Thèse d'exercice : Médecine. Médecine générale : Université de Nantes : 2004. / Bibliogr. f. 101-109 [97 réf.].

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

abstract / Biological Sciences / Doctoral / Doctor of Philosophy

No description available.

Fusarium head blight (FHB) is an important disease of wheat and other small grain cereals. The principal pathogen associated with FHB in Manitoba is Gibberella zeae (Schwein.) Petch (anamorph = Fusarium graminearum Schwabe). Currently there no biological control agents registered for the control of FHB. The overall objects of the project were to identify isolates that may be used in the biological control of FHB and to investigate the interaction between the identified biocontrol agents and G. zeae.In this study, 150 bacteria and 29 fungi were isolated from soil, wheat heads and crop debris from southern Manitoba. An additional 10 isolates of Trichoderma harzianum were obtained from the Canadian Collection of Fungal Cultures, CCFC (Ottawa, Ontario). The T. harzianum isolate, T-22 (RootShield) was included as a positive control. All were screened for inhibition of Gibberella zeae using confrontation plate assays in vitro and seed, wheat head, and straw assays in planta. Only 6% of bacterial and 45% of fungal isolates tested in the confrontation plate assays. Of the 6% of the bacterial isolates none significantly reduced FHB disease on wheat heads or reduced perithecial production on wheat straw, and had a negative effect on seed germination, therefore were not further evaluated. Chrysosporium sp. and Penicillium spp. and Trichoderma harzianum, were the fungal species that inhibited the growth of G. zeae by more than 50%. Of which Trichoderma isolates were the most effective and were able to over-grow G. zeae. Fourteen of the 18 isolates tested, including six Trichoderma and two Chrysosporium isolates, significantly reduced perithecial production on wheat straw by 52-89% compared to the control. From this study, Trichoderma harzianum was identified as most potentially effective candidate for the biocontrol of Gibberella zeae. Spore suspensions and cell-free filtrates of Trichoderma harzianum isolates were evaluated for their effectiveness in reducing perithecial and ascospore production of Gibberella zeae on wheat straw. Five T. harzianum isolates, including T-22 (RootShieldTM), reduced perithecial formation by 70% or more. Perithecial reduction was highest (96-99%) when T. harzianum spore suspension or cell-free filtrate was applied to straw 24 hours prior to inoculation with G. zeae. Control was less effective when T. harzianum was applied at the same time (co-inoculated) or 24 hours after G. zeae. Field trials showed significant reduction of perithecia on residues treated with T. harzianum prior to placement on the soil surface. Identification of those compounds in the cell-free filtrate most likely to affect biocontrol was accomplished through the use of cluster analysis, ordination and regression methods. It was found that isolates that produce similar levels of biocontrol had similar chemical composition. Ultrastructural changes were observed primarily in the exterior cells of the outer cell wall. Cytoplasmic degradation, invagination of the plasma cell membrane and thin cell walls were observed in the treated samples. Immature perithecia were overgrown by T. harzianum 15 days after co-inoculation. Few perithecia were overgrown at later stages. The perithecia affected by T. harzianum collapsed 21 days after inoculation (dai), compared to the perithecia in the untreated samples which collapsed 28 dai.

Fusarium head blight (FHB) is a serious disease of wheat, primarily caused by the pathogen Fusarium graminearum. FHB results in yield losses and decreased grain quality due to the ability of the pathogen to produce the mycotoxin deoxynivalenol (DON) as well as acetylated derivatives of DON such as 3-acetyl DON (3-ADON) and 15-acetyl DON (15-ADON). Research shows that the 15-ADON chemotype is being replaced by the 3-ADON chemotype in eastern and central Canada. The first study investigated the potential for differences between the two chemotypes in terms of disease progression, effect on yield, Fusarium damaged kernels (FDK) and DON levels. Results showed that 3-ADON isolates were able to produce significantly more DON and FDK, and had significantly greater negative effects on yield than 15-ADON isolates, although there were no differences in symptom disease progression. The second study investigated if there were differences in resistance for the two chemotypes on 3BS and 4B quantitative trait loci (QTL) lines for disease severity and FDK levels. No differences were detected between chemotypes for disease progression but there were for FDK levels. One 3BS line was identified as partially resistant with significantly lower disease severity and FDK levels than the other QTL and null lines.

Fusarium head blight (FHB) is an important disease of wheat and other small grain cereals. The principal pathogen associated with FHB in Manitoba is Gibberella zeae (Schwein.) Petch (anamorph = Fusarium graminearum Schwabe). Currently there no biological control agents registered for the control of FHB. The overall objects of the project were to identify isolates that may be used in the biological control of FHB and to investigate the interaction between the identified biocontrol agents and G. zeae.In this study, 150 bacteria and 29 fungi were isolated from soil, wheat heads and crop debris from southern Manitoba. An additional 10 isolates of Trichoderma harzianum were obtained from the Canadian Collection of Fungal Cultures, CCFC (Ottawa, Ontario). The T. harzianum isolate, T-22 (RootShield) was included as a positive control. All were screened for inhibition of Gibberella zeae using confrontation plate assays in vitro and seed, wheat head, and straw assays in planta. Only 6% of bacterial and 45% of fungal isolates tested in the confrontation plate assays. Of the 6% of the bacterial isolates none significantly reduced FHB disease on wheat heads or reduced perithecial production on wheat straw, and had a negative effect on seed germination, therefore were not further evaluated. Chrysosporium sp. and Penicillium spp. and Trichoderma harzianum, were the fungal species that inhibited the growth of G. zeae by more than 50%. Of which Trichoderma isolates were the most effective and were able to over-grow G. zeae. Fourteen of the 18 isolates tested, including six Trichoderma and two Chrysosporium isolates, significantly reduced perithecial production on wheat straw by 52-89% compared to the control. From this study, Trichoderma harzianum was identified as most potentially effective candidate for the biocontrol of Gibberella zeae. Spore suspensions and cell-free filtrates of Trichoderma harzianum isolates were evaluated for their effectiveness in reducing perithecial and ascospore production of Gibberella zeae on wheat straw. Five T. harzianum isolates, including T-22 (RootShieldTM), reduced perithecial formation by 70% or more. Perithecial reduction was highest (96-99%) when T. harzianum spore suspension or cell-free filtrate was applied to straw 24 hours prior to inoculation with G. zeae. Control was less effective when T. harzianum was applied at the same time (co-inoculated) or 24 hours after G. zeae. Field trials showed significant reduction of perithecia on residues treated with T. harzianum prior to placement on the soil surface. Identification of those compounds in the cell-free filtrate most likely to affect biocontrol was accomplished through the use of cluster analysis, ordination and regression methods. It was found that isolates that produce similar levels of biocontrol had similar chemical composition. Ultrastructural changes were observed primarily in the exterior cells of the outer cell wall. Cytoplasmic degradation, invagination of the plasma cell membrane and thin cell walls were observed in the treated samples. Immature perithecia were overgrown by T. harzianum 15 days after co-inoculation. Few perithecia were overgrown at later stages. The perithecia affected by T. harzianum collapsed 21 days after inoculation (dai), compared to the perithecia in the untreated samples which collapsed 28 dai.

Fusarium head blight (FHB) is a serious disease of wheat, primarily caused by the pathogen Fusarium graminearum. FHB results in yield losses and decreased grain quality due to the ability of the pathogen to produce the mycotoxin deoxynivalenol (DON) as well as acetylated derivatives of DON such as 3-acetyl DON (3-ADON) and 15-acetyl DON (15-ADON). Research shows that the 15-ADON chemotype is being replaced by the 3-ADON chemotype in eastern and central Canada. The first study investigated the potential for differences between the two chemotypes in terms of disease progression, effect on yield, Fusarium damaged kernels (FDK) and DON levels. Results showed that 3-ADON isolates were able to produce significantly more DON and FDK, and had significantly greater negative effects on yield than 15-ADON isolates, although there were no differences in symptom disease progression. The second study investigated if there were differences in resistance for the two chemotypes on 3BS and 4B quantitative trait loci (QTL) lines for disease severity and FDK levels. No differences were detected between chemotypes for disease progression but there were for FDK levels. One 3BS line was identified as partially resistant with significantly lower disease severity and FDK levels than the other QTL and null lines.

No description available.

The ascomycete Fusarium verticillioides (Sacc.) Nirenberg (teleomorph: Gibberella moniliformis Wineland) causes stalk and ear rots on maize worldwide. In addition to the economic losses due to reduced yield, the fungus produces fumonisins on infected corn. One of the unanswered questions in mycotoxin research is how fungi perceive and respond to various extracellular stimuli and produce mycotoxins. To date, extensive research has been performed on important signaling pathways that regulate mycotoxin biosynthesis, but little is known about the downstream target genes, notably transcription factors (TFs). While the roles of TFs have shown to be critical in eukaryotic transcription regulation, only a few have been characterized in F. verticillioides. TFs with zinc fingers have been reported in all living organisms, and in fungal species, members of the Cys2-Hys2 (C2H2) zinc finger TF family are predicted to be involved in cell differentiation, carbon utilization, and development. Using the available genomic resources, I constructed a library of C2H2 TF deletion mutants, and identified SDA1, FvFLBC and CHT1 genes with a potential role in carbon utilization, development and fumonisin B1 (FB1) biosynthesis. The Δsda1 strain showed complete growth inhibition when using sorbitol as the sole carbon source and produced higher levels of FB1 when grown on corn kernels. In addition, the Δsda1 strain produced less number of conidia compared to the wild-type progenitor. Through gene complementation, I also demonstrated that F. verticillioides SDA1 and Trichoderma reesei ACE1 are functionally conserved. FvFLBC acts as a regulator of asexual development but not FB1 biosynthesis. I also discovered that the FvFlbC N-terminus is critical for conidia production. CHT1 is associated with asexual development, fumonisin biosynthesis and pigmentation. Characterization of key signal transduction pathways, and more importantly the function of SDA1, FvFLBC and CHT1, should facilitate the elucidation of the mechanisms and regulations of growth, development, and secondary metabolism in F. verticillioides. The outcome of this study may help us determine how to minimize F. verticillioides contamination of crops and the resulting mycotoxins, providing safer and higher value corn in the US and worldwide.