Bm. Moerschbacher et al., Small oligomers of galacturonic acid are endogenous suppressors of diseaseresistance reactions in wheat leaves, J EXP BOT, 50(334), 1999, pp. 605-612
Plants infected by a phytopathogenic fungus appear to recognize the presenc
e of the pathogen by the molecular recognition of fungal cell wall fragment
s, termed 'elicitors', or of breakdown products of their own cell walls, te
rmed 'endogenous elicitors'. Successful pathogens are thought to counteract
this elicitation of active resistance reactions by the production of 'supp
ressors'. Evidence is presented here that fragments of the host cell wall,
presumably produced enzymatically during fungal penetration, may act as 'en
dogenous suppressors' of resistance reactions in wheat. Pectic fractions we
re extracted from wheat cell walls by a variety of methods: Ca2+-chelators
(CDTA and imidazole), a commercial mixture of pectic enzymes (Pectolyase Y2
3), a highly purified recombinant endopolygalacturonase (EPG), and solvolys
es of the cell walls in anhydrous hydrogen fluoride at low temperatures fol
lowed by imidazole extraction. All of these fractions suppressed elicitor-i
nduced activities of phenylalanine ammonia-lyase and peroxidases when co-in
jected with a glycoproteogalactanelicitor, isolated from germ tubes of the
wheat stem rust fungus, into the intercellular spaces of wheat leaves. Supp
ressor activity was correlated with the content of galacturonic acid in the
extracts. of the oligogalacturonides tested (monomer to hexamer), the dime
r and trimer proved to be most active. This was not only true for suppressi
on of elicitor-induced responses, but also for suppression of the hypersens
itive resistance reaction in infected, genetically resistant host plants. A
s 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.