Virulence of Selected Phytopathogenic Pseudomonads in Arabidopsis thaliana. Keith R. Davis. The Ohio State Biotechnology Center and Department of Plant Biology, The Ohio State University, Columbus 43210. Eric Schott(2), and Frederick M. Ausubel(2). (1)The Ohio State Biotechnology Center and Department of Plant Biology, The Ohio State University, Columbus 43210; and (2)Department of Genetics, Harvard Medical School, Boston, MA 02115 and Department of Molecular Biology, Massachusetts General Hospital, Boston 02114.. MPMI 4:477-488. Accepted 1 May 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

We are developing a model pathogenesis system that utilizes Arabidopsis thaliana as a host for infection by a variety of phyto-pathogenic Xanthomonas and Pseudomonas strains. We divided 51 different strains into four categories based on the symptoms they elicited when infiltrated into A. thaliana leaves at two doses, 10⁶ and 10⁷ cfu/ml. Highly virulent and weakly virulent strains elicited spreading water-soaked lesions within 48 hr at doses of 10⁶and 10⁷ cfu/ml, respectively. Avirulent strains elicited either a hypersensitive response (dry necrotic lesion) within 12-24 hr at doses of 10⁶ and 10⁷ cfu/ml or pitting with mild chlorosis within 48 hr at a dose of 10⁷ cfu/ml. Null strains elicited no visible symptoms at a dose of 10⁷ cfu/ml. Several Pseudomonas strains were chosen for additional characterization. The highly virulent strains, P. syringae pv. tomato DC3000 and P. s. pv. maculicola 795, elicited spreading water-soaked lesions with chlorotic margins, multiplied 10³-to 10⁴-fold in A. thaliana leaves, induced a five- to 10-fold transient accumulation of mRNA corresponding to phenylalanine ammonia-lyase (PAL), and had little effect on the pH of the medium when added to Arabidopsis tissue culture cells. The avirulent strain, P. cichorii 83-1, elicited a localized, dry lesion typical of a hypersensitive response within 12-24 hr, failed to multiply in A. thaliana leaves, induced a 15- to 30-fold accumulation of PAL mRNA, and caused an alkalinization of the medium when added to Arabidopsis tissue cultures. P. aureofaciens strain 923 elicited a null response, did not multiply in A. thaliana leaves, induced a five- to sixfold accumulation of PAL mRNA, and caused an acidification of the medium when added to Arabidopsis tissue culture cells. To facilitate the monitoring of the induction of A. thaliana PAL mRNA in response to infiltration with phytopathogenic bacteria, we cloned and partially sequenced an A. thaliana genomic DNA sequence encoding PAL. Southern blot analysis showed that the A. thaliana genome contains two or three additional sequences that are at least partially homologous to the cloned PAL gene.