Plant defensins (formerly ''gamma-thionins)'' are a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of small,

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

-rich

defensins Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate and invertebrate animals, plants, and fungi. They are host defense peptides, with members displaying either direct antimicrobial activity, immune sig ...

found in plants that serve to defend them against

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

s and

parasites Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

History

The first plant defensins were discovered in

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pr ...

and

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

in 1990 and were initially designated as a γ-thionins. In 1995 the name was changed to 'plant defensin' when it was identified that they are evolutionarily unrelated to other

thionin Thionins are a family of small proteins found solely in higher plants. Typically, a thionin consists of 45–48 amino acid residues. 6–8 of these are cysteine forming 3–4 disulfide bonds. They include phoratoxins and viscotoxins. Alpha- an ...

s and were more similar to

from insects and mammals.

Function

Plant defensins are a large component of the plant innate immune system. A plant

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

typically contains large numbers of different defensin genes that vary in their efficacies against different pathogens and the amount they are expressed in different tissues.

Antimicrobial activity

The modes of action of different defensins depends on the type of fungus they are interacting with. Most characterized plant defensins are

antimicrobial peptides Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for a ...

. Both

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as crypto ...

and

antibacterial An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

plant defensins have been identified, although their exact

mechanisms of action In pharmacology, the term mechanism of action (MOA) refers to the specific biochemical interaction through which a drug substance produces its pharmacological effect. A mechanism of action usually includes mention of the specific molecular targe ...

vary.

Enzyme inhibition

Some plant defensins have also been identified as

enzyme inhibitor An enzyme inhibitor is a molecule that binds to an enzyme and blocks its activity. Enzymes are proteins that speed up chemical reactions necessary for life, in which substrate molecules are converted into products. An enzyme facilitates a sp ...

s of

α-amylase α-Amylase is an enzyme (EC 3.2.1.1; systematic name 4-α-D-glucan glucanohydrolase) that hydrolyses α bonds of large, α-linked polysaccharides, such as starch and glycogen, yielding shorter chains thereof, dextrins, and maltose: :Endohydr ...

trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting these long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...

. It is believed that these are

antifeedant Antifeedants are organic compounds produced by plants to inhibit attack by insects and grazing animals. These chemical compounds are typically classified as secondary metabolites in that they are not essential for the metabolism of the plant, but i ...

activities to deter insects.

Anti-cancer

An additional promiscuous activity of some plant defensins is stopping the growth or disrupting the membranes of

cancer cell Cancer cells are cells that divide continually, forming solid tumors or flooding the blood with abnormal cells. Cell division is a normal process used by the body for growth and repair. A parent cell divides to form two daughter cells, and these d ...

s in ''

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

'' experiments.

Structure

file:Plant_defensin_secondary_structure.svg, Secondary structure of main types of plant defensins (with cysteines numbered). The 8-cysteine variant is the most common. Alpha helix in red, beta strands in blue, disulphide bonds in yellow., 300x300px Defensin proteins are produced as a Protein precursor, precursor protein with one or two prodomains that are removed to make the final mature protein. In their mature form, they generally consist of about 45 to 50 amino-acid residues. The folded structure is characterised by a well-defined 3-stranded anti-parallel beta-sheet and a short alpha-helix. The structure of most plant defensins is cross-linked by four disulfide bridges: three in core and one linking the N- and

C-termini The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...

. Some plant defensins have only the core three disulphides, and a few have been found with an additional one (resulting in five total bridges).

Evolution

Plant defensins are members of the

protein superfamily A protein superfamily is the largest grouping (clade) of proteins for which common ancestry can be inferred (see homology (biology), homology). Usually this common ancestry is inferred from structural alignment and mechanistic similarity, even if n ...

called the ''cis''-defensins or CSαβ fold. This superfamily includes

arthropod defensin Arthropod defensins are a family defensin proteins found in mollusks, insects, and arachnids. These cysteine-rich antibacterial peptides are primarily active against Gram-positive bacteria and fungi in vitro. However ''Drosophila'' fruit flies m ...

s and

fungal defensin A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

s (but not defensins found in mammals). It also includes several families of proteins not involved in the immune system, including plant S-locus 11 proteins involved in

self-incompatibility Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals ...

during reproduction, and toxin proteins in scorpion

venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...

Examples

The following plant proteins belong to this family: * The flower-specific ''

Nicotiana alata ''Nicotiana alata'' is a species of tobacco. It is called jasmine tobacco, sweet tobacco, winged tobacco, ''tanbaku'', and Persian tobacco. ''Nictoiana alata'' is mainly grown as an ornamental plant; numerous cultivars and hybrids are derived ...

'' defensin (NaD1) * Gamma-thionins from ''

Triticum aestivum Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield. Ta ...

'' (wheat) endosperm (gamma-purothionins) and gamma-hordothionins from ''

Hordeum vulgare Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pr ...

'' (barley) are toxic to animal cells and inhibit protein synthesis in cell free systems. * A flower-specific thionin (FST) from ''

Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an annually grown herbaceous plant of the ''Nicotiana'' genus. The plant is tropical in origin, is commonly grown throughout the world, and is often found in cultivation. It grows to heights bet ...

'' (common tobacco). * Antifungal proteins (AFP) from the seeds of Brassicaceae species such as radish, mustard, turnip and ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'' (thale cress). * Inhibitors of insect alpha-amylases from sorghum. * Probable protease inhibitor P322 from ''

Solanum tuberosum The potato is a starch#Food, starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial plant, perennial in the nightshade family Solanaceae. Wild potato species can be fo ...

'' (potato). * A germination-related protein from ''

Vigna unguiculata The cowpea (''Vigna unguiculata'') is an annual herbaceous legume from the genus ''Vigna''. Its tolerance for sandy soil and low rainfall have made it an important crop in the semiarid regions across Africa and Asia. It requires very few inputs, ...

'' (cowpea). * Anther-specific protein SF18 from sunflower. SF18 is a protein that contains a gamma-thionin domain at its N-terminus and a proline-rich C-terminal domain. * ''

Glycine max Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...

'' (soybean) sulfur-rich protein SE60. * ''

Vicia faba ''Vicia faba'', commonly known as the broad bean, fava bean, or faba bean, is a species of vetch, a flowering plant in the pea and bean family Fabaceae. It is widely cultivated as a crop for human consumption, and also as a cover crop. Variet ...

'' (broad bean) antibacterial peptides fabatin-1 and -2.

Databases

A database for antimicrobial peptides, including defensins is available: PhytAMP (http://phytamp.hammamilab.org).

References

Subfamilies

*Gamma Purothionin {{InterPro content, IPR008176 Protein domains Peripheral membrane proteins Defensins