VIRULENCE OF SELECTED PHYTOPATHOGENIC PSEUDOMONADS IN ARABIDOPSIS-THALIANA

We are developing a model pathogenesis system that utilizes Arabidopsis thaliana as a host for infection by a variety of phytopathogenic 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, 10(6) and 10(7) cfu/ml. Highly virulent and weakly virulent strains elicited spreading water-soaked lesions within 48 hr at doses of 10(6) and 10(7) cfu/ml, respectively. Avirulent strains elicited either a hypersensitive response (dry necrotic lesion) within 12-24 hr at doses of 10(6) and 10(7) cfu/ml or pitting with mild chlorosis within 48 hr at a dose of 10(7) cfu/ml. Null strains elicited no visible symptoms at a dose of 10(7) 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 10(3)- to 10(4)-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.