The growth of Xanthomonas oryzae pv. oryzae (Xoo), causal agent of bacterial leaf blight and typical vascular pathogen, was investigated in tissues and guttation fluids of rice leaves to elucidate the mechanisms of susceptibility in the host-parasite interaction. Light and scanning electron-microscopic observation showed that, in the compatible cultivar-bacterial strain combination, bacterial masses were abundant in the lumen of xylem vessels, but were absent in other vascular elements. The inner surface of spiral and ring vessels where the bacterium multiplied appeared to be digested by the causal organism. Histochemical tests also revealed that the xylem vessel walls were degraded after infection with Xoo. Based on the histological study, the growth of Xoo was examined in guttation fluids collected from rice leaves. Unexpectedly, the growth rate of Xoo was very low in an aqueous solution of freeze-dried guttation fluid. When 0.5% sucrose was added, however, the bacterium could grow extensively in a solution containing freeze-dried guttation fluids. In addition, the growth of Xoo was also promoted by substances in the pectic fraction extracted from cell walls of rice leaves. The results suggested that nutritional factors such as pectic substances from affected xylem vessel walls were involved in the multiplication of Xoo in the lumen of xylem vessels.
The Phytopathological Society of Japan