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7437370 
Journal Article 
Induced and Preformed Antimicrobial Proteins 
Broekaert, WF; Terras, FRG; Cammue, BPA; , 
2000 
Springer Netherlands 
Dordrecht 
Mechanisms of Resistance to Plant Diseases 
371-477 
To protect themselves from invasion by pathogenic microorganisms, plants produce a wide array of proteins that exert direct antimicrobial activity. Based on homology at the level of the amino acid sequence and/or three-dimensional folding pattern, these antimicrobial proteins (AMPs) can be classified into over 18 distinct protein families. Some information is emerging on how these proteins interfere with the growth of microorganisms, fungi in particular. Hydrolases such as PR-3-type chitinases and PR-2-type glucanases, and possibly also PR-4-type proteins, affect fungal growth by disturbing the structural integrity of their cell wall. Thionins, 2S albumins, lipid transfer proteins and puroindolines have been demonstrated to partially lyse artificial phospholipid vesicles and are therefore believed to interfere with the phospholipid bilayer of the microbial plasma membrane. Other AMPs, such as PR-5-type proteins and plant defensins are proposed to affect plasma membrane receptors, although the evidence is still circumstantial. For many different types of AMPs it has been shown that particular combinations by two result in synergistic antimicrobial effects, suggesting that maximal antimicrobial potency of AMPs is achieved when they act in concert. Most plant tissues express simultaneously a number of AMP genes. In uninfected vegetative tissues expression is predominant in gateways for microbial invasion such as epidermal cells, stomata, hydathodes and cells in vascular strands. Infection of vegetative tissues by viruses, bacteria or fungi results in the coordinate activation of sets of AMP genes via multiple signalling pathways. These genes are in some but not all cases the same as those that confer basal expression in microbial gateway cells. At the subcellular level, AMPs are usually either deposited in the extracellular space or stored in vacuoles, in which case they are released when host cell lysis occurs. Vacuolar and extracellular AMP isoforms are usually products of different genes. The contribution of AMP genes to the resistance of plants has been studied extensively by overexpression or antisense down-regulation of AMP genes in transgenic plants, or by using mutant or transgenic plants showing either up- or down-regulation of signalling pathways leading to coordinate AMP gene expression.