PLASMID-BORNE DETERMINANTS OF PIGMENTATION, EXOPOLYSACCHARIDE PRODUCTION, AND VIRULENCE IN XANTHOMONAS-CAMPESTRIS PV MALVACEARUM

Three plasmids of 55.0, 31.2, and 7.4 kb, designated as pXCM18-I, pXCM18-II, and pXCM18-III, respectively, were identified and isolated from strain HR13B/2 of Xanthomonas campestris is pv. malvacearum. The bacterium was cured of all three plasmids in high frequency (3.1%) with heat (42 degrees C). The cured strains were nonmucoid and light yellow and were greatly impaired in exopolysaccharide production and virulence. Transformation of a cured strain using a mixture of purified plasmid DNA from HR13B/2 resulted in colonies carrying all six possible combinations of the three plasmids. Of the transformants, two morphologically distinct colony types were observed. The first type of transformants were mucoid and deep yellow, not distinguished from wild-type colonies in morphology, and fully complemented for exopolysaccharide production. These transformants invariably carried pXCM18-II. The second type of transformants were not distinguished from the untransformed strain in morphology but were found to be transformed after screening for plasmid content. These transformants carried either pXCM18-I, pXCM18-III, or both, but never pXCM18-II. In pathogenicity tests, each of the three plasmids restored virulence significantly, and additively, with plasmid pXCM18-I playing a predominant role.