Many plants synthesize antimicrobial proteins, which confer resistance to pathogenic microorganisms. These proteins are known as pathogenesis related (PR) proteins.
These include chitinase, a -1, 3-glucanase, protease inhibitors, Non-specific lipid transfer proteins, ribosome inactivating proteins (RIP) and several other antimicrobial proteins. Specific genes in some plants encode for these proteins.
Transgenic plants have been genetically engineered by transferring chitinase and a -J, 3-glucanase genes. These transformed plants have the ability to resist pathogenic fungi. These genes have been isolated from a number of sources including plants (rice and barley), bacteria (Serratia marcescens) and even fungi (Tricoderma harziannum). RIPs have also been used against fungal pathogens. These are enzymes, which inhibit the assembly of ribosomes during protein synthesis.
There are many antimicrobial proteins having antifungal and antibacterial activity. Genes encoding these proteins have been introduced into plants with varying success. Lysozyme is an enzyme that digests the chitin and peptidoglycan constituents of fungal and bacterial cell walls, respectively. Potato plants expressing the lysozyme gene have been raised conferring resistance to Erwinia carotovora.
Thionin protein genes have been introduced into plants with mixed success. Barley a-thionin gene expressed in transgenic tobacco plant has an increased resistance to Pseudomonas syringae.
Defensins are small antimicrobial peptides, which display lytic activity on binding to the plasma membrane of the pathogenic microorganism. One such protein is alfalfa, encoded by the alfalfa gene. This gene has been introduced into the potato plant and its activity monitored in the field. The transgenic potato plants had a significant resistance to the fungal pathogen. Verticil Hum dahlia. An artificial defensin gene has been made by fusing two genes, cecropin and mellitin oBTained from a giant silk moth and a bee, respectively.
This gene has been introduced,into the potato plant. Such transgenic potato plants are resistant to the bacterial pathogen, Erwinia carotovora.