Role of extracellular polysaccharides of Xanthomonas campestris pv. citri in the early stage of infection.

Strains of Xanthomonas campestris pv. citri did not produce detectable amount of ethylene in yeast-extract peptone broth and a synthetic medium containing methionine. Citrus leaves inoculated with the bacterium, however, produced ethylene at three different stages, i.e., early (1 to 6h after inoculation), intermediate (10 to 24h after inoculation) and late (72 to 120h after inoculation until defoliation), respectively. Ethylene biosynthesis in the early stage was also demonstrated by the infiltration of extracellular polysaccharides (EPS) of the bacterium and various polysaccharides such as carboxymethylcellulose, xylan, polyethyleneglycol, star h, inulin, glycogen, pectin, and lipopolysaccharides extracted from the bacterium. The amount of ethylene produced with EPS was significantly higher than those with other polysaccharides as well as living cells of the bacterium. Citrus leaf tissues infiltrated with EPS showed no significant changes in the electrolyte leakage and amino acid composition. Ethylene production at the early stage of infection was considered to associate with the “early selective protection” of citrus leaf tissues. When the leaf water potential was increased by allowing detached leaves to absorb free water from the cut end of leaf petioles, water-congestion was reproduced even after 3 weeks on citrus leaves which had previously been injected with polysaccharides. Although the same phenomenon was observed with other polysaccharides, the degree of recovered water-soaking was most conspicuous with EPS. Bacterial cells suspended in water was effectively protected by EPS resulting in the marked increase in their survival rates.