Small oligomers of galacturonic acid are endogenous suppressors of disease resistance reactions in wheat leaves
1999
Plants infected by a phytopathogenic fungus appear to recognize the presence of the pathogen by the molecular recognition of fungal cell wall fragments, termed 'elicitors', or of breakdown products of their own cell walls, termed 'endogenous elicitors'. Successful pathogens are thought to counteract this elicita tion of active resistance reactions by the production of 'suppressors'. Evidence is presented here that fragments of the host cell wall, presumably produced enzymatically during fungal penetration, may act as 'endogenous suppressors' of resistance reactions in wheat. Pectic fractions were extracted from wheat cell walls by a variety of methods: Ca(2+)-chelators (CDTA and imidazole), a commercial mixture of pectic enzymes (Pectolyase Y23), a highly purified recombinant endopolygalacturonase (EPG), and solvolyses of the cell walls in anhydrous hydrogen fluoride at low temperatures followed by imidazole extraction. All of these fractions suppressed elicitor-induced activities of phenylalanine ammonia-lyase and peroxidases when co-injected with a glycoproteogalactan-elicitor, isolated from germ tubes of the wheat stem rust fungus, into the intercellular spaces of wheat leaves. Suppressor activity was correlated with the content of galacturonic acid in the extracts. Of the oligogalacturonides tested (monomer to hexamer), the dimer and trimer proved to be most active. This was not only true for suppression of elicitor-induced responses, but also for suppression of the hypersensitive resistance reaction in infected, genetically resistant host plants. As a consequence of reduced host cell necrosis in suppressor-treated leaves, the fungus developed larger colonies than in water-treated control leaves. Small oligomers of galacturonic acid, thus, are endogenous suppressors of resistance reactions in wheat leaves.
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