Cleavage of chitinous elicitors from the ectomycorrhizal fungus Hebeloma crustuliniforme by host chitinases prevents induction of K+ and Cl- release, extracellular alkalinization and H2O2 synthesis of Picea abies cells

Rapid reactions comprising efflux of K+ and Cl-, phosphorylation of a 63-kDa protein (pp63), extracellular alkalinization and synthesis of H2O2 are equally induced in cells of Picea abies (L.) Karst. by chitotetraose, colloidal chitin and cell wall elicitors from the ectomycorrhizal fungus Hebeloma crustuliniforme (Bull. ex Fries.) Quel. an ectomycorrhizal partner of spruce. Cleavage of fungal cell wall elicitors and of artificial chitin elicitors to monomeric and dimeric fragments by apoplasmic spruce chitinases (36-kDa class I chitinase, pI 8.0, and 28-kDa chitinase, pI 8.7; EC 3.2.1.14) equally prevented induction of these rapid reactions. Also, N-acetylglucosamine oligomers and elicitors from the fungal cell walls showed a similar dependence of their activity on the degree of polymerisation. From these results it is suggested that, during ectomycorrhiza formation, only some of the chitin-derived elicitors reach their receptors at the plant plasma membrane, initiating reactions of the hypersensitive response in the host cells. The remaining fungal elicitors will be degraded to varying extents by wall-localized chitinases of the host root, reducing the defence reactions of the plant and allowing symbiotic interactions of both organisms.