Genetic characterization of plant-pathogen interactions between Xanthomonas campestris pv. vesicatoria and tomato (Lycopersicon esculentum L.)

Bacterial spot caused by Xanthomonas campestris pv. vesicatoria (Xcv) is one of the most important diseases of tomato ( Lycopersicon esculentum). A new source of resistance to tomato race 3 of this pathogen was found in the wild species L. pennellii. Genetic segregation of the resistance was determined with an F₂ progeny of 245 plants derived from a cross between the tomato line L. esculentum Hawaii 7998, susceptible to this race, and the resistant parent L. pennellii LA716. Segregation ratios near 3:1 confirmed that a single dominant gene controlled the inheritance of this trait. Fifty lines representing the introgression of the entire genome of L. pennellii into L. esculentum were screened. The resistance gene was located on chromosome 3. Linkage analysis of the resistance with RFLP and CAPS markers flanking that region of the genome indicates that the position of Xv4 maps to an approximately 21.9 cM interval defined on the centromeric side by TG599 at 9.3 cM and, on the telomeric side by TG134 at 11.1 cM. The role and characterization of the corresponding avirulence gene, avrXv4, are also discussed.; A gene-for-gene system controlling the resistance to this pathogen in tomato was previously described. In order to elucidate the possible role of avrXv3 in eliciting the hypersensitive reaction (HR), a collection of mutated avrXv3 constructs were screened for their ability to elicit HR using the Xcv background and Agrobacterium-mediated transient expression. The mutants and wild type gene were also assessed for their ability to activate transcription in yeast. Mutants were generated by PCR-mediated deletion mutagenesis of putative domains inferred from the hydrophobicity analysis of the predicted protein, and the modification of the termini of the protein by the addition of 6 histidine residues. The results suggest that AvrXv3 protein has transcription activation activity in yeast. It is not conclusive whether or not this activity is associated with the ability to elicit the HR in tomato. Besides, modifications of the termini of this protein seem to block the secretion of this Avr protein into the host cell.