CLONING AND CHARACTERIZATION OF A MEMBER OF THE XANTHOMONAS AVR PTH GENE FAMILY THAT EVADES ALL COMMERCIALLY UTILIZED COTTON R-GENES IN THEUNITED-STATES/

The highly virulent African strains of Xanthomonas campestris pv. malv acearum are quarantined pathogens in the United States and can evade o r overcome all commercially utilized resistance (R) genes in cotton gr own in the United States including the entire set of host differential lines used to distinguish 19 races of the pathogen. Nevertheless, the African strains carry multiple DNA fragments that strongly hybridize with members of the Xanthomonas avirulence (avr)/pathogenicity (pth) g ene family. Since all previously tested members of the gene family con fer avirulence against one or more R genes in cotton, strains carrying multiple members might not be expected to evade so many different R g enes. The hybridizing DNA fragments were cloned from African strain Xc mN and found to confer water-soaking ability to a nearly asymptomatic mutant strain of the pathogen. Restriction mapping, Southern hybridiza tion, and DNA sequencing of the cloned fragments from XcmN were used t o identify two water-soaking genes, pthN and pthN2, as new members of the Xanthomonas avr/pth gene family. The complete DNA sequence of pthN was obtained, and it is >94% identical with all other sequenced membe rs of the gene family. Gene fusions of pthN with avrb6 (another family member) and other experiments revealed that the ability of African st rain XcmN to water-soak cotton and avoid recognition by commercially u sed cotton R genes is determined by the specific repeats of multiple f unctional members of the Xanthomonas avr/pth gene family.