CHARACTERIZATION OF A FUNGAL GLYCOPROTEIN THAT ELICITS A DEFENSE RESPONSE IN FRENCH BEAN

A glycoprotein elicitor of phytoalexin accumulation in leaves of Phaseolus vulgaris, produced well before lysis in the medium of cultures of Colletotrichum lindemuthianum race δ, has been purified to apparent homogeneity. The glycoprotein was a monomer of molecular weight 28 kDa with a pI of 4·25. The glycosyl side chains which accounted for 43% of the weight of the holoprotein, were composed principally of galactose, mannose and rhamnose, exhibited a minimum degree of polymerization of 8 and were apparently O-linked to abundant serine and/or threonine residues of the peptide backbone. In a P. vulgaris leaf injection bioassay the purified glycoprotein had activity easily detectable at nanomolar concentrations and induced browning of the treated tissue and the accumulation of both phenylalanine ammonia-lyase and the isoflavonoid phytoalexin phaseollinisoflavan. For these three linked defence responses, suboptimal concentrations of the glycoprotein induced respectively 4·2, 7·6 and 9·7 fold more activity in the cultivar resistant to race δ (ev. Kievit) than in a cultivar susceptible to that race (ev. Pinto). Protein integrity was not required for elicitor activity and glycosyl side-chains isolated from the protein were shown to be active elicitors. The role of this elicitor in the plant/pathogen interaction is discussed. © 1992.