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Sclerotinia sclerotiorum oxalate-minus mutants accumulate fumaric acid in a pH-responsive manner and remain pathogenic on most host plants

Weidong Chen: USDA ARS

<div>The necrotrophic pathogen <em>Sclerotinia sclerotiorum</em> has a wide host range of >400 plant species. It produces and accumulates high levels of oxalic acid in culture and in diseased tissue, which is considered to be required for <em>S. sclerotiorum</em> to cause disease. Using genetically defined mutants defective at the oxaloacetate acetylhydrolase (<em>oah</em>) gene, we recently showed that the <em>oah</em> mutants lost oxalate production, but instead accumulated fumaric acid in culture, and remained pathogenic on some legume plants. This study was designed to determine the kinetics of fumaric acid accumulation in the <em>oah</em> mutants and test the pathogenicity of the <em>oah</em> mutants on additional hosts of several plant families (Asteraceae, Brassicaceae, Cucurbitaceae and Solanaceae). Production of fumaric acid and other acids under pH 3, 5, and 7 was monitored using HPLC. The <em>oah</em> mutants produced increasingly higher levels of fumaric acid as ambient pH increased, showing the same pH response of oxalic acid production found in the wild type. Interestingly, both wild type and the <em>oah</em> mutants produced more kinds of organic acids as the ambient pH increased. Pathogenicity assays showed that the <em>oah</em> mutants were not only able to initiate infection, but also caused disease levels similar to that of the wild type on most host plants. The results showed that the <em>oah</em> mutants remained pH-responsive in acid production and can cause significant disease in the absence of oxalic acid at least on some plants.</div>