DEFENSE ACTIVATION AND ENHANCED PATHOGEN TOLERANCE INDUCED BY H2O2 INTRANSGENIC TOBACCO

Transgenic tobacco deficient in the H2O2-removing enzyme catalase (Cat 1AS) was used as an inducible and noninvasive system to study the role of H2O2 as an activator of pathogenesis-related (PR) proteins in plan ts, Excess H2O2 in Cat1AS plants was generated by simply increasing li ght intensities, Sustained exposure of Cat1AS plants to excess H2O2 pr ovoked tissue damage, stimulated salicylic acid and ethylene productio n, and induced the expression of acidic and basic PR proteins with a t iming and magnitude similar to the hypersensitive response against pat hogens. Salicylic acid production was biphasic, and the first peak of salicylic acid as well as the peal of ethylene occurred within the fir st hours of high light, which is long before the development of tissue necrosis, Under these conditions, accumulation of acidic PR proteins was also seen in upper leaves that were not exposed to high light, ind icating systemic induction of expression, Short exposure of Cat1AS pla nts to excess H2O2 did not cause damage, induced local expression of a cidic and basic PR proteins, and enhanced pathogen tolerance. However, the timing and magnitude of PR protein induction was inn this case mo re similar to that in upper uninfected leaves than to that in hypersen sitive-response leaves of pathogen-infected plants, Together, these da ta demonstrate that sublethal levels of H2O2 activate expression of ac idic and basic PR proteins and lead to enhanced pathogen tolerance. Ho wever, rapid and strong activation of PR protein expression, as seen d uring the hypersensitive response, occurs only when excess H2O2 is acc ompanied by leaf necrosis.