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G protein-coupled receptors (GPCRs) are a large transmembrane receptor superfamily that is involved in many cellular signaling pathways. In the present study, GPCR-family genes from the toxigenic and necrotrophic plant pathogen Alternaria alternata were cloned and characterized. Three GPCR-encoding genes, AaGPR1, AaGPR2, and AaGPR3 were identified in the draft genomic data of the A. alternata tomato pathotype, which produces the host-specific AAL-toxin. AaGPR1, AaGPR2, and AaGPR3 each encodes a protein containing a seven transmembrane domain that is characteristic of GPCRs. Targeted deletion of AaGPR1, AaGPR2, or AaGPR3 in the A. alternata tomato pathotype was conducted to understand the influence of G-protein signaling mechanisms on developmental processes and virulence of this pathogen. No changes in colony morphology or AAL-toxin production were observed for the deletion strain ΔAaGPR1, 2, and 3, compared with the wild-type strain. However, one deletion strain, ΔAaGPR3, exhibited aberrant conidial morphology including decreased conidial length and beak formation. The ability to induce the formation of necrotic lesions on susceptible leaves also significantly decreased in ΔAaGPR3, indicating a reduction in virulence. These defects are similar to the phenotypes found for the Gα gene (AGA1) mutant of A. alternata. These results indicate that the G-protein signal transduction pathway appears to be involved in conidial development and virulence of A. alternata.

The Southern corn leaf blight (SCLB) epidemic of 1970 devastated fields of T-cytoplasm corn planted in monoculture throughout the eastern United States. The epidemic was driven by race T, a previously unseen race of Cochliobolus heterostrophus. A second fungus, Phyllosticta zeae-maydis, with the same biological specificity, appeared coincidentally. Race T produces T-toxin, while Phyllosticta zeae-maydis produces PM-toxin, both host-selective polyketide toxins necessary for supervirulence. The present abundance of genome sequences offers an opportunity to tackle the evolutionary origins of T- and PM- toxin biosynthetic genes, previously thought unique to these species. Using the C. heterostrophus genes as probes, we identified orthologs in six additional Dothideomycete and three Eurotiomycete species. In stark contrast to the genetically fragmented race T Tox1 locus that encodes these genes, all newly found Tox1-like genes in other species reside at a single collinear locus. This compact arrangement, phylogenetic analyses, comparisons of Tox1 protein tree topology to a species tree, and Tox1 gene characteristics suggest that the locus is ancient and that some species, including C. heterostrophus, gained Tox1 by horizontal gene transfer. C. heterostrophus and Phyllosticta zeae-maydis did not exchange Tox1 DNA at the time of the SCLB epidemic, but how they acquired Tox1 remains uncertain. The presence of additional genes in Tox1-like clusters of other species, although not in C. heterostrophus and Phyllosticta zeae-maydis, suggests that the metabolites produced differ from T- and PM-toxin.

Trichoderma harzianum, a biocontrol agent for various plant pathogens, is known to degrade fungal cell walls; this mycoparasitism is believed to require secretion of cell-wall-degrading enzymes against host pathogens. In this study, we identified a homologue of yeast SNF1 (sucrose nonfermenting 1) encoding protein kinase in T. harzianum (ThSNF1) by draft genome sequencing of strain T36. Targeted gene disruption of ThSNF1 was performed using the PEG method with fusion PCR products. Growth of mutant ΔThSNF1 was markedly less than for the wild-type strain on minimal medium with chitin as a carbon source. The mutant exhibited reduced expression of the genes encoding chitinase and polygalacturonase and markedly reduced spore production. Mycoparasitism against plant pathogens such as Fusarium oxysporum f. sp. cubense (Panama disease) and Fusarium graminearum (Fusarium head blight) was clearly impaired in the mutant. The results suggest that ThSNF1 is critical for asexual development, utilization of certain carbon sources and virulence on fungi, and is therefore important for the biocontrol ability of T. harzianum.

Native Trichoderma spp. were isolated from agricultural fields in several regions of Ecuador. These isolates were characterized via morphological observation as well as molecular phylogenetic analysis based on DNA sequences of the rDNA internal transcribed spacer region, elongation factor-1α gene and RNA polymerase subunit II gene. Fifteen native Trichoderma spp. were identified as T. harzianum, T. asperellum, T. virens and T. reesei. Some of these strains showed strong antagonistic activities against several important pathogens in Ecuador, such as Fusarium oxysporum f. sp. cubense (Panama disease) and Mycosphaerella fijiensis (black Sigatoka) on banana, as well as Moniliophthora roreri (frosty pod rot) and Moniliophthora perniciosa (witches’ broom disease) on cacao. The isolates also showed inhibitory effects on in vitro colony growth tests against Japanese isolates of Fusarium oxysporum f. sp. lycopersici, Alternaria alternata and Rosellinia necatrix. The native Trichoderma strains characterized here are potential biocontrol agents against important pathogens of banana and cacao in Ecuador.

The tomato pathotype of Alternaria alternata (A. arborescens) produces the dark brown to black pigment melanin, which accumulates in the cell walls of hyphae and conidia. Melanin has been implicated as a pathogenicity factor in some phytopathogenic fungi. Here, two genes of the tomato pathotype for melanin biosynthesis, ALM1 and BRM2-1, which encode a polyketide synthetase and a 1,3,8-trihydroxynaphthalene (THN) reductase, respectively, have been cloned and disrupted in the pathogen. The gene-disrupted mutants, alm1 and brm2-1, had albino and brown phenotypes, respectively. The wild-type and the mutants caused the same necrotic lesions on the leaves after inoculation with spores. These results suggest that melanin is unlikely to play a direct role in pathogenicity in the tomato pathotype A. alternata. Scanning electron microscopy revealed that the conidia of both mutants have much smoother surfaces in comparison to the wild-type. The conidia of those mutants were more sensitive to UV light than those of the wild-type, demonstrating that melanin confers UV tolerance.

To analyze the genetics of host-specific toxin production and its relation to the specific pathogenicity of a mitosporic fungus Alternaria alternata, we developed a protoplast fusion system. Protoplasts of drug-resistant transformants of the A. alternata tomato pathotype (AAL-toxin producer) and A. alternata strawberry pathotype (AF-toxin producer) were fused by electrofusion. Of five fusion strains examined, two strains were pathogenic on both tomato and strawberry host plants, whereas the rest of the fusion strains were pathogenic only on tomato. Pulsed-field gel electrophoresis analysis demonstrated that the hybrid strains pathogenic on both tomato and strawberry carry 1.0- and 1.05-Mb conditionally dispensable (CD) chromosomes derived, respectively, from the parental strains of the tomato and strawberry pathotypes. On the other hand, the fusion strains appeared to maintain only a single homologous chromosome derived from one of the parental strain in the case of essential chromosomes (A chromosomes). The results suggest that fusion strains between two different pathotypes of A. alternata might be haploid resulting from the deletion of extra sets of essential chromosomes in the fused nuclei, whereas the CD chromosomes derived from each parental strain could be maintained stably in a new genetic background with an expanded range of pathogenicity.