MOLECULAR RECOGNITION OF PATHOGEN ATTACK OCCURS INSIDE OF PLANT-CELLSIN PLANT-DISEASE RESISTANCE SPECIFIED BY THE ARABIDOPSIS GENES RPS2 AND RPM1

The Arabidopsis thaliana disease resistance genes RPS2 and RPM1 belong to a class of plant disease resistance genes that encode proteins tha t contain an N-terminal tripartite nucleotide binding site (NBS) and a C-terminal tandem array of leucine-rich repeats, RPS2 and RPM1 confer resistance to strains of the bacterial phytopathogen Pseudomonas syri ngae carrying the avirulence genes avl Rpt2 and avrB, respectively, in these gene-for-gene relationships, it has been proposed that pathogen avirulence genes generate specific ligands that are recognized by cog nate receptors encoded by the corresponding plant resistance genes, To test this hypothesis, it is crucial to know the site of the potential molecular recognition. Mutational analysis of RPS2 protein and in vit ro translation/translocation studies indicated that RPS2 protein is lo calized in the plant cytoplasm. To determine whether avirulence gene p roducts themselves are the ligands for resistance proteins, we express ed the avrRpt2 and avrB genes directly in plant cells using a novel qu antitative transient expression assay, and found that expression of av rRpt2 and avrB elicited a resistance response in plants carrying the c orresponding resistance genes, This observation indicates that no bact erial factors other than the avirulence gene products are required for the specific resistance response as long as the avirulence gene produ cts are correctly localized. We propose that molecular recognition of P. syringae in RPS2- and RPM1-specified resistance occurs inside of pl ant cells.