EFFECT OF LIGHT ON INCOMPATIBLE INTERACTIONS BETWEEN XANTHOMONAS-ORYZAE PV ORYZAE AND RICE

When leaves of rice (Oryza sativa L.) cultivars possessing gene Xa-10 for resistance to Xanthomonas oryzae pv. oryzae were exposed to at least 8 h light after infiltration with avirulent strains of the pathogen or strains containing the cloned avirulence gene avrXa10, a dark brown lesion characteristic of the hypersensitive response (HR) was observed by 24-48 h. The spatial and temporal patterns of increase in the activity of a cationic peroxidase and accumulation of lignin-like polymers were correlated with a decrease in bacterial multiplication in the tissues undergoing the HR. However, when the plants were exposed to less than 8 h of light after inoculation, water-soaked lesions characteristic of a compatible response appeared in the inoculation site. Under these conditions, little cationic peroxidase was detected, lignin did not accumulate, and final bacterial numbers were higher than in tissues exposed to more than 8 h of light. These results suggest that light is essential for rice plant defence responses, and that the HR and the accumulation of peroxidase and lignin-like compounds may be important for resistance in rice to X. oryzae pv. oryzae . © 1993 Academic Press, Inc.