Three hydroxyproline-rich glycopeptides derived from a single petunia polyprotein precursor activate defensin I, a pathogen defense response gene

Hydroxyproline-rich glycopeptides (HypSys peptides) are recently discovered 16 -20-amino acid defense signals in tobacco and tomato leaves that are derived from cell wall-associated precursors. The peptides are powerful wound signals that activate the expression of defensive genes in tobacco and tomato leaves in response to herbivore attacks. We have isolated a cDNA from petunia (Petunia hybrida) leaves encoding a putative protein of 214 amino acids that is a homolog of tobacco and tomato HypSys peptide precursors and is inducible by wounding and MeJA. The deduced protein contains a leader sequence and four predicted proline-rich peptides of 18-21 amino acids. Three of the four peptides were isolated from leaves, and each peptide contained hydroxylated prolines and glycosyl residues. Each of the peptides has a -GR-motif at its N terminus, indicating that it may be the substrate site for a processing enzyme. The peptides were active in a petunia suspension culture bioassay at nanomolar concentrations, but they did not induce the expression of defense genes that are directed against herbivores, as found in tobacco and tomato leaves. They did, however, activate expression of defensin 1, a gene associated with inducible defense responses against pathogens.