Restoration of pathogenicity of a penetration-deficient mutant of Collectotrichum lagenarium by DNA complementation

Microwounding is a pivotal factor for the induction of actin-dependent penetration resistance against fungal attack

Induced penetration resistance is triggered by failed penetration attempts of nonpathogenic fungi. The resistance mechanism is an important nonhost reaction in plants that can block the invasion of filamentous pathogens such as fungi and oomycetes. However, it remains unclear whether the mechanical stimuli accompanying fungal penetration play a role in induced penetration resistance, whereas the perforation of the cell wall may provide significant stimuli to plant cells. Here, we used microneedles or biolistic bombardment to mimic fungal penetration pegs and a micromanipulation transfer technique of the bio-probe, a germling of Blumeria graminis hordei, to the wounded cells to demonstrate that microwounds derived from fungal penetration attempts may trigger induced penetration resistance in plant cells. When preinoculated with the nonpathogenic fungi Erysiphe pisi and Colletotrichum orbiculare, which were unable to penetrate a barley cell, the penetration of a bio-probe that was transferred by micromanipulation onto the same cell was completely blocked. Fungal penetration was essential to the triggering of induced penetration resistance because a penetration-peg-defective mutant of C. orbiculare completely lacked the ability to trigger resistance. The artificial microwounds significantly, but not completely, blocked the penetration of the bio-probe. Treatment with the actin polymerization inhibitor cytochalasin A or expression of the actin depolymerizing protein HvPro1 caused complete ablation of the induced penetration resistance triggered by either failed fungal penetration or artificial microwounds. These results strongly suggest that microwounding may trigger actin-dependent induced penetration resistance. Manipulation of induced penetration resistance may be a promising target to improve basic disease resistance in plants.

Plant pathology: a story about biology

Disease is a universal feature of life for multicellular organisms, and the study of disease has contributed to the establishment of key concepts in the biological sciences. This implies strong connections between plant pathology and basic biology, something that could perhaps be made more apparent to undergraduate students interested in the life sciences. To that end, we present an instructional narrative that begins with a simple question: Why are there diseases? Responses and follow-up questions can facilitate exploration of such topics as the evolution of parasitism, plant adaptations to parasitism, impacts of parasites on native plant communities, and ways in which human intervention can foster the emergence of aggressive plant pathogens. This approach may help to attract students who would not have found their way to plant pathology through traditional pathways. Packaging the narrative as a game may render it more interesting and accessible, particularly to a younger audience.

Protein kinase A subunits of the ascomycete pathogen Mycosphaerella graminicola regulate asexual fructification, filamentation, melanization and osmosensing

SUMMARY As in many fungi, asexual reproduction of Mycosphaerella graminicola in planta is a complex process that requires proper differentiation of the infectious hyphae in the substomatal cavities of foliar tissue before pycnidia with conidia can be formed. In this study, we have investigated the role of the cAMP signalling pathway in development and pathogenicity of this pathogen by disruption of the genes encoding the catalytic (designated MgTpk2) and regulatory subunit (designated MgBcy1) of protein kinase A. The MgTpk2 and MgBcy1 mutants showed altered phenotypes in vitro when grown under different growth conditions. On potato dextrose agar (PDA), MgBcy1 mutants showed altered osmosensitivity and reduced melanization, whereas the MgTpk2 mutants showed accelerated melanization when compared with the M. graminicola IPO323 wild-type strain and ectopic transformants. MgTpk2 mutants also secreted a dark-brown pigment into yeast glucose broth medium. In germination and microconidiation assays, both mutants showed a germination pattern similar to that of the controls on water agar, whereas on PDA filamentous growth of MgTpk2 mutants was impaired. Pathogenicity assays showed that the MgTpk2 and MgBcy1 mutants were less virulent as they caused only limited chlorotic and necrotic symptoms at the tips of the inoculated leaves. Further analyses of the infection process showed that MgTpk2 and MgBcy1 mutants were able to germinate, penetrate and colonize mesophyll tissue, but were unable to produce the asexual fructifications, which was particularly due to inappropriate differentiation during the late stage of this morphogenesis-related process.

Genetic Mapping of Pathogenicity and Aggressiveness of Gibberella zeae (Fusarium graminearum) Toward Wheat

ABSTRACT Gibberella zeae is the major fungal pathogen of Fusarium head blight of wheat and produces several mycotoxins that are harmful to humans and domesticated animals. We identified loci associated with pathogenicity and aggressiveness on an amplified fragment length polymorphism based genetic map of G. zeae in a cross between a lineage 6 nivalenol producer from Japan and a lineage 7 deoxynivalenol producer from Kansas. Ninety-nine progeny and the parents were tested in the greenhouse for 2 years. Progeny segregated qualitatively (61:38) for pathogenicity:nonpathogenicity, respectively. The trait maps to linkage group IV, which is adjacent to loci that affect colony pigmentation, perithecium production, and trichothecene toxin amount. Among the 61 pathogenic progeny, the amount of disease induced (aggressiveness) varied quantitatively. Two reproducible quantitative trait loci (QTL) for aggressiveness were detected on linkage group I using simple interval analysis. A QTL linked to the TRI5 locus (trichodiene synthase in the trichothecene pathway gene cluster) explained 51% of the variation observed, and a second QTL that was 50 centimorgans away explained 29% of the phenotypic variation. TRI5 is tightly linked to the locus controlling trichothecene toxin type. The two QTLs, however, were likely part of the same QTL using composite interval analysis. Progeny that produced deoxynivalenol were, on average, approximately twice as aggressive as those that produced nivalenol. No transgressive segregation for aggressiveness was detected. The rather simple inheritance of both traits in this interlineage cross suggests that relatively few loci for pathogenicity or aggressiveness differ between lineage 6 and 7.

CgDN3: an essential pathogenicity gene of colletotrichum gloeosporioides necessary to avert a hypersensitive-like response in the host Stylosanthes guianensis

A gene of Colletotrichum gloeosporioides that is induced by nitrogen starvation in axenic culture and is expressed at the early stages of infection of the host Stylosanthes guianensis has been identified and its role in pathogenicity tested. The sequence of this gene, named CgDN3, indicated that it encodes a protein of 74 amino acids that contains a predicted 18 amino acid signal sequence for secretion of a basic 54 amino acid mature protein with weak homology to an internal region of plant wall-associated receptor kinases. Mutants of C. gloeosporioides were produced by homologous recombination in which part of the coding sequence and promoter region of the CgDN3 gene was replaced with a hygromycin-resistance gene cassette. Mutations in the CgDN3 gene were confirmed in two independent transformants and Northern (RNA) analysis demonstrated the disrupted CgDN3 gene was not expressed. The mutants had faster mycelial growth rates in vitro but produced spores that germinated to form appressoria normally on the leaf surface. However, the CgDN3 mutants were unable to infect and reproduce on intact host leaves. Microscopic analysis revealed small clusters of necrotic host cells at inoculation sites on leaves, suggesting that these mutants elicited a localized, host hypersensitive-like response. The mutants were able to grow necrotrophically and reproduce on leaves when conidia were inoculated directly onto wound sites. The putative promoter region of the CgDN3 gene was fused to a gene encoding a modified jellyfish green fluorescent protein and introduced into the fungus. Following inoculation, strong expression of green fluorescent protein was observed in primary infection vesicles in infected epidermal cells with weaker expression evident in hyphae growing within infected leaf tissue. These findings indicate that CgDN3 encodes a novel pathogenicity determinant associated with the biotrophic phase of primary infection and required to avert a hypersensitive-like response by a compatible host.

The Colletotrichum lagenarium MAP kinase gene CMK1 regulates diverse aspects of fungal pathogenesis

The infection process of Colletotrichum lagenarium, the causal agent of cucumber anthracnose disease, involves several key steps: germination; formation of melanized appressoria; appressorial penetration; and subsequent invasive growth in host plants. Here we report that the C. lagenarium CMK1 gene encoding a mitogen-activated protein (MAP) kinase plays a central role in these infection steps. CMK1 can complement appressorium formation of the Pmk1 MAP kinase mutant of Magnaporthe grisea. Deletion of CMK1 causes reduction of conidiation and complete lack of pathogenicity to the host plant. Surprisingly, in contrast to M. grisea pmk1 mutants, conidia of cmk1 mutants fail to germinate on both host plant and glass surfaces, demonstrating that the CMK1 MAP kinase regulates conidial germination. However, addition of yeast extract rescues germination, indicating the presence of a CMK1-independent pathway for regulation of conidial germination. Germinating conidia of cmk1 mutants fail to form appressoria and the mutants are unable to grow invasively in the host plant. This strongly suggests that MAP kinase signaling pathways have general significance for infection structure formation and pathogenic growth in phytopathogenic fungi. Furthermore, three melanin genes show no or slight expression in the cmk1 mutant when conidia fail to germinate, suggesting that CMK1 plays a role in gene expression required for appressorial melanization.

Colletotrichum trifolii mutants disrupted in the catalytic subunit of cAMP-dependent protein kinase are nonpathogenic

Colletotrichum trifolii is the fungal pathogen of alfalfa that causes anthracnose disease. For successful plant infection, this fungus must undergo a series of morphological transitions following conidial attachment, including germination and subsequent differentiation, resulting in appressorium formation. Our previous studies with pharmacological effectors of signaling pathways have suggested the involvement of cyclic AMP (cAMP)-dependent protein kinase (PKA) during these processes. To more precisely evaluate the role of PKA in C. trifolii morphogenesis, the gene encoding the catalytic (C) subunit of PKA (Ct-PKAC) was isolated, sequenced, and inactivated by gene replacement. Southern blot analysis with C. trifolii genomic DNA suggested that Ct-PKAC is a single-copy gene. Northern (RNA) blot analysis with total RNA from different fungal growth stages indicated that the expression of this gene was developmentally regulated. When Ct-PKAC was insertionally inactivated by gene replacement, the transformants showed a small reduction in growth relative to the wild type and conidiation patterns were altered. Importantly, PKA-deficient strains were unable to infect intact alfalfa (host) plants, though only a slight delay was observed in the timing for conidial germination and appressorial formation in the Ct-PKAC disruption mutants. Moreover, these mutants were able to colonize host tissues following artificial wounding, resulting in typical anthracnose disease lesions. Coupled with microscopy, these data suggest that the defect in pathogenicity is likely due to a failure in penetration. Our results demonstrate that PKA has an important role in regulating the transition between vegetative growth and conidiation, and is essential for pathogenic development in C. trifolii.

Cloning and structural analysis of the melanin biosynthesis gene SCD1 encoding scytalone dehydratase in Colletotrichum lagenarium

Appressoria of the phytopathogenic fungus Colletotrichum lagenarium contain melanin, which has been implicated as an important factor in the penetration of host plants. A cDNA clone containing the melanin biosynthetic gene encoding scytalone dehydratase (SCD1) from C. lagenarium was identified by hybridization with a heterologous cDNA probe from Magnaporthe grisea. The cDNA clone was used to identify a cosmid containing SCD1 in a genomic library of C. lagenarium, and the nucleotide sequence was determined for both the cDNA and genomic clones. The SCD1 gene contained one open reading frame composed of 188 codons and two deduced introns of 57 and 67 nucleotides. The deduced amino acid sequence of the N-terminal region of SCD1 showed high similarity to the amino acid sequence of scytalone dehydratase from Cochliobolus miyabeanus. A plasmid containing the SCD1 gene transformed the melanin-deficient mutant 9201Y (Scd-) to the wild phenotype but did not complement the conditional scytalone dehydratase-deficient mutant C. lagenarium 8015. Genomic DNA analysis indicated that the SCD1 gene is a single locus in C. lagenarium. Transcripts of the SCD1 gene were detected 2 h after the start of conidial germination.

Morphogenesis and mechanisms of penetration by plant pathogenic fungi

Infection structures of phytopathogenic fungi are modified hyphae specialized for the invasion of plant tissues. Initial events are adhesion to the cuticle and directed growth of the germ tube on the plant surface. At the site of penetration, appressoria are often formed that may have melanized walls and develop high turgor pressure to support the penetration process. The penetration hypha accumulates components of the cytoskeleton in the tip and secretes a variety of cell wall-degrading enzymes in a highly regulated fashion in order to penetrate the cuticle and the plant cell wall. This article reviews recent papers on the cytology, physiology, and molecular biology of the penetration process.

Structural analysis of PKS1, a polyketide synthase gene involved in melanin biosynthesis in Colletotrichum lagenarium

Albino mutants (Pks-) of Colletotrichum lagenarium form nonmelanized appressoria and possess little penetrating ability on the host plant. The defect in albino mutant 79215 (Pks-) is considered to lie in pentaketide biosynthesis and/or pentaketide cyclization during melanin biosynthesis. The cosmid pAC7, carrying the PKS1 gene, when transformed into the albino mutant restores the wild-type melanin phenotype. We have determine the DNA sequence and the transcriptional organization of the PKS1 gene. The PKS1 gene contains one open reading frame, consisting of 3 exons separated by two short introns. The predicted PKS1 polypeptide consists of 2187 amino acids and shows significant similarities with other polyketide synthases, particularly that encoded by wA in Aspergillus nidulans, involved in conidial pigmentation. The PKS1 gene contains highly conserved beta-ketoacyl synthase, acetyl/malonyl transferase, and acyl carrier protein domains. We propose that the C. lagenarium PKS1 gene encodes a polyketide synthase involved in melanin biosynthesis.