Systemic Acquired Resistance Induced by A New Kind of 90 kD Extracellular Elicitor Protein

Systemic acquired resistance to TMV, Pseudomonas syringae pv. tabaci and Alternaria alternata in tobacco elicited by 90 kD extracellular elicitor protein, a hypersensitive responses (HR) elicitor on tobacco leaves purified from the culture filtrate of Phytophthora boehmeriae was studied. Inoculation of TMV at 24 h after infiltration of tobacco leaves (cv. Samsun harboring N gene) with the elicitor at dosage of 10 nmol/L, resulted in fewer and smaller visible necrotic lesions on treated leaves thatn that on buffer-infiltrated ones. Fewer lesions were also observed on the 1st and 2nd upper or lower leaves of the elicitor-treated leaf on the same plant when TWV was inoculated, but the average diameter of necrotic lesion did not significantly differ from that of the control leaves. The reduction rates in lesion number were about 40.9%-53.1% and the induced resistance lasted for about seven days. All the dosages ranging from 0.5 to 100 nmol/L tested could trigger the resistance significantly and the resistance level (34.9%-58.2%) was positively correlated with the dosages used. Whereas, the elicitor only provided low level of induced resistance to TMV (4%) when the treated leaves were inoculated immediately or at seven day after elicitor applied. Similar induced resistance by elicitor protein against Pseudomonas syringae pv. tabaci and A. alternata was also found in this study. When inoculation with P. syringae pv. tabaci or A. alternata performed on tobacco plants (cv. W38) at 48h after elicitor treatment, smaller or fewer lesions which significantly differed from the control were observed on treated leaves and their upper and lower nonelicitor-treated leaves. Results showed that the induced resistance of tobacco by 90 kD extracellular elicitor protein from P. boehmeriae was typically a systemic acquired resistance (SAD) characterized by systemicity, durability and a broad-spectrum, which indicated the 90 kD protein elicitor is a kind of natural specific trigger for SAR. Results also indicated an endogenous mobile signaling mechanism be elicited and functioned between treated leaves and their upper or lower non-elicitor-treated ones, where it induced strong responses without HR.