Inhibition of fungal and bacterial plant pathogens by synthetic peptides: in vitro growth inhibition, interaction between peptides and inhibition of disease progression

Four synthetic cationic peptides, pep6, pep7, pep11 and pep20, were tested alone and in combinations for their antimicrobial activities against econom ically important plant pathogenic fungi (Phytophthora infestans and Alterna ria solani) and bacteria (Erwinia carotovora subsp, carotovora and E, carot ovora subsp, atroseptica). In in vitro studies, P, infestans and A. solani were inhibited by all four peptides, while E, carotovora subsp, carotovora and E, carotovora subsp, atroseptica were inhibited only by pep11 and pep20 , All peptides completely inhibited E! infestans and A. solani on potato le aves and P, infestans on tubers at concentrations comparable to the in vitr o IC50 (effective concentration for 50% growth inhibition) values, suggesti ng that these peptides are more potent in preventing infection than in inhi biting hyphal growth in vitro, Microscopic observations of E! infestans and A. solani when treated with these peptides revealed hyphal anomalies. In t uber-infectivity assays, pep11 and pep20 reduced bacterial soft-rot symptom s by 50% at 2.0 to 2.30 mu M and by 100% at 20 phl, In assays involving two -way combinations of these peptides, growth inhibitions of fungi and bacter ia by the combinations were no more than the sum of growth inhibitions by e ach peptide when used alone, indicating that they act additively. pep11 and pep20 are not phytotoxic to potato plants at 200 mu M, With strong and bro ad-spectrum antimicrobial activities of pep11 and pep20 against fungi and b acteria, and with no antagonistic activities, the expression of these pepti des in transgenic potato plants could lead to enhanced disease resistance a gainst these pathogens.