Minisymposium 25: Pathogen virulence mechanisms

Abs # 46003: Sclerotinia sclerotiorum produces oxalic acid to deregulate guard cells during infection

Presenter:	Guimaraes, Rejane L., rejaneg@hotmail.com
Authors	Guimaraes, Rejane L. (A)   Stotz, Henrik U. (A)
Affiliations:	(A): Department of Horticulture, Oregon State University
Web Site:	http://www.science.oregonstate.edu/~stotzhe/index.html

Sclerotinia sclerotiorum is a devastating pathogen that infects more than 400 plant species, including bean, soybean, canola, and sunflower, and causes major economic losses worldwide. Disease symptoms vary with the host, but often include water-soaking lesions and wilting of leaves and stems. In this work, we provide compelling evidence for effects of oxalate on guard cell physiology in response to a fungal pathogen. We visualized open stomata in dark-adapted leaves of Vicia faba infected with a GFP-tagged S. sclerotiorum strain. Guard cells respond to fungal infection in advance of mycelial invasion. GFP-tagged S. sclerotiorum penetrated open stomatal pores. In contrast to wild type S. sclerotiorum, an oxalate-deficient mutant caused significantly narrower stomatal aperture in dark-adapted V. faba leaves. Measurements in whole plants demonstrate that wild type S. sclerotiorum causes a significant increase in stomatal conductance (gs: 160 mmolm-2s-1) and transpiration rate (3.3 mmolm-2s-1) as well as a considerable decrease in plant biomass compared to the oxalate-deficient mutant (gs: 96 mmolm-2s-1, E: 2.1 mmolm-2s-1). These data suggest that S. sclerotiorum produces oxalate to alter stomatal behavior, facilitate infection, and increase plant stress. Using detached epidermis from V. faba leaves, we show that oxalate induces stomatal opening at different pH. At physiological concentrations of 10 mM, oxalate treatment triggers guard cell potassium ion accumulation and starch breakdown, suggesting osmotic alterations of guard cells. Also, 10 mM oxalate interferes with ABA-induced stomatal closure when detached epidermis is studied. Because mutants deficient in stomatal closure and ABA signaling would be expected to have increased susceptibility to fungal infection, we challenged Arabidopsis thaliana mutants with S. sclerotiorum. We used the oxalate-deficient fungal mutant to avoid a confounding influence of oxalate on disease development. Statistical analysis using the repeated measures procedure of a general linear model suggests that abi1, abi3, abi4 and aba2 mutants are significantly more susceptible to fungal attack than their corresponding wild type plants. This indicates that defects in stomatal closure are important in the S. sclerotiorum-host interaction. We conclude that oxalate may alter guard cell movement by 1) increasing osmotic pressure via accumulation of potassium ions and starch breakdown and by 2) inhibiting ABA-induced stomatal closure.