An Antimicrobial Peptide from the Australian Native Provides the First Functionally Characterised Member of a Subfamily of Plant Defensins

An antimicrobial peptide ( Hv AMP1) was isolated from seeds of the Australian native legume Hardenbergia violacea (Schneev.) Stearn. The peptide is 47 amino acid residues in length, contains 8 cysteines, and has a molecular weight of 5392 and a predicted pI of 10.41. Hv AMP1 inhibited the growth of several plant pathogenic fungi at concentrations as low as 1 µM in vitro and produced distinct hyphal distortion and increased branching. This antimicrobial activity was greatly diminished in the presence of 1 mM CaCl 2 and 50 mM KCl. The purified peptide at 40 µM did not inhibit three different a-amylase enzymes. Aeukaryotic cell-free translation system showed inhibition approaching 50% in the presence of ~100 µM of Hv AMP1. The viability of plant and mammalian cells cultured in vitro was not adversely affected by concentrations of Hv AMP1 as high as 40 mM. The amino acid sequence of Hv AMP1 contained the consensus amino acids that define the plant defensin family of peptides. The Hv AMP1 amino acid sequence showed 87% and 57% identity with the amino acid sequences deduced from cDNA sequences from defensins of Vigna unguiculata and Pisum sativum , respectively. Other plant defensin sequences showed less than 33% amino acid identity to the peptide. Therefore, Hv AMP1 and the putative plant defensins of cowpea and pea define a distinct sequence subfamily of plant defensins which is at present limited to members of the Fabaceae. Hv AMP1 is the first member of this subfamily to be purified and functionally characterised. The antimicrobial activity of Hv AMP1 suggests a defensive role for this subfamily of peptides. Keywords: Plant defence, cowpea, pea, thionins, α -amylase inhibitors, protein synthesis inhibitor, antifungal activity, Fabaceae. Australian Journal of Plant Physiology 24(5) 571 - 578 Full text doi:10.1071/PP97075 © CSIRO 1997