Hydrogen peroxide from the oxidative burst is neither necessary nor sufficient for hypersensitive cell death induction, phenylalanine ammonia lyase stimulation, salicylic acid accumulation, or scopoletin consumption in cultured tobacco cells treated with elicitin

H2O2 from the oxidative burst, cell death, and defense responses such as th e production of phenylalanine ammonia lyase (PAL), salicylic acid (SA), and scopoletin were analyzed in cultured tobacco (Nicotiana tabacum) cells tre ated with three proteinaceous elicitors: two elicitins (alpha-megaspermin a nd beta-megaspermin) and one glycoprotein. These three proteins have been i solated from Phytophthora megasperma H20 and have been previously shown to be equally efficient in inducing a hypersensitive response (HR) upon infilt ration into tobacco leaves. However, in cultured tobacco cells these elicit ors exhibited strikingly different biological activities. beta-Megaspermin was the only elicitor that caused cell death and induced a strong, biphasic H2O2 burst. Both elicitins stimulated PAL activity similarly and strongly, while the glycoprotein caused only a slight increase. Only elicitins induc ed SA accumulation and scopoletin consumption, and beta-megaspermin was mor e efficient. To assess the role of H2O2 in HR cell death and defense respon se expression in elicitin-treated cells, a gain and loss of function strate gy was used. Our results indicated that H2O2 was neither necessary nor suff icient for HR cell death, PAL activation, or SA accumulation, and that extr acellular H2O2 was not a direct cause of intracellular scopoletin consumpti on.