Protein and Peptide Letters, 2005, 12, 11-16 11 0929-8665/05 $50.00+.00 © 2005 Bentham Science Publishers Ltd. Plant Defense and Antimicrobial Peptides Mariana S. Castro 1,2* and Wagner Fontes 1 1 Brazilian Center for Protein Research, Department of Cell Biology, University of Brasilia and 2 Toxinology Laboratory, Department of Physiological Sciences, University of Brasilia, Brasilia/DF, Brazil, 70.910-900. Abstract: Plants are constantly exposed to a large array of pathogenic organisms and the survival in these conditions demands quick defense responses which include the synthesis of defense peptides and proteins with antimicrobial properties. The main groups of antimicrobial peptides found in plants are thionins, defensins and lipid transfer proteins. They constitute interesting candidates to engineer disease resistance in plants. Keywords: plant defense, antimicrobial peptides, thionins, defensins, lipid transfer proteins. MECHANISMS OF ATTACK AND DEFENSE IN PLANTS The increase in agricultural productivity has been a constant concern in recent years, specially due to the dramatic growth of human global population. To compensate this phenomenon, innumerous strategies are being developed with the objective of an increase in food production associated with improvements in product quality. Among the most commonly used strategies are: the use of artificially selected plant varieties more resistant to diseases; better agricultural practices such as crop rotation; the use of fertilizers and pesticides. However, many of those strategies have shown undesirable results, what makes necessary a yet bigger development in the related fields of study, above all, in the defense of plant to infections. Plants are constantly exposed to a great variety of potentially pathogenic organisms, such as viruses, fungi, bacteria, protozoa, mycoplasma and nematodes, and can be affected by adverse environment conditions. When considering plant and pathogen interactions, while the parasite uses a series of chemical compounds, such as toxins and hydrolytic enzymes, plants have numerous defense substances besides various forms of physical barriers. During the infection process, the pathogen utilizes substances which contribute to f facilitate its passage through the physical barriers presented by the plants, making sure that it can obtain nutrients from the host and promoting the neutralization of its defense responses. The substances secreted by the pathogen while spreading through the host tissues are mainly: 1) hydrolytic enzymes that degrade the structural components which constitute the host cell wall, allowing its penetration in these tissues [1,2]; 2) toxins that interfere with the host metabolic functions, altering the cell membrane permeability or *Address correspondence to this author at the Brazilian Center for Protein Research, Department of Cell Biology, University of Brasilia, Brazil; Tel: 55-61-307-2142; Fax: 55-61-274-1251; E-mail: mscastro@unb.br inactivating enzymes, interrupting, in some cases, essential metabolic pathways [3,4]; 3) growth regulators (such as auxins, ethylene, gibberellins, among others) that promote an imbalance in the host hormonal system, causing physiological responses that are incompatible with the normal plant development pattern [5,6]; 4) polysaccharides, probably responsible for the blockage of water translocation mechanisms throughout the vascular system [7,8]. Among the classes of substances secreted by pathogens, it is useful to point out the important contribution of the hydrolytic enzymes to the pathogen dissemination. Among the hydrolytic enzymes produced by the pathogens during its invasion to the host tissue, it can be brought out [1,2]: 1) cutinases, enzymes capable of degrading cutin, one of the main components of plant cuticle. Many fungi present a basal production of cutinases, which, in contact with the substrate (cutin) promote its degradation, releasing monomers that, in its turn, induce the acute expression and production of cutinases by the fungus, intensifying the invasive process; 2) pectinases, enzymes that degrade pectic components of plant cell walls, among which there are: polygalacturonases, pectin lyases and pectin methyl esterases; 3) cellulases, hemicellulases and ligninases, which promote the final degradation of the cell wall, allowing, in many cases, the pathogen installation and dissemination. On the other hand, there are at least two mechanisms of pathogen resistance in plants, the first being represented by the structures and compounds synthesized during the plant normal development (constitutive resistance factors) and the second mechanism activated only after contact with the pathogen (induced resistance factors). The induction mechanism involves the recognition of elicitors and signal transduction, with the response normally regulated via expression of genes related to defense [9]. This mechanism