We examined the role of salicylic acid (SA) accumulation in the tobacco hypersensitive response (HR) and systemic acquired resistance (SAR) induced by PB90, a 90 kDa protein elicitor that is secreted by Phytophthora boehmeriae. The elicitor induced HR of a consistent shape and size on tobacco plants expressing the bacterial gene nahG. Salicylate hydroxylase is encodec by nahG and inactivates SA by converting it into catechol. The mutant NahG does not accumulate SA. In contrast, infiltration with of a wild type tobacco (cv. Xanthi nc) leaves with the elicitor caused the increase of SA levels. The same SA levels were observed in leaves not treated by the elicitor. HR appears to be mediated by a SA-independent signaling pathway. PB90 treatment resulted in enhanced resistance of wild-type plants to infection by black shank fungus, P. nicotianae, and TMV, but not in NahG plants. Moreover, the elicitor-induced expression of an SAR marker gene encoding PR-1a was suppressed in NahG plants. These results indicate that SA mediates SAR but not HR in tobacco treated with PB90. During plant-elicitor interactions, HR and SAR may be regulated by distinct signal pathways, or SA may function as an intermediate signal upstream of SAR but downstream of HR, and HR may not be a direct defense mechanism against pathogen infection.

野生型烟草Bel-W3叶片经棉疫病菌90kD蛋白激发子处理后，处理叶及其上位叶在24h内均发生2次氧化迸发产生H2O2，且第二次H2O2迸发高峰期同时出现，均出现在第12小时，处理部位细胞死亡高峰期比第二次H2O2迸发高峰期滞后8h。引起过敏反应剂量的激发子诱发反义抑制抗坏血酸过氧化物酶anti-APX烟草的过敏性坏死枯斑比野生型的大而且出现得早；不能诱发野生型烟草HR的剂量可以诱导anti-APX烟草发生HR。经激发子处理后anti-APX烟草对烟草疫霉（Phytophthora nicotianae）和TMV产生的诱导抗性比其野生型高。上述结果表明，H2O2可能是一种重要的信号分子，在棉疫病菌90kD蛋白激发子诱发烟草的HR和SAR中具有重要作用，但可能不是一种可以系统移动的信号分子。