Plant Antimicrobial Peptides

Abstract

Antimicrobial peptides (AMPs) are natural antibiotics produced by all living organisms to resist infection by pathogens. They are important effector molecules of the innate immune system both in animals and plants. AMPs are diverse in structure and mode of action and display broad-spectrum antimicrobial activity and thus show promise for engineering pathogen resistance in crops and development of novel pharmaceuticals. A variety of AMP classes have been discriminated, which include defensins, thionins, lipid-transfer proteins, hevein- and knottin-like peptides, and macrocyclic peptides. In this review, the role of AMPs in the plant immune system is briefly discussed, and different families of plant AMPs with respect to their structural peculiarities and biological role are considered. Special emphasis is given to AMPs of wild plants. Defensins belong to the largest AMP family widely distributed throughout the plant and animal kingdoms with a wide range of in vitro biological activities. Current evidence indicates that they interact with specific molecules on the fungal membranes and act on intracellular targets. Thionins show high inhibitory activity against diverse fungi and bacteria including human pathogens and are toxic to some other types of cells, such as mammalian, insect, and plant cells. The antimicrobial effect of thionins is associated with membrane permeabilization through pore formation. Lipid-transfer proteins are not only antimicrobial agents but are also involved in signaling. Hevein-like AMPs comprise peptides with a conserved chitin-binding domain, a variable number of disulfide bridges, and divergent precursor structures. Knottin-like and macrocyclic AMPs form the cystine knot and exhibit insecticidal, antimicrobial, anti-HIV, hemolytic, and uterotonic activities. Due to exclusive structural and chemical stability, plant cyclotides are regarded as templates into which diverse biological activities may be introduced.