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ARS Home » Pacific West Area » Pullman, Washington » Grain Legume Genetics Physiology Research » Research » Publications at this Location » Publication #314769

Research Project: Genetic Improvement of Cool Season Food Legumes

Location: Grain Legume Genetics Physiology Research

Title: pH Dependency of sclerotial development and pathogenicity revealed by using genetically defined oxalate-minus mutants of Sclerotinia sclerotiorum

Author
item Xu, Laingsheng - Washington State University
item Xiang, Meichun - Washington State University
item White, David - Washington State University
item Chen, Weidong

Submitted to: Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2015
Publication Date: 8/20/2015
Citation: Xu, L., Xiang, M., White, D., Chen, W. 2015. pH Dependency of sclerotial development and pathogenicity revealed by using genetically defined oxalate-minus mutants of Sclerotinia sclerotiorum. Environmental Microbiology. 17:2896-2909.

Interpretive Summary: The fungal pathogen Sclerotinia sclerotiorum causes white mold, stem rot and other various diseases on more than 400 plant species including all grain legume crops. It produces copious amount of oxalic acid, which, for over a quarter century, has been claimed as the pathogenicity determinant based on UV-induced mutants that concomitantly lost oxalate production and pathogenicity, but also harbor a developmetal defect in sclerotial production. However, in this study, we generated oxalate-minus mutants of S. sclerotiorum using two independent mutagenesis techniques –T-DNA insertion and targeted gene replacement. We tested the resulting mutants for growth at different pHs and for pathogenicity on four legume host plants. The genetically defined oxalate-minus mutants completely lost oxalic acid production, but accumulated fumaric acid, produced functional sclerotia and have reduced ability to acidify the environment. The oxalate-minus mutants are also pathogenic on plants, but their virulence varied depending on the pH and buffering capacity of host tissue. These results suggest that it is low pH, not oxalic acid itself, that establishes the optimum conditions for growth, reproduction, pathogenicity and virulence expression of S. sclerotiorum. This research fundamentally revises the oxalate pathogenesis theory of S. sclerotiorum that was based on genetically undefined UV-mutants of 25 years. The distinction between low pH and oxalate per se is very important. It will stimulate research into identifying additional candidates as pathogenicity factors toward better understanding and managing Sclerotinia diseases.

Technical Abstract: The devastating plant pathogen Sclerotinia sclerotiorum produces copious (up to 50mM) amounts of oxalic acid, which, for over a quarter century, has been claimed as the pathogenicity determinant based on UV-induced mutants that concomitantly lost oxalate production and pathogenicity. Such a claim was made without fulfilling the molecular Koch’s postulates because the UV-mutants are genetically undefined and harbor a developmental defect in sclerotial production. Here we generated oxalate-minus mutants of S. sclerotiorum using two independent mutagenesis techniques, and tested the resulting mutants for growth at different pHs and for pathogenicity on four host plants. The oxalate-minus mutants accumulated fumaric acid, produced functional sclerotia and have reduced ability to acidify the environment. The oxalate-minus mutants retained pathogenicity on plants, but their virulence varied depending on the pH and buffering capacity of host tissue. Acidifying the host tissue enhanced virulence of the oxalate-minus mutants, whereas supplementing with oxalate did not. These results suggest that it is low pH, not oxalic acid itself, that establishes the optimum conditions for growth, reproduction, pathogenicity and virulence expression of S. sclerotiorum. Exonerating oxalic acid as the primary pathogenicity determinant will stimulate research into identifying additional candidates as pathogenicity factors toward better understanding and managing Sclerotinia diseases.