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Metabolite Profiling of Arabidopsis Inoculated with Alternaria brassicicola Reveals That Ascorbate Reduces Disease Severity

¹Department of Plant Biology and Microbial and Plant Genomics Institute, University of Minnesota, Rm 250 BioSciences Center, 1445 Gortner Ave., St. Paul 55108, U.S.A.; ²Department of Biological Sciences, Chicago State University, Rm 310 Williams Science Center, 9501 S. King Drive, Chicago 60628; ³Department of Biochemistry, University of California, Riverside 92521-0129, U.S.A.; and ⁴University of California at Davis Genome Center, GBSF Building Room 1315, 451 East Health Sciences Drive, Davis 95616-8816, U.S.A.

The interaction between the pathogenic ascomycete Alternaria brassicicola and Arabidopsis was investigated by metabolite profiling. The effect of A. brassicicola challenge on metabolite levels was substantial, with nearly 50% of detected compounds undergoing significant changes. Mutations blocking ethylene, jasmonic acid, or ethylene signaling had little effect on metabolite levels. The effects of altering levels of some metabolites were tested by exogenous application during A. brassicicola inoculation. Gamma amino-butyric acid (GABA) or xylitol promoted, while trehalose and ascorbate inhibited, disease severity. GABA promoted, and ascorbate strongly inhibited, fungal growth in culture. Arabidopsis vtc1 and vtc2 mutants, that have low levels of ascorbate, were more susceptible to A. brassicicola. Ascorbate levels declined following A. brassicicola inoculation while levels of dehydroascorbate increased, resulting in a shift of the redox balance between these compounds in the direction of oxidation. These results demonstrate that ascorbate is an important component of resistance to this pathogen.