Characterization of a New Antifungal Chitin-Binding Peptide from Sugar Beet Leaves

The intercellular washing fluid (IWF) from leaves of sugar beet (Beta vulgaris L.) contains a number of proteins exhibiting in vitro antifungal activity against the devastating leaf pathogen Cercospora beticola (Sacc.). Among these, a potent antifungal peptide, designated IWF4, was identified. The 30-amino-acid residue sequence of IWF4 is rich in cysteines (6) and glycines (7) and has a highly basic isoelectric point. IWF4 shows homology to the chitin-binding (hevein) domain of chitin-binding proteins, e.g. class I and IV chitinases. Accordingly, IWF4 has a strong affinity to chitin. Notably, it binds chitin more strongly than the chitin-binding chitinases. A full-length IWF4 cDNA clone was obtained that codes for a preproprotein of 76 amino acids containing an N-terminal putative signal peptide of 21 residues, followed by the mature IWF4 peptide of 30 residues, and an acidic C-terminal extension of 25 residues. IWF4 mRNA is expressed in the aerial parts of the plant only, with a constitutive expression in young and mature leaves and in young flowers. No induced expression of IWF4 protein or mRNA was detected during infection with C. beticola or after treatment with 2,6-dichloroisonicotinic acid, a well-known inducer of resistance in plants.