Resistance of tomato and pepper to T3 strains of Xanthomonas campestris pv. vesicatoria is specified by a plant-inducible

Tomato race 3 (T3) of Xanthomonas campestris pv, vesicatoria (Xcv) elicits a hypersensitive response (HR) in leaves of Lycopersicon esculentum near-is ogenic line (NIL) 216 and pepper genotypes, One cosmid clone (35 kb) select ed from a genomic library of a T3 strain induced an HR in all resistant pla nts. A 1.5-kb active subclone containing the putative avirulence (avr) gene , designated avrXv3, was sequenced. The avrXv3 gene encodes a 654-bp open r eading frame (ORF) with no homology to any known gene. Expression studies w ith a fusion of this gene and uidA indicated that avrXv3 is plant inducible and controlled by the hypersensitivity and pathogenicity (hrp) regulatory system. Mutational analysis and transcription activation assays revealed th at AvrXv3 has transcription activation activity in yeast, and that the puta tive domain responsible for that activity is located at the C terminus of t he AvrXv3 protein. Agrobacterium tumefaciens-mediated transient expression confirmed the direct role of AvrXv3 in eliciting the HR in tomato NIL 216 a nd supported the hypothesis that Avr proteins must be present inside the pl ant host cell to trigger the HR.