Characterization of Hypersensitive Resistance to Bacterial Spot Race T3 (Xanthomonas perforans) from Tomato Accession PI 128216.

Bacterial spot of tomato is caused by four species of Xanthomonas. The accession PI 128216 (Solanum pimpinellifolium) displays a hypersensitive reaction (HR) to race T3 strains (predominately Xantho- monas perforans). We developed an inbred backcross (IBC) population (BCzSs, 178 families)derived from PI 128216 and OH88119 (S. lycopersicum) as the susceptible recurrent parent for simultaneous intro- gression and genetic analysis of the HR response. These IBC families were evaluated in the greenhouse for HR to race T3 strain Xcv761. The 1BC population was genotyped with molecular markers distributed throughout the genome in order to identify candidate loci conferring resistance. We treated the IBC population as a hypothesis forming generation to guide validation in subsequent crosses. Nonparametric analysis identified an association between HR and markers clustered on chromosome 11 (P < 0.05 to 0.0001) and chromosome 6 (0.04 > P > 0.002)..Further analysis of the 1BC population suggested that markers on chromosome 6 and I1 failed to assort independently, a phenomenon known as gametic phase disequilibrium. Therefore, to validate marker- trait linkages, resistant IBC plants were crossed with OH88119 and BC3F2 progeny were evaluated for HR in the greenhouse. In these sub- sequent populations, the HR response was associated with the chromo- some 11 markers (P < 0.0002) but not with the markers on chromosome 6 (P > 0.25). Independent F2 families were developed by crossing resistant IBC lines to OH8245, OH88119, and OH7530. These populations were genotyped, organized into classes based on chromosome I I markers, and evaluated for resistance in the field. The PI 128216 locus on chromosome I I provided resistance that was dependent on gene dosage and genetic background. These results define a single locus, Rx-4, from P1 128216, which provides resistance to bacterial spot race T3, has additive gene action, and is located on chromosome 11. [ABSTRACT FROM AUTHOR]