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First published online October 22, 2004; 10.1104/pp.104.049650 Plant Physiology 136:3703-3711 (2004) © 2004 American Society of Plant Biologists Oxalate Production by Sclerotinia sclerotiorum Deregulates Guard Cells during Infection1,[w]Department of Horticulture, Oregon State University, Corvallis, Oregon 97331
Oxalic acid is a virulence factor of several phytopathogenic fungi, including Sclerotinia sclerotiorum (Lib.) de Bary, but the detailed mechanisms by which oxalic acid affects host cells and tissues are not understood. We tested the hypothesis that oxalate induces foliar wilting during fungal infection by manipulating guard cells. Unlike uninfected leaves, stomatal pores of Vicia faba leaves infected with S. sclerotiorum are open at night. This cellular response appears to be dependent on oxalic acid because stomatal pores are partially closed when leaves are infected with an oxalate-deficient mutant of S. sclerotiorum. In contrast to oxalate-deficient S. sclerotiorum, wild-type fungus causes an increase in stomatal conductance and transpiration as well as a decrease in plant biomass. Green fluorescent protein-tagged S. sclerotiorum emerges through open stomata from the uninfected abaxial leaf surface for secondary colonization. Exogenous application of oxalic acid to the detached abaxial epidermis of V. faba leaves induces stomatal opening. Guard cells treated with oxalic acid accumulate potassium and break down starch, both of which are known to contribute to stomatal opening. Oxalate interferes with abscisic acid (ABA)-induced stomatal closure. The Arabidopsis (Arabidopsis thaliana) L. Heynh. mutants abi1, abi3, abi4, and aba2 are more susceptible to oxalate-deficient S. sclerotiorum than wild-type plants, suggesting that Sclerotinia resistance is dependent on ABA. We conclude that oxalate acts via (1) accumulation of osmotically active molecules to induce stomatal opening and (2) inhibition of ABA-induced stomatal closure.
1 This work was supported by the U.S. Department of Agriculture Sclerotinia Initiative Special Grants program (grant no. 5854422257) and by the Department of Horticulture at Oregon State University. [w] The online version of this article contains Web-only data. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.049650. * Corresponding author; e-mail stotzhe{at}science.oregonstate.edu; fax 5417373479. Received July 13, 2004; returned for revision August 23, 2004; accepted August 24, 2004. This article has been cited by other articles:
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