We have collected airborne fungi in houses and public facilities in Japan and currently kept about 500 strains of genus Aspergillus, which are the subjects of this study. From the collection, we chose Aspergillus ochraceus and Aspergillus fumigatus, which were isolated from the air during the period of 2003-2009 , to survey their mycotoxin production by LC/MS/MS. A. ochraceus strains were served to investigate the production of ochratoxin A and ochratoxin B, while A. fumigatus strains were to examine the production of gliotoxin and verruculogen. The results confirmed that many strains are capable of producing mycotoxins. It should be noted that some airborne spores of A. ochraceus strains were found to produce relatively high level of ochratoxin A.
The tomato pathotype of Alternaria alternata causes Alternaria stem canker of tomato (Solanum lycopersicum L.) by producing a host-specific AAL-toxin. The chemical structure and biological activities of AAL-toxin are analogous to those of the mycotoxin fumonisin. Determination of pathogen development and toxin production in infected tomato plants is of prime importance for pathological evaluation and risk assessment of AAL-toxin. On a resistant cultivar, AAL-toxin and the pathogen were restricted to a small region surrounding only the initial inoculation site. On the other hand, widespread distribution of the toxin and the pathogen were detected in a susceptible cultivar. The pathological significance and contamination risk from the toxin were supported by the results of in planta production of the toxin in diseased tomato plants.
Fusarium head blight (FHB) is a widespread, destructive disease of wheat and barley that is primarily caused by Fusarium graminearum Schwabe. F. graminearum produces mycotoxins such as deoxynivalenol (DON) and nivalenol (NIV) in infected grain. Fungicide application is the most practical way to control FHB, but it is not consistently effective. To obtain better chemical control, the timing of application is important. In both wheat and barley, FHB fungicides have been usually applied at or near anthesis (flowering stage) regardless of the cultivar because this stage in wheat is most susceptible. However, we found that in barley the most critical time for F. graminearum infection and mycotoxin accumulation differs among cultivars. Whereas chasmogamous (open-flowering) cultivars were most susceptible at anthesis, cleistogamous (closed-flowering) cultivars showed good resistance at anthesis, but were relatively susceptible after ‘spent’ anther extrusion (SAE). As expected from these observations, field experiments using thiophanate-methyl fungicide indicated that the optimal timing for chemical control in cleistogamous barley is around the beginning of SAE, rather than at anthesis. For wheat, the manner in which DON and NIV accumulate in grain infected with F. graminearum and the influence of the time of infection on mycotoxin accumulation were investigated. High levels of DON and NIV were produced in grain beyond 20 days after anthesis. This indicates the importance of the late stage of grain development in mycotoxin contamination in wheat, suggesting that control strategies that cover both the early and late stages of grain development should be developed.
The plant pathogenic fungus Fusarium graminearum (teleomorph: Gibberella zeae) is a crucial pathogen of major cereal crops and the fungus has taken on greater importance throughout the world. Taking advantage of rapid progress on fungal molecular techniques, we have studied biosynthetic pathways of harmful secondary metabolites of F. graminearum such as trichothecenes, zearalenone, and aurofusarin. We revealed that both Tri13 and Tri7 genes are requried for the chemotype determination of trichothecenes for nivalenol and 4 -acetyl-nivalenol production, respectively. We also constructed efficient chemotype determination system by utilizing distrupted Tri7 gene sequences of deoxinivalenol producers. Forward and reverse genetic approaches enabled us to characterize the gene clusters responsible for the biosynthesis of two polyketide compounds, zearalenone and aurofusarin. Both gene clusters cover genes encoding polyketide synthases and transcription activators. We also characterized an ABC transporter, ZRA 1, related with zearaleone production and constructed a zearalenone conditional gene expression system from the microarray analyses. These results increased our understanding on Fusarium genetics and mycotoxicology. Our future works will focus on revealing the regulatory mechanisms of toxin production and biological functions of these secondary metabolites.