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Manganese Oxidation States in Gaeumannomyces-Infested Wheat Rhizospheres Probed by Micro-XANES Spectroscopy. Darrell G. Schulze, Department of Agronomy, Purdue University, 1150 Lilly Hall, West Lafayette, IN 47907; Tina McCay-Buis(2), Stephen R. Sutton(3), and Don M. Huber(4). (2)(4)Department of Botany and Plant Pathology, Purdue University, 1155 Lilly Hall, West Lafayette, IN 47907; (3)Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago 60637. Phytopathology 85:990-994. Accepted for publication 26 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-990.

The take-all disease, caused by Gaeumannomyces graminis var. tritici, is one of the world’s most damaging root diseases of wheat. It has been hypothesized that the fungus reduces the host’s defense mechanism prior to invasion by catalyzing the oxidation of soluble Mn²⁺ to insoluble Mn⁴⁺ on the rhizoplane and in the soil surrounding the root. For the first time, a direct test of this hypothesis has been accomplished using micro-X-ray absorption near edge structure (XANES) spectroscopy to obtain information about the spatial distribution of Mn oxidation states in and around live wheat roots growing in agar infected with G. graminis var. tritici. Mn in clear agar occurred only as Mn²⁺, whereas Mn around dark roots infected with G. graminis var. tritici was predominately present as Mn⁴⁺. The distribution of Mn oxidation states clearly showed the presence of Mn⁴⁺-containing precipitates in the interior of a root infected with G. graminis var. tritici. This was consistent with a map of Mn concentration that showed a relative accumulation of total Mn in the interior of the root as a result of G. graminis var. tritici-catalyzed biomineralization. Given the penetrating nature of X rays, the micro-XANES technique should be applicable to roots growing in soil, thus providing a technique to measure Mn oxidation states during pathogenesis under conditions that closely simulate the natural soil environment.