Differential Expression of Fungal Genes Determines the Lifestyle of Plectosphaerella Strains During Arabidopsis thaliana Colonization.

The fungal genus Plectosphaerella comprises species and strains with different lifestyles on plants, such as P. cucumerina, which has served as model for the characterization of Arabidopsis thaliana basal and nonhost resistance to necrotrophic fungi. We have sequenced, annotated, and compared the genomes and transcriptomes of three Plectosphaerella strains with different lifestyles on A. thaliana, namely, PcBMM, a natural pathogen of wild-type plants (Col-0), Pc2127, a nonpathogenic strain on Col-0 but pathogenic on the immunocompromised cyp79B2 cyp79B3 mutant, and P0831, which was isolated from a natural population of A. thaliana and is shown here to be nonpathogenic and to grow epiphytically on Col-0 and cyp79B2 cyp79B3 plants. The genomes of these Plectosphaerella strains are very similar and do not differ in the number of genes with pathogenesis-related functions, with the exception of secreted carbohydrate-active enzymes (CAZymes), which are up to five times more abundant in the pathogenic strain PcBMM. Analysis of the fungal transcriptomes in inoculated Col-0 and cyp79B2 cyp79B3 plants at initial colonization stages confirm the key role of secreted CAZymes in the necrotrophic interaction, since PcBMM expresses more genes encoding secreted CAZymes than Pc2127 and P0831. We also show that P0831 epiphytic growth on A. thaliana involves the transcription of specific repertoires of fungal genes, which might be necessary for epiphytic growth adaptation. Overall, these results suggest that in-planta expression of specific sets of fungal genes at early stages of colonization determine the diverse lifestyles and pathogenicity of Plectosphaerella strains.

Disruption of abscisic acid signaling constitutively activates Arabidopsis resistance to the necrotrophic fungus Plectosphaerella cucumerina.

Plant resistance to necrotrophic fungi is regulated by a complex set of signaling pathways that includes those mediated by the hormones salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and abscisic acid (ABA). The role of ABA in plant resistance remains controversial, as positive and negative regulatory functions have been described depending on the plant-pathogen interaction analyzed. Here, we show that ABA signaling negatively regulates Arabidopsis (Arabidopsis thaliana) resistance to the necrotrophic fungus Plectosphaerella cucumerina. Arabidopsis plants impaired in ABA biosynthesis, such as the aba1-6 mutant, or in ABA signaling, like the quadruple pyr/pyl mutant (pyr1pyl1pyl2pyl4), were more resistant to P. cucumerina than wild-type plants. In contrast, the hab1-1abi1-2abi2-2 mutant impaired in three phosphatases that negatively regulate ABA signaling displayed an enhanced susceptibility phenotype to this fungus. Comparative transcriptomic analyses of aba1-6 and wild-type plants revealed that the ABA pathway negatively regulates defense genes, many of which are controlled by the SA, JA, or ET pathway. In line with these data, we found that aba1-6 resistance to P. cucumerina was partially compromised when the SA, JA, or ET pathway was disrupted in this mutant. Additionally, in the aba1-6 plants, some genes encoding cell wall-related proteins were misregulated. Fourier transform infrared spectroscopy and biochemical analyses of cell walls from aba1-6 and wild-type plants revealed significant differences in their Fourier transform infrared spectratypes and uronic acid and cellulose contents. All these data suggest that ABA signaling has a complex function in Arabidopsis basal resistance, negatively regulating SA/JA/ET-mediated resistance to necrotrophic fungi.

Tryptophan-derived secondary metabolites in Arabidopsis thaliana confer non-host resistance to necrotrophic Plectosphaerella cucumerina fungi.

A defence pathway contributing to non-host resistance to biotrophic fungi in Arabidopsis involves the synthesis and targeted delivery of the tryptophan (trp)-derived metabolites indol glucosinolates (IGs) and camalexin at pathogen contact sites. We have examined whether these metabolites are also rate-limiting for colonization by necrotrophic fungi. Inoculation of Arabidopsis with adapted or non-adapted isolates of the ascomycete Plectosphaerella cucumerina triggers the accumulation of trp-derived metabolites. We found that their depletion in cyp79B2 cyp79B3 mutants renders Arabidopsis fully susceptible to each of three tested non-adapted P. cucumerina isolates, and super-susceptible to an adapted P. cucumerina isolate. This assigns a key role to trp-derived secondary metabolites in limiting the growth of both non-adapted and adapted necrotrophic fungi. However, 4-methoxy-indol-3-ylmethylglucosinolate, which is generated by the P450 monooxygenase CYP81F2, and hydrolyzed by PEN2 myrosinase, together with the antimicrobial camalexin play a minor role in restricting the growth of the non-adapted necrotrophs. This contrasts with a major role of these two trp-derived phytochemicals in limiting invasive growth of non-adapted biotrophic powdery mildew fungi, thereby implying the existence of other unknown trp-derived metabolites in resistance responses to non-adapted necrotrophic P. cucumerina. Impaired defence to non-adapted P. cucumerina, but not to the non-adapted biotrophic fungus Erysiphe pisi, on cyp79B2 cyp79B3 plants is largely restored in the irx1 background, which shows a constitutive accumulation of antimicrobial peptides. Our findings imply differential contributions of antimicrobials in non-host resistance to necrotrophic and biotrophic pathogens.

New virulence groups in Fusarium oxysporum f. sp. phaseoli: the expression of the gene coding for the transcription factor ftf1 correlates with virulence.

Fusarium oxysporum f. sp. phaseoli strains isolated from runner bean plants showing Fusarium wilt symptoms were characterized. The analysis of the genetic diversity of these strains and the comparison with strains formerly isolated from diseased common bean plants grown in the same region of Spain indicated a close genetic similarity among them. Pathogenicity assays carried out on runner bean plants showed virulence differences that allowed the classification of these strains into three groups: super virulent, highly virulent, and weakly virulent. However, all the analyzed strains behaved as highly virulent when inoculated on common bean plants, indicating that virulence is specific of the host-pathogen interaction. We also analyzed the number of copies and expression of the gene encoding the transcription factor ftf1, which has been shown to be specific of virulent F. oxysporum strains and highly up-regulated during plant infection. In planta real-time quantitative polymerase chain reaction expression analysis showed that expression of ftf1 was correlated with the degree of virulence. The comparative analysis of the polymorphic copies of ftf1 detected in the strains here characterized and those detected in the genome sequence of F. oxysporum f. sp. lycopersici strain 4287 indicates that some of the copies are likely nonfunctional.

fost12, the Fusarium oxysporum homolog of the transcription factor Ste12, is upregulated during plant infection and required for virulence.

We have identified a Fusarium oxysporum homolog of the Ste12 transcription factor that regulates mating and filamentation in Saccharomyces cerevisiae. The corresponding gene, fost12, from a highly virulent strain of F. oxysporum f. sp. phaseoli, was confirmed to share a high level of similarity and possessed the STE and C2H2 domains characteristic of the fungal Ste12 transcription factor family of proteins. Disruption of fost12 resulted in no visible alterations of colony morphology or in vitro growth characteristics. However, the disruption mutants showed a substantial reduction in virulence when inoculated in common bean seedlings. In planta transcription of fost12 is drastically increased between 12 and 24h after inoculation, as detected by real-time RT-PCR. The results of the transcriptional analyses carried out in several F. oxysporum strains during axenic growth suggest that the fost12 gene product is a virulence factor required to deal with the nutritional stress confronted by the pathogen during host plant colonization.

The FTF gene family regulates virulence and expression of SIX effectors in Fusarium oxysporum.

The FTF (Fusarium transcription factor) gene family comprises a single copy gene, FTF2, which is present in all the filamentous ascomycetes analysed, and several copies of a close relative, FTF1, which is exclusive to Fusarium oxysporum. An RNA-mediated gene silencing system was developed to target mRNA produced by all the FTF genes, and tested in two formae speciales: F. oxysporum f. sp. phaseoli (whose host is common bean) and F. oxysporum f. sp. lycopersici (whose host is tomato). Quantification of the mRNA levels showed knockdown of FTF1 and FTF2 in randomly isolated transformants of both formae speciales. The attenuation of FTF expression resulted in a marked reduction in virulence, a reduced expression of several SIX (Secreted In Xylem) genes, the best studied family of effectors in F. oxysporum, and lower levels of SGE1 (Six Gene Expression 1) mRNA, the presumptive regulator of SIX expression. Moreover, the knockdown mutants showed a pattern of colonization of the host plant similar to that displayed by strains devoid of FTF1 copies (weakly virulent strains). Gene knockout of FTF2 also resulted in a reduction in virulence, but to a lesser extent. These results demonstrate the role of the FTF gene expansion, mostly the FTF1 paralogues, as a regulator of virulence in F. oxysporum and suggest that the control of effector expression is the mechanism involved.

A DNA-Based Procedure for In Planta Detection of Fusarium oxysporum f. sp. phaseoli.

ABSTRACT We have characterized strains of Fusarium oxysporum from common bean fields in Spain that were nonpathogenic on common bean, as well as F. oxysporum strains (F. oxysporum f. sp. phaseoli) pathogenic to common bean by random amplified polymorphic DNA (RAPD) analysis. We identified a RAPD marker (RAPD 4.12) specific for the highly virulent pathogenic strains of the seven races of F. oxysporum f. sp. phaseoli. Sequence analysis of RAPD 4.12 allowed the design of oligonucleotides that amplify a 609-bp sequence characterized amplified region (SCAR) marker (SCAR-B310A280). Under controlled environmental and greenhouse conditions, detection of the pathogen by polymerase chain reaction was 100% successful in root samples of infected but still symptomless plants and in stem samples of plants with disease severity of >/=4 in the Centro Internacional de Agricultura Tropical (CIAT; Cali, Colombia) scale. The diagnostic procedure can be completed in 5 h and allows the detection of all known races of the pathogen in plant samples at early stages of the disease with no visible symptoms.

Functional genomics tools to decipher the pathogenicity mechanisms of the necrotrophic fungus Plectosphaerella cucumerina in Arabidopsis thaliana.

The analysis of the interaction between Arabidopsis thaliana and adapted (PcBMM) and nonadapted (Pc2127) isolates of the necrotrophic fungus Plectosphaerella cucumerina has contributed to the identification of molecular mechanisms controlling plant resistance to necrotrophs. To characterize the pathogenicity bases of the virulence of necrotrophic fungi in Arabidopsis, we developed P. cucumerina functional genomics tools using Agrobacterium tumefaciens-mediated transformation. We generated PcBMM-GFP and Pc2127-GFP transformants constitutively expressing the green fluorescence protein (GFP), and a collection of random T-DNA insertional PcBMM transformants. Confocal microscopy analyses of the initial stages of PcBMM-GFP infection revealed that this pathogen, like other necrotrophic fungi, does not form an appressorium or penetrate into plant cells, but causes successive degradation of leaf cell layers. By comparing the colonization of Arabidopsis wild-type plants and hypersusceptible (agb1-1 and cyp79B2cyp79B3) and resistant (irx1-6) mutants by PcBMM-GFP or Pc2127-GFP, we found that the plant immune response was already mounted at 12-18 h post-inoculation, and that Arabidopsis resistance to these fungi correlated with the time course of spore germination and hyphal growth on the leaf surface. The virulence of a subset of the PcBMM T-DNA insertional transformants was determined in Arabidopsis wild-type plants and agb1-1 mutant, and several transformants were identified that showed altered virulence in these genotypes in comparison with that of untransformed PcBMM. The T-DNA flanking regions in these fungal mutants were successfully sequenced, further supporting the utility of these functional genomics tools in the molecular characterization of the pathogenicity of necrotrophic fungi.

Repression of the auxin response pathway increases Arabidopsis susceptibility to necrotrophic fungi.

In plants, resistance to necrotrophic pathogens depends on the interplay between different hormone systems, such as those regulated by salicylic acid (SA), jasmonic acid (JA), ethylene, and abscisic acid. Repression of auxin signaling by the SA pathway was recently shown to contribute to antibacterial resistance. Here, we demonstrate that Arabidopsis auxin signaling mutants axr1, axr2, and axr6 that have defects in the auxin-stimulated SCF (Skp1-Cullin-F-box) ubiquitination pathway exhibit increased susceptibility to the necrotrophic fungi Plectosphaerella cucumerina and Botrytis cinerea. Also, stabilization of the auxin transcriptional repressor AXR3 that is normally targeted for removal by the SCF-ubiquitin/proteasome machinery occurs upon P. cucumerina infection. Pharmacological inhibition of auxin transport or proteasome function each compromise necrotroph resistance of wild-type plants to a similar extent as in non-treated auxin response mutants. These results suggest that auxin signaling is important for resistance to the necrotrophic fungi P. cucumerina and B. cinerea. SGT1b (one of two Arabidopsis SGT1 genes encoding HSP90/HSC70 co-chaperones) promotes the functions of SCF E3-ubiquitin ligase complexes in auxin and JA responses and resistance conditioned by certain Resistance (R) genes to biotrophic pathogens. We find that sgt1b mutants are as resistant to P. cucumerina as wild-type plants. Conversely, auxin/SCF signaling mutants are uncompromised in RPP4-triggered resistance to the obligate biotrophic oomycete, Hyaloperonospora parasitica. Thus, the predominant action of SGT1b in R gene-conditioned resistance to oomycetes appears to be at a site other than assisting SCF E3-ubiquitin ligases. However, genetic additivity of sgt1b axr1 double mutants in susceptibility to H. parasitica suggests that SCF-mediated ubiquitination contributes to limiting biotrophic pathogen colonization once plant-pathogen compatibility is established.

The gene coding for a new transcription factor (ftf1) of Fusarium oxysporum is only expressed during infection of common bean.

We report the isolation and analysis of the gene encoding ftf1 (Fusarium transcription factor 1), a previously undescribed putative transcription factor from highly virulent strains of Fusarium oxysporum f.sp. phaseoli that is transcribed specifically during early stages of infection of its host common bean (Phaseolus vulgaris L.). The predicted 1080 amino acid ftf1 protein contains a Zn(II)2-Cys6 binuclear cluster DNA-binding motif. ftf1 expression during axenic growth in culture was not detected by either Northern or RT-PCR. On the contrary, in planta transcription of ftf1 is increased about 24h after plant inoculation, as detected by real-time RT-PCR. This result suggests that ftf1 has a role in the establishment of the fungus within the plant and/or the progress of the disease. Multiple copies of ftf1 are present in highly virulent strains of F. oxysporum f.sp. phaseoli.