Foc TR4 by Trichoderma reesei isolate CSR-T-3 was 85.19% indicating a higher antagonistic potential than other Trichoderma isolates used in the study. Further, in in vivo assays, the banana plants treated with the isolate CSR-T-3 T. reesei had a significant reduction in the disease severity index (0.75) and also had increased phenological indices with respect to Foc TR4 treated plants. Enhanced activity of defense enzymes, such as β-1, 3-glucanase, peroxidase, chitinase, polyphenol oxidase, and phenylalanine ammonia lyase with higher phenol contents were found in the Trichoderma isolate CSR-T-3 treated banana plants challenge-inoculated with Foc TR4. Fusarium toxins, such as fusaristatin A, fusarin C, chlamydosporal, and beauveric acid were identified by LC-MS in Foc TR4-infected banana plants while high intensity production of antifungal compounds, such as ß-caryophyllene, catechin-o-gallate, soyasapogenol rhamnosyl glucoronide, peptaibols, fenigycin, iturin C19, anthocyanin, and gallocatechin-o-gallate were detected in T. reesei isolate CSR-T-3 treated plants previously inoculated with Foc TR4. Gene expression analysis indicated the upregulation of TrCBH1/TrCBH2, TrXYL1, TrEGL1, TrTMK1, TrTGA1, and TrVEL1 genes in CSR-T-3 treatment. LC-MS and gene expression analysis could ascertain the upregulation of genes involved in mycoparasitism and the signal transduction pathway leading to secondary metabolite production under CSR-T-3 treatment. The plants in the field study showed a reduced disease severity index (1.14) with high phenological growth and yield indices when treated with T. reesei isolate CSR-T-3 formulation. We report here an effective biocontrol-based management technological transformation from lab to the field for successful control of Fusarium wilt disease caused by Foc TR4 in bananas.

FcStuA from Fusarium culmorum controls wheat foot and root rot in a toxin dispensable manner

Fusarium culmorum is one of the most harmful pathogens of durum wheat and is the causal agent of foot and root rot (FRR) disease. F. culmorum produces the mycotoxin deoxynivalenol (DON) that is involved in the pathogenic process. The role of the gene FcStuA, a StuA ortholog protein with an APSES domain sharing 98.5% homology to the FgStuA protein (FGSG10129), was determined by functional characterisation of deletion mutants obtained from two F. culmorum wild-type strains, FcUk99 (a highly pathogenic DON producer) and Fc233B (unable to produce toxin and with a mild pathogenic behavior). The ΔFcStuA mutants originating from both strains showed common phenotypic characters including stunted vegetative growth, loss of hydrophobicity of the mycelium, altered pigmentation, decreased activity of polygalacturonic enzymes and catalases, altered and reduced conidiation, delayed conidial germination patterns and complete loss of pathogenicity towards wheat stem base/root tissue. Glycolytic process efficiency [measured as growth on glucose as sole carbon (C) source] was strongly impaired and growth was partially restored on glutamic acid. Growth on pectin-like sources ranked in between glucose and glutamic acid with the following order (the lowest to the highest growth): beechwood xylan, sugarbeet arabinan, polygalacturonic acid, citrus pectin, apple pectin, potato azogalactan. DON production in the mutants originating from FcUK99 strain was significantly decreased (−95%) in vitro. Moreover, both sets of mutants were unable to colonise non-cereal plant tissues, i.e. apple and tomato fruits and potato tubers. No differences between mutants, ectopic and wild-type strains were observed concerning the level of resistance towards four fungicides belonging to three classes, the demethylase inhibitors epoxiconazole and tebuconzole, the succinate dehydrogenase inhibitor isopyrazam and the cytochrome bc1 inhibitor trifloxystrobin. StuA, given its multiple functions in cell regulation and pathogenicity control, is proposed as a potential target for novel disease management strategies.

Altered trichothecene biosynthesis in TRI6-silenced transformants of Fusarium culmorum influences the severity of crown and foot rot on durum wheat seedlings

An RNA silencing construct was used to alter mycotoxin production in the plant pathogenic fungus Fusarium culmorum, the incitant of crown and foot rot on wheat. The transformation of a wild-type strain and its nitrate reductase-deficient mutant with inverted repeat transgenes (IRTs) containing sequences corresponding to the trichothecene regulatory gene TRI6 was achieved using hygromycin B resistance as a selectable marker. Southern analysis revealed a variety of integration patterns of the TRI6 IRT. One transformant underwent homologous recombination with deletion of the endogenous TRI6 gene, whereas, in another transformant, the TRI6 IRT was not integrated into the genome. The TRI6 IRT did not alter the physiological characteristics, such as spore production, pigmentation or growth rate, on solid media. In most transformants, a high TRI6 amplification signal was detected by quantitative reverse transcription-polymerase chain reaction, corresponding to a TRI6-hybridizing smear of degraded fragments by Northern analysis, whereas TRI5 expression decreased compared with the respective nontransformed strain. Four transformants showed increased TRI5 expression, which was correlated with a dramatic (up to 28-fold) augmentation of deoxynivalenol production. Pathogenicity assays on durum wheat seedlings confirmed that impairment of deoxynivalenol production in the TRI6 IRT transformants correlated with a loss of virulence, with decreased disease indices ranging from 40% to 80% in nine silenced strains, whereas the overproducing transformants displayed higher virulence compared with the wild-type.

Generation and identification of DNA sequence flanking T-DNA integration site of Trichoderma atroviride mutants with high dichlorvos-degrading capacity

A protocol for efficient Agrobacterium tumefaciens-mediated transformation (ATMT) of biocontrol fungus Trichoderma atroviride strain T23 was developed to construct mutants with improved dichlorvos-degradation ability. A transformation frequency of 5x10(-6) was achieved. Among 110 genetically stable T-DNA transformants of T. atroviride T23, two transformants, AMT-12 and AMT-28, confirmed by Southern blot analysis to have single-copy inserts of T-DNA, showed an increase in dichlorvos-degradation ability of more than 10% compared to that of the wild type, exhibited similar tolerance to the pesticide, but lower spore formation ability. Five transformants exhibited a reduction in degradation of more than 70%, exhibited wild-type spore formation, and tolerated up to 800 microg/mL of dichlorvos. The left-flanking sequence of the insertion site in AMT-12 was cloned as a 1845-bp fragment and shown to have 89% identity to the DNA from T. atroviride IMI 206040; however, the involvement of this DNA in dichlorvos degradation remains still to be determined. This study can promote both a more efficient isolation of DNA sequence flanking T-DNA integration site in T. atroviride mutants and a more rational utilization of these transformants in dichlorvos degradation.

Phylogeny and evolution of mating-type genes from Pyrenophora teres, the causal agent of barley "net blotch" disease

The main aim of this study was to test the patterns of sequence divergence and haplotype structure at the MAT locus of Pyrenophora teres, the causal agent of barley 'net blotch' disease. P. teres is a heterothallic ascomycete that co-occurs in two symptomatological forms, the net form (NF) and the spot form (SF). The mating-type genes MAT1-1-1 and MAT1-2-1 were sequenced from 22 NF isolates (12 MAT1-1-1 and 10 MAT1-2-1 sequences) and 17 SF isolates (10 MAT1-1-1 and seven MAT1-2-1 sequences) collected from Sardinian barley landrace populations and worldwide. On the basis of a parsimony network analysis, the two forms of P. teres are phylogenetically separated. More than 85% of the total nucleotide variation was found between formae speciales. The two forms do not share any polymorphisms. Six diagnostic nucleotide polymorphisms were found in the MAT1-1-1 intron (1) and in the MAT1-1-1 (3) and MAT1-2-1 (2) exons. Three diagnostic non-synonymous mutations were found, one in MAT1-1-1 and two in MAT1-2-1. For comparison with P. teres sequence data, the mating-type genes from Pyrenophora graminea were also isolated and sequenced. Divergence between P. graminea and P. teres is of a similar magnitude to that between NF and SF of P. teres. The MAT genes of P. graminea were closer to those of SF than to NF, with the MAT1-2-1 SF peptide not different from the MAT1-2-1 peptide of P. graminea. Overall, these data suggest long genetic isolation between the two forms of P. teres and that hybridization is rare or absent under field conditions, with each form having some particular niche specialization. This indicates that research on resistance to P. teres should consider the two forms separately, as different species.

Population genetic structure of Pyrenophora teres Drechs. the causal agent of net blotch in Sardinian landraces of barley (Hordeum vulgare L.)

Monoconidial cultures of Pyrenophora teres, the causal agent of barley net blotch, were isolated from leaves collected from six populations of the barley landrace "S'orgiu sardu" growing in five agro-ecological areas of Sardinia, Italy, and genotyped using AFLPs. The 150 isolates were from lesions of either the "net form" (P. teres f. sp. teres) or the "spot form" (P. teres f. sp. maculata) of the disease. Of 121 AFLP markers, 42%, were polymorphic. Cluster analysis resolved the isolates into two strongly divergent groups (F(ST) = 0.79), corresponding to the net (45% of the isolates) and the spot (55% of the isolates) forms (designated the NFR and SFR groups, respectively). The absence of intermediate genotypes and the low number of shared markers between the two groups indicated that hybridization between the two formae is rare or absent under the field condition of Sardinia. Five of the barley populations hosted both forms but in different proportions. The SFR populations were similar in overall polymorphism to the NFR populations. However, compared to the SFR form, the NFR occurred in all fields sampled and showed a higher population divergence (F(ST) = 0.43 versus F(ST) = 0.09 with all isolates; F(ST) = 0.37 versus F(ST) = 0.06 with clone corrected samples) probably due to a lower migration rate. AFLP fingerprints resolved 117 distinct genotypes among the 150 isolates sampled (78%), 87% in SFR and 68% in NFR isolates. Although the absolute numbers may be a function of the number of AFLP markers assayed, the relative difference suggests that clonality is more prevalent among the NFR isolates (with 11 of 46 haplotypes observed more than once), compared with SFR isolates (7 of 71 haplotypes). Both digenic and multilocus linkage disequilibrium analyses suggested that sexual reproduction occurs at significant levels within the NFR and SFR populations, and that the relative contribution of sexual and asexual reproduction varies among different environments.

Recovery of Fusarium oxysporum Fo47 Mutants Affected in Their Biocontrol Activity After Transposition of the Fot1 Element

ABSTRACT To investigate the biocontrol mechanisms by which the antagonistic Fusarium oxysporum strain Fo47 is active against Fusarium wilt, a Fot1 transposon-mediated insertional mutagenesis approach was adopted to generate mutants affected in their antagonistic activity. Ninety strains in which an active Fot1 copy had transposed were identified with a phenotypic assay for excision and tested for their biocontrol activity against F. oxysporum f. sp. lini on flax in greenhouse experiments. Sixteen strains were affected in their capacity to protect flax plants, either positively (more antagonistic than Fo47) or negatively (less antagonistic). The molecular characterization of these mutants confirms the excision of Fot1 and its reinsertion in most of the cases. Moreover, we demonstrate that other transposable elements such as Fot2, impala, and Hop have no transposition activity in the mutant genomes. The phenotypic characterization of these mutants shows that they are affected neither in their in vitro growth habit nor in their competitiveness in soil compared with wild-type strain Fo47. These results show that mutants are not impaired in their saprophytic phase and suggest that the altered biocontrol phenotype should likely be expressed during the interaction with the host plant.

Rock phosphate solubilization and colonization of maize rhizosphere by wild and genetically modified strains of Penicillium rugulosum

Maize root colonization and phosphate solubilizing activity of the fungus Penicillium rugulosum were assessed in a greenhouse trial using soil-plant microcosms. The bacterial gene hph conferring resistance to hygromicin B was introduced by electroporation in the wild-type strain IR-94MF1 of P. rugulosum and one transformant, w-T3, was selected. Maize plants were grown for 5 weeks in a P-poor soil and fertilized with a Florida apatite mineral, with Navay, an apatite rock deposit from Venezuela, or with simple superphosphate. Inoculation treatments included strain IR-94MF1, transformant w-T3 and two IR-94MF1 UV-induced mutants with enhanced (Mps++) or reduced (Mps-) in vitro mineral phosphate solubilizing activity. In the absence of P fertilization, inoculation with any P. rugulosum isolate significantly reduced the size of the total and P-solubilizing bacterial community present in maize rhizosphere. The bacterial community significantly increased in maize inoculated with IR-94MF1 and w-T3 when P was added as apatites Navay or Florida. All P. rugulosum strains were able to stimulate the growth of maize plants as indicated by 3.6 to 28.6% increases in dry matter yields. In the presence of rock phosphate, P uptake by maize plants inoculated with the two mutants Mps++ and Mps- was not always in agreement with their P-solubilizing phenotypes. Strain IR-94MF1 and transformant w-T3 increased P assimilation by the plants fertilized with Navay rock phosphate by 26 and 38%, respectively. In this treatment, w-T3 showed its highest significant maize rhizosphere colonization. With the simple superphosphate treatment, w-T3 increased P uptake in plants by 8% over the uninoculated control and also decreased significantly the community size of total bacteria, total fungi, and P-solubilizing fungi in the rhizosphere.

PCR Detection of Fusarium oxysporum f. sp. basilici on Basil

Sixty-nine amplified DNA fragments, generated from different isolates of Fusarium oxysporum f. sp. basilici, were tested for F. oxysporum f. sp. basilici-specificity in a dot blot assay. One 1,038-bp fragment hybridized to DNA from all F. oxysporum f. sp. basilici isolates but not to DNA obtained from F. oxysporum isolates nonpathogenic to basil or representatives of other formae speciales of F. oxysporum, or from isolates of F. redolens, F. tabacinum, Rhizoctonia solani, Sclerotinia sclerotiorum, S. minor, and Pythium ultimum obtained from diseased basil. This fragment was cloned and sequenced, and three pairs of F. oxysporum f. sp. basilici-specific primers were designed, giving rise to amplification products of 943, 382, and 330 bp. A nested PCR assay allowed detection of F. oxysporum f. sp. basilici in diseased seedlings and in artificially and naturally contaminated seeds. The theoretical detection limit of this system was 10² fungal propagules per 100 seeds on artificially contaminated samples, while on naturally contaminated commercial seed lots, 32 propagules per 100 seeds were detected.

Agrobacterium-Mediated Transformation of Fusarium oxysporum: An Efficient Tool for Insertional Mutagenesis and Gene Transfer

ABSTRACT Agrobacterium tumefaciens-mediated transformation (ATMT) has long been used to transfer genes to a wide variety of plants and has also served as an efficient tool for insertional mutagenesis. In this paper, we report the construction of four novel binary vectors for fungal transformation and the optimization of an ATMT protocol for insertional mutagenesis, which permits an efficient genetic manipulation of Fusarium oxysporum and other phytopathogenic fungi to be achieved. Employing the binary vectors, carrying the bacterial hygromycin B phosphotrans-ferase gene (hph) under the control of the Aspergillus nidulans trpC promoter as a selectable marker, led to the production of 300 to 500 hygromycin B resistant transformants per 1 x 10(6) conidia of F. oxysporum, which is at least an order of magnitude higher than that previously accomplished. Transformation efficiency correlated strongly with the duration of cocultivation of fungal spores with Agrobacterium tumefaciens cells and significantly with the number of Agrobacteruium tumefaciens cells present during the cocultivation period (r = 0.996; n = 3; P < 0.01). All transformants tested remained mitotically stable, maintaining their hygromycin B resistance. Growing Agrobacterium tumefaciens cells in the presence of acetosyringone (AS) prior to cocultivation shortened the time required for the formation of transformants but decreased to 53% the percentage of transformants containing a single T-DNA insert per genome. This increased to over 80% when Agrobacterium tumefaciens cells grown in the absence of AS were used. There was no correlation between the average copy number of T-DNA per genome and the colony diameter of the transformants, the period of cocultivation or the quantity of Agrobacterium tumefaciens cells present during cocultivation. To isolate the host sequences flanking the inserted T-DNA, we employed a modified thermal asymmetric interlaced PCR (TAIL-PCR) technique. Utilizing just one arbitrary primer resulted in the successful amplification of desired products in 90% of those transformants analyzed. The insertion event appeared to be a random process with truncation of the inserted T-DNA, ranging from 1 to 14 bp in size, occurring on both the right and left border sequences. Considering the size and design of the vectors described here, coupled with the efficiency and flexibility of this ATMT protocol, it is suggested that ATMT should be regarded as a highly efficient alternative to other DNA transfer procedures in characterizing those genes important for the pathogenicity of F. oxysporum and potentially those of other fungal pathogens.

Detection of Fusarium oxysporum f. sp. dianthi in Carnation Tissue by PCR Amplification of Transposon Insertions

ABSTRACT Strains of the carnation wilt pathogen, Fusarium oxysporum f. sp. dianthi, can be distinguished by DNA fingerprint patterns, using the fungal transposable elements Fot1 and impala as probes for Southern hybridization. The DNA fingerprints correspond to three groups of F. oxysporum f. sp. dianthi strains: the first group includes isolates of races 1 and 8; the second group includes isolates of races 2, 5 and 6; and the third group includes isolates of race 4. Genomic DNAs flanking race-associated insertion sites of Fot1 (from races 1, 2, and 8) or impala (from race 4) were amplified by the inverse polymerase chain reaction (PCR) technique. These regions were cloned and sequenced, and three sets of primers overlapping the 3' or 5' end of the transposon and its genomic insertion were designed. Using fungal genomic DNA as template in PCR experiments, primer pairs generated amplification products of 295, 564 and 1,315 bp, corresponding to races 1 and 8; races 2, 5, and 6; and race 4, respectively. When multiplex PCR was performed with genomic DNA belonging to races 1 and 8, 2, or 4, single amplimers were generated, allowing clear race determination of the isolate tested. PCR was successfully performed on DNA extracted from susceptible carnation cv. Indios infected with isolates representative of races 1, 2, 4, and 8.

Transformants of Trichoderma longibrachiatum Overexpressing the beta-1,4-Endoglucanase Gene egl1 Show Enhanced Biocontrol of Pythium ultimum on Cucumber

ABSTRACT Nine transformants of Trichoderma longibrachiatum with extra copies of the egl1 gene were studied for mitotic stability, endoglucanase production, and biocontrol activity against Pythium ultimum on cucumber seedlings. The transformants showed a significantly higher level of expression of the egl1 gene in comparison to the wild type under both inducing and noninducing growth conditions. Transformants with the egl1 gene under the control of a constitutive promoter had the highest enzymatic activity. Both the endoglucanase activity and the transforming sequences were stable under nonselective conditions. When applied to cucumber seeds sown in P. ultimum-infested soil, T. longibrachiatum transformants with increased inducible or constitutive egl1 expression generally were more suppressive than the wild-type strain.