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All organisms, including plants and humans, are vulnerable to infections by microorganisms such as bacteria and fungi. Defensins are proteins of the innate immune system that defend hosts by direct contact and subsequent perforation of invading pathogen cells. Due to their direct effect on increasing chance of survival, these small positively charged anti-microbial peptides are present in all eukaryotes including plants and humans and have evolved multiple subtypes such as human alpha and beta defensins. Although both organisms are extremely different, striking similarities in the structure, function, location and expression of defensins exist and greatly outweigh their differences.
The structure of plant and human beta defensins are extremely similar, however human alpha defensins are not. The overall structure of plant defensins is a three stranded anti-parallel beta sheet with a superimposed alpha helix. Essential to the stabilization of this structure are eight cysteine residues. These cysteine residues stabilize the structure through formation of disulfide bonds between the alpha helix and beta sheets. This structure of plant defensins is very similar to human beta defensins which also consist of a three stranded anti-parallel beta sheet with a superimposed alpha helix. Human alpha defensin, on the other hand, are less similar as they lack the superimposed alpha helix. The human beta defensins unlike the plant defensins contains only six cysteine residues to form stabilizing disulfide bonds. The amino acid sequence although containing positive basic amino acids is also different between all mentioned defensins. Despite differences between the human beta defensins and the plant defensins, experiments of transgeinic Arabidopsis plants expressing human beta-2-defensin show fungal resistance that is proportional to the level of gene expression and exceeding that of non-transgenic plants. This experiment strongly suggests that due to strong similarity in structure between the human defensin and endogenous plant defensins, human defensins were able to adopt plant defensin anti-fungal roles. Thus although differences may be present in structure, they are insignificant and do not effect the proteins function.
Due to the fact that a proteins structure determines its function, it should not be surprising that both plant and human defensins possess anti-microbial action against both bacteria and fungi. It is important to note however that plant and human defensins disproportionately favor separate targets; majority of plant defensins target fungi while majority of human alpha and beta defensins target bacteria. These target preferences are hypothesized to be related to the fact that plants and humans are susceptible to different pathogens and thus their defensins are exposed to different selective pressures. For example, plants are much more susceptible to fungal infections and thus fungal pathogenicity would select for survivors with anti-fungal defensins. Similarly humans are much more susceptible to bacterial infections and thus bacterial pathogenicity would select for survivors with anti-bacterial defensins. Although both human and plant defensins have anti-microbial function, human defensins possess other functions that plant defensins do not. For example human defensins can act a chemotactic signals that attract immune cells, they can promote cell proliferation and can inhibit neoplasm formation.
Plant and human defensins due to their anti-microbial function are generally distributed in locations that contact or are close to the outside environment or entry sites of microorganisms. It makes sense to conserve this epithelial distribution among plants and animals as microorganisms entering the host will be confronted with innate immunity preventing further spread and internalization of the infection. Although general distribution of plant and human defensins are the same, specific locations of defensins are different due to the obvious difference in plant and human biology. Human alpha defensins such as human neutrophil peptides 1-4 are located in neutrophils and human defensin 5-6 are located in the panth cells of the gastrointestinal tract. Human beta defensins such as human beta-defensin-2 is located in the epithelial layer of the gastrointestinal tract, reproductive system, urinary tract, skin and lungs. Plants defensins are distributed in the peripheral layers of reproductive organs, leaves, stems, roots, seeds and stomata.
Expression of plant and human defensins in the above locations are stimulated by direct contact with invading pathogens and wounding. Although majority of plant defensins are anti-fungal, some plant defensins are anti-insect. When a plant senses damage from herbivorous insects, the expression of these anti-insect defensins occurs. Anti-insect defensins, by inhibiting the insect digestive enzyme alpha amylase, prevent further feeding and damage of the plant. Similarly, human skin when wounded increases the expression of human beta-2-defensin. Through the use of human beta-2 antibodies, the expression of human beta-2-defensin was visualized and indicated a significant increase in expression throughout wounded skin samples. Normal skin in contrast had minimal to undetectable levels of human beta-2-defensin expression. Human beta-2-defensin expression is also stimulated by inflammation. Inflammation is a sign of microorganism infection and a product of acquired immunity which plants lack.
Human defensin expression, unlike plants, does not have to be stimulated by anything. The panth cells of the gastrointestinal tract express some alpha defensins, such as human defensin 5-6, continuously. Although continual expression consumes energy and would be wasteful in a plant, humans unlike plants cannot make their own food and risk the ingestion of food contaminated with microorganisms. As humans are continuously taking in food, the gastrointestinal tract becomes a prime entry site for microorganisms and thus continual expression is very adaptive.
Innate immune defensin proteins, by directly killing microorganisms, defend the survival of two very different hosts, plants and humans. Despite organism differences, defensins due to their adaptive nature are extremely conserved with regards to their beta sheet with a super imposed alpha helix structure, anti-microbial function, external distribution and expression induced by both contact and wounding. Although slight differences in structure, anti-microbial targets, specific anatomical location and expression did exist, these differences proved to be a result of organism differences rather than differences in the protein itself. Defensins, the only innate immune protein conserved throughout the eukaryotic kingdom, will continue throughout evolution to enhance the fitness and survival of all host types.