Ramifications of microbial interactions conditioning susceptibility to take-all of wheat
Abstract
Manganese chemistry plays a crucial role in the etiology of take-all root, crown, and stem rot of wheat caused by Gaeumannomyces graminis var. tritici (Ggt). The purpose of this field and growth chamber research was to establish the impact of rhizosphere organisms and cultural practices on Mn availability, and verify the role of Mn in take-all. This research confirmed the positive correlation of Mn oxidation as a virulence factor of Ggt, and identified temperature as a significant interactive factor with both Mn oxidation and virulence. Among the rhizophere microbes interacting with take-all, Bacillus cereus was found to inhibit Mn oxidation by Ggt, reduce take-all, and increase wheat yield. In contrast, Agrobacterium radiobacter, a strong Mn oxidizer, reduced Mn uptake by wheat and increased take-all. These important findings explain differences reported by researchers from different wheat production areas. However, oxidation-reduction reactions with other organisms was not consistently correlated with an effect on take-all. Light and electron microscopy evaluation, combined with energy dispersive X-ray microanalysis, confirmed the presence of Mn oxides on and near fungal mycelium and infection structures. The extra-cellular Mn-oxidative factor produced by Ggt oxidized Mn in advance of the mycelium. Barrier type plant defense reactions were not observed following oxidation of Mn in the infection court. In contrast, membrane-bound Mn oxidation limited to the lobed hyphopodia and adjacent infection structures of the closely related G. graminis var. graminis (Ggg), facilitated epiphytic growth, but limited penetration of Ggg to epidermal cortical tissues. This may explain Ggg's failure to induce extensive necrosis or vascular occlusion characteristic of take-all. Micro-X-ray Absorption Near Edge Structure spectroscopy (XANES), a new high energy X-ray fluorescence technique, was used to establish that direct oxidation of Mn occurred during Ggt pathogenesis on wheat roots. Manganese accumulated inside the root as a result of Ggt-catalyzed mineralization. These findings provide new and exciting direct evidence that Ggt oxidizes Mn in the rhizosphere of wheat seedlings, and provides the ground work for future in-situ research using this powerful technique to study microbial interactions in the rhizosphere involved in micronutrient availability, disease suppression, and biological control.
Degree
Ph.D.
Advisors
Huber, Purdue University.
Subject Area
Plant pathology|Microbiology|Ecology
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