A soluble carbohydrate elicitor from Blumeria graminis f. sp tritici is recognized by a broad range of cereals

Wheat plants rapidly recognize pathogenic and non-pathogenic conidia of the powdery mildew fungus Blumeria (syn. Erysiphe) graminis on their leaf surf aces. This suggests that a chemical signal emanates from conidia at the pre -penetration stage of infection. Conidia of B. graminis f. sp. tritici were found to contain an elicitor that was easily washed off their surface. The elicitor activity is heat stable and could not be removed by phenol extrac tion. By contrast, elicitor activity is sensitive to periodate oxidation an d partial acid hydrolysis suggesting that the elicitor activity resides in a carbohydrate moiety. Analysis of carbohydrates revealed mostly glucose, w ith smaller amounts of xylose and mannose. The glucosyl residues of the B. graminis elicitor were found to be linked (1 --> 2)-, (1 --> 4), and (1 --> 6)-, with (1 --> 4, 1 --> 6)- branch point residues, and no 3-linked gluco se residues were detected. As treatment with beta-mannanase significantly r educed elicitor activity, mixed-linkage (1 --> 4), (1 --> 6)-mannosyl resid ues appeared to be important for elicitor activity. The B. graminis elicito r induced the expression of all defence-related genes tested in wheat and a lso induced resistance to subsequent attack by B. graminis f. sp. tritici. In contrast, a hypersensitive response was not induced by the elicitor in t he absence or the presence of a challenging inoculum of B. graminis f. sp. trilici. The elicitor also induced the accumulation of thaumatin-like prote ins in barley, oat, rye, rice and maize, but did not induce necrosis in any of these species. This suggests that the B. graminis elicitor represents a host non-specific determinant of non-self recognition in cereals activatin g general defence responses other than the hypersensitive reaction. (C) 200 0 Academic Press.