The biological role of Fusarium graminearum mycotoxins

Abstract

F. graminearum is an important pathogen that causes head blight and root rot in wheat and ear and root rot in maize. Because of its toxigenic nature most of the diseased crops are found to accumulate large pool of mycotoxins. Trichothecenes type B deoxynivalenol (DON) its acetylated derivatives, nivalenol (NIV) and estrogenic zearalenone (ZEN) are major mycotoxins produced by the fungus. The DON occurrence is high in cereals while NIV and ZEN are potential contaminants in maize. The biological role of these mycotoxins is not fully elucidated yet. We investigated the mixed colony incubation and ecological role of ZEN in rhizosphere colonization of maize and wheat root infection. The G. roseum which detoxifies ZEN and its knock-out non-detoxifying mutants were generated and tested earlier in our research group. We found out that mixed incubation of colonies of G. roseum wild-type and Zes2 mutant started to grow the same time on pink-covered surface of F. graminearum on YES medium. The wild-type colony size however increased multiple-fold and appeared as hollowed spots or digested pits on pathogen surface and pigmentation changed at borders from light-pale to dark-orange over 3-5 days incubation. In contrast, Zes2 mutant colony diameter stayed compact as no large expansion was measured. In rhizosphere colonization trials 5 aggressive F. graminearum strains co-inoculated with G. roseum wild-type and Zes2 mutant on maize and wheat root. Significant differences were observed between G. roseum wild-type and Zes2 mutant co-i! noculated with F. graminearum. The co-inoculated seedlings with G. roseum wild-type and pathogen showed increased emergence, larger yields in root and shoot indexes and lesser disease symptoms while Zes2 mutant resulted in partial protection in seedlings of maize and wheat. We may conclude that ZEN is involved in substrate colonization and inhibition would leave its producer in hostile ecological environments to protect the resource from its competitors. Twenty-two F. graminearum strains were investigated in wheat-maize-pathosystem. The greater disease severity was observed on wheat heads; however increased root infection took place in maize. The strains virulence was positively correlated among wheat heads/maize ears and root infections of wheat and maize. The DON contents were also positively correlated with disease severity in both wheat and maize ears. However, no positive correlation was found between DON contents and fungal DNA in maize because of different chemotypes. In wheat, tight correlation was proved between DON and DNA contents. NIV was detected in lower amount than DON while all maize ears were quantified for ZEN accumulation. In our study some of the strains were also found to produce 15ADON on maize which is of great concern. In both wheat and maize ears the amount of mycotoxins was a decisive factor in pathogen virulence. We may conclude that wheat heads are susceptible than root while opposite is true in m! aize that both ears and roots are observed susceptible to Fusarium infection. We investigated the role of DON and NIV in wheat and maize root infection. We inoculated the seeds of both plants with two F. graminearum wild-type strains specific to DON and NIV chemotypes and two individual deficient mutants correspond to each chemotypes. Wheat inoculated with NIV wild-type showed poor emergence, reduced root and shoot indexes and increased disease severity compared to the deficient mutants. The DNA contents showed larger amount in NIV wild-type infected wheat root or shoot than traces of DNA detected in deficient mutants. NIV was detected in traceable amounts but with great variability among samples of wheat or below the detection limits. In maize, either NIV wild-type or deficient mutants both produced same magnitude of disease. DON wild-type inoculated wheat suffered extreme losses in germination, resulted in reduced biomass indexes and greater disease severity than deficient mutants. No significant differences observed between DON wild-type and defici! ent mutants on maize root. We conclude that NIV and DON are virulent factor on wheat not on maize root infection.