HOST-SPECIFIC SYMPTOMS AND INCREASED RELEASE OF XANTHOMONAS-CITRI AND XANTHOMONAS-CAMPESTRIS PV MALVACEARUM FROM LEAVES ARE DETERMINED BY THE 102-BP TANDEM REPEATS OF PTHA AND AVRB6, RESPECTIVELY

Six avirulence genes (avrB4, avrb6, avrb7, avrBIn, avrB102, and avrB102) found in Xanthomonas campestris pv. malvacearum strain XcmH1005 and a host-specific pathogenicity gene (pthA) found in X. citri belong to an avr/pth gene family and are characterized by tandemly arranged, 102-bp repeats in the central portions of the genes. Marker exchange mutagenesis and complementation experiments revealed that avrb6 was required for XcmH1005 to cause severe water-soaking and subsequent necrosis in susceptible Acala-44 cotton lines, An average of 240 times more bacteria were released onto the cotton leaf surface from water-soaked spots caused by XcmH1005 than from those caused by an isogenic avrb6(-) strain, strongly indicating a role for avrb6 in pathogen dispersal. However, avrb6 did not affect in planta bacterial growth rate or yield. By constructing chimeric genes among pthA, avrB4, avrb6, avrb7, avrBIn, avrB101, and avrB102, the 102-bp tandem repeats of the genes were found to determine the gene-for-genes specificity of the avirulence reactions on cotton. Tn addition, the repeat regions of avrb6 and pthA determined their specificity in enhancing water-soaking of cotton and causing cankers on citrus, respectively. When the native promoters of each gene were replaced by the Escherichia coli lacZ promoter, the hypersensitive response elicited in resistant host lines was stronger in all cases tested, while the pathogenic specificities of avrb6 for cotton and pthA for citrus were unaltered. These results indicate that some members of this avr/pth gene family may help condition host range by increasing the release of Xanthomonas cells from the mesophyll to the leaf surface, leading to increased dispersal on specific hosts.