GzSNF1 is required for normal sexual and asexual development in the ascomycete Gibberella zeae

A novel transcriptional factor important for pathogenesis and ascosporogenesis in Fusarium graminearum

Fusarium head blight or scab caused by Fusarium graminearum is an important disease of wheat and barley. The pathogen not only causes severe yield losses but also contaminates infested grains with mycotoxins. In a previous study, we identified several pathogenicity mutants by random insertional mutagenesis. One of these mutants was disrupted in the ZIF1 gene, which encodes a b-ZIP transcription factor unique to filamentous ascomycetes. The Δzif1 mutant generated by gene replacement was significantly reduced in deoxynivalenol (DON) production and virulence on flowering wheat heads. It was defective in spreading from inoculated florets to the rachis and other spikelets. Deletion of the ZIF1 ortholog MoZIF1 in the rice blast fungus also caused reductions in virulence and in invasive growth. In addition, the Δzif1 mutant is defective in sexual reproduction. Although it had normal male fertility, when selfed or mated as the female in outcrosess, the Δzif1 mutant produced small, pigmented perithecia that were sterile (lack of asci and ascospores), suggesting a female-specific role for ZIF1 during fertilization or ascus development. Similar female-specific defects in sexual reproduction were observed in the ΔMozif1 mutant. When mated as the female, the ΔMozif1 perithecia failed to develop long necks and asci or ascospores. The ZIF1 gene is well conserved in filamentous ascomycetes, particularly in the b-ZIP domain, which is essential for its function. Expression of ZIF1 in Magnaporthe oryzae complemented the defects of the ΔMozif1 mutant. These results indicate that this b-ZIP transcription factor is functionally conserved in these two fungal pathogens for plant infection and sexual reproduction.

Identification and functional characterization of genes involved in the sexual reproduction of the ascomycete fungus Gibberella zeae

We previously reported that G protein alpha subunit 1 (GPA1) is essential for sexual reproduction in the homothallic ascomycete fungus Gibberella zeae. In this study we performed microarray analyses on a GPA1 deletion mutant of G. zeae (Δgpa1) to identify genes involved in the sexual reproduction of this fungus. In the Δgpa1 strain, 645 genes were down-regulated and 550 genes were up-regulated during sexual reproduction when compared to the wild-type strain. One hundred of the down-regulated genes were selected for further investigation based on orthologous group clusters and differences in transcript levels. Quantitative real time-PCR was used to determine transcriptional profiles of these genes at various sexual and vegetative stages. We observed that transcript levels of 78 of these genes were dramatically increased in the wild-type strain during sexual reproduction compared to levels observed during vegetative growth, and were down-regulated in Δgpa1 compared to the wild-type strain. We deleted 57 of these genes and found that four of the deletion mutants lost self-fertility and five produced fewer perithecia compared to the wild-type strain. Two mutants produced wild-type numbers of perithecia, but maturation of perithecia and ascospores was delayed. In all we identified 11 genes that are involved in sexual reproduction of G. zeae and present evidence that some of these genes function at distinct stages during sexual reproduction in the fungus.

An in vitro method for the analysis of infection-related morphogenesis in Fusarium graminearum

Fusarium graminearum is a significant pathogen of many cereal crops. With its genetic tractability, ease of culture, genome sequence availability and economic significance, F. graminearum has become the subject of intensive molecular research. Although molecular tools have been developed to enhance research into virulence determinants of F. graminearum, simple assays for infection-related development are lacking. As such, the objective of this study was to develop an in vitro protocol for the analysis of infection-related morphogenesis in F. graminearum. We demonstrate that two morphologically distinct hyphal structures are produced by F. graminearum during the invasion of detached wheat glumes: subcuticular hyphae and bulbous infection hyphae. Specialized wheat epidermal cells (papillae) appear to act as sites of invasion by F. graminearum on the adaxial side of detached wheat glumes. In addition, the development of bulbous infection hyphae is dependent on the pathogenicity mitogen-activated protein kinase Gpmk1, further supporting the infection-related nature of these structures. This relatively simple assay will contribute to the tractability of the F. graminearum system and help to uncover molecular requirements for infection-related development.