Virulence, and Bacterial Multiplication and Movement in Rice Leaves of Xanthomonas campestris pv. oryzae Mutants Impaired in Productivity of Extracellular Polysaccharide.

A series of mutants impaired in productivity of extracellular polysaccharide (EPS) were generated by transposon Tn4431 mutagenesis in Xanthomonas campestris pv. oryzae. EPS production in the mutants on potato semisynthetic agar medium ranged from 2 to 41 of that in their parent strain T7174R. Disease index number of the mutants on rice plants (Oryza sativa cv. IR24) ranged from 0.4 to 3.2 28 days after inoculation, which were significantly lower than that of T7174R (5.4). Furthermore, chemical composition of EPS from the mutants was altered by the mutation. However, any mutants with reduced productivity of EPS did not completely lose their virulence. Although the mutants multi plied almost to the same level of T7174R in area close to the inoculation point, distribution of the mutants tended to be restricted in the area. Our results suggest that EPS productivity is not an essential factor for multiplication of X. c. pv. oryzae in rice plants but is related with movement of bacteria in infected xylem vessels. The reduced movement of the mutants may reflect the reduced virulence of the mutants.