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Rust (fungus)
 
 
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Rusts are fungi of the order Uredinales. Many of these species are plant parasites. Some are superficially similar to the smuts
Smut (fungus)

The smuts are fungus, mostly Ustilaginomycetes , that cause plant disease.Smuts affect grasses, notably including cereal crops such as maize. They initially attack the plant's reproductive system, forming galls which darken and burst, releasing fungal spores which infect other plants nearby....
, although their relation to each other is not clear. The taxonomy of Urediniomycota, as a whole, is in a state of flux.

Many of the rusts have two or more hosts (heteroecious
Heteroecious

A heteroecious parasite is one that requires at least two Host s. The primary host is the host in which the parasite spends its adult life; the other is the secondary host....
) and up to five spore stages. However they most commonly reproduce via asexual
Asexual reproduction

Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction....
 spore
Spore

In biology, a spore is a reproduction structure that is adapted for biological dispersal and surviving for extended periods of time in unfavorable conditions....
 production.






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Rusts are fungi of the order Uredinales. Many of these species are plant parasites. Some are superficially similar to the smuts
Smut (fungus)

The smuts are fungus, mostly Ustilaginomycetes , that cause plant disease.Smuts affect grasses, notably including cereal crops such as maize. They initially attack the plant's reproductive system, forming galls which darken and burst, releasing fungal spores which infect other plants nearby....
, although their relation to each other is not clear. The taxonomy of Urediniomycota, as a whole, is in a state of flux.

Many of the rusts have two or more hosts (heteroecious
Heteroecious

A heteroecious parasite is one that requires at least two Host s. The primary host is the host in which the parasite spends its adult life; the other is the secondary host....
) and up to five spore stages. However they most commonly reproduce via asexual
Asexual reproduction

Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction....
 spore
Spore

In biology, a spore is a reproduction structure that is adapted for biological dispersal and surviving for extended periods of time in unfavorable conditions....
 production. Their spores are airborne and can travel great distances. They mostly cause foliar infections.

The group received its common name from the fact that some species have a reddish spore
Spore

In biology, a spore is a reproduction structure that is adapted for biological dispersal and surviving for extended periods of time in unfavorable conditions....
 stage, which resembles the corrosion process known as rust
Rust

Rust is a general term for a series of iron oxides, usually red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture....
.

One economically important rust is wheat leaf rust
Wheat leaf rust

Wheat leaf rust, also known as brown rust, is a serious fungal disease affecting wheat and rye caused by the Rust Puccinia triticina....
, Puccinia triticina, a serious fungal disease affecting wheat
Wheat

Wheat , is a worldwide cultivated Poaceae from the Levant region of the Middle East. Globally, after maize, wheat is the second most-produced food among the cereal just above rice....
 and rye
Rye

Rye is a Poaceae grown extensively as a grain and forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some rye whiskey, some vodkas, and animal fodder....
, which has caused serious epidemics in North America
North America

North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and almost totally in the western hemisphere....
, Mexico
Mexico

The United Mexican States , commonly known as Mexico , is a federalism constitutionalism republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of Mexico....
 and South America
South America

South America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere....
.

Infection process


Rust occurs on many species of plant, but in most cases any one species of rust can only infect one species of plant. This can make rust useful in biological control. The following describes the infection process of asexual spores. A picture summarizing the process can be found in the gallery below.

Spore attachment


When a rust conidium
Conidium

Conidia, sometimes termed conidiospores, are Asexual reproduction, non-motile spores of a fungus; they are also called mitospores due to the way they are generated through the cellular process of mitosis....
 lands on a plant surface, it needs to attach to it, or it would simply be washed off. First, weak, hydrophobic interactions are formed between the spore and the cutin
Cutin

Cutin is one of two waxy polymers that are the main components of the plant cuticle which covers all aerial surfaces of plants. The other major cuticle polymer, which is much more readily taphonomy, is cutan....
 of the plant cell surface. Then unknown signals cause the production of hydrophobic mucilaginous macromolecules called adhesins. These will stick the spore irreversibly to the plant surface. Once attached, the spore will germinate.

Germ tube elongation


Rust fungi penetrate the plant by using the natural opening of the stoma
Stoma

In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening....
, but first the growing germ tube
Germ tube

A germ tube is an outgrowth produced by certain species of spore-releasing fungi during germination.The germ tube differentiates, grows, and develops by mitosis to create a somatic hyphae....
 must locate it. Rust fungi have evolved to more efficiently locate stomata by the use of thigmotropism
Thigmotropism

Thigmotropism is a movement in which an organism moves or grows in response to touch or contact stimulus . The prefix thigmo- comes from the Greek language for "touch"....
. The germ tube grows in a random manner until it reaches a ridge between epidermal cells
Epidermis (botany)

The epidermis is a single-layered group of cells that covers plants leaf, flowers, roots and Plant stem. It forms a boundary between the plant and the external world....
. At this point, it will start to grow perpendicular
Perpendicular

In geometry, two line or plane , are considered perpendicular to each other if they form congruence adjacent angles angles . The term may be used as a noun or adjective....
 to the ridge, greatly increasing its chances of locating a stoma.

Appresorium formation


The stoma is the site of appresorium
Appresorium

Appresoria, the plural of appresorium, are the tips of infectious hyphae, often from a germinating spore, that make contact with their host cell wall and that flatten out into disc, fan or lobed shapes tightly affixed or appressed on the host cell wall....
 formation, a structure that functions to both firmly anchor the fungus and aid in penetration. In the rust fungi appresorial formation is controlled by a process of thigmodifferentiation. Appresoria are formed when the germ tube detects ridges that match the dimensions of the stomatal lips of its host
Host (biology)

In biology, a host is an organism that harbors a virus or parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter....
 species.

It has been proposed that this process is mediated by a mechanosensitive calcium
Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the earth's Crust ....
 ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
 channel that is located at the germ tube tip. This ion channel would transduce the stretching of the cell membrane
Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
 caused by changes in leaf
Leaf

In botany, a leaf is an above-ground plant Organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the cells containing chloroplast to light over a broad area, and to allow light to penetrate fully into the tissues....
 topography
Topography

Topography is the study of Earth's surface shape and features or those ofplanets, Natural satellite, and asteroids. It is also the description of such surface shapes and features ....
 into ion fluxes that lead to changes in gene
Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
 expression and appresorium formation.. This theory is supported by experiments that show that applying Ca2+ externally to the germ tube causes differentiation
Differentiation

Differentiation can mean the following:* The act of finding the derivative in mathematics* Differentiated instruction in education,* Cellular differentiation in biology...
.

From the appresorium an infection peg grows down into the plant and between the mesophyll cells.

The haustorium


Rust fungi are biotrophs, meaning they gain their nutrients from living cells. This requires a specialised entension of the fungus into a living plant cell called a haustorium
Haustorium

In botany, a haustorium is the hyphal tip of a parasitic fungus or of the root of a parasitic plant , that penetrates the host's tissue, but stays outside the host cell membrane....
. This develops from a haustorial mother cell. The plant cell membrane invaginates around the main haustorial body and the space between the two membranes becomes known as the extra-haustorial matrix. An iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 and phosphorus
Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. The name comes from the and . A Valency nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate minerals....
 rich neck band bridges the plant and fungal membranes and acts as a seal preventing the escape of nutrient
Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment....
s into the plant apoplast
Apoplast

Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
. The haustorium contains amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 and hexose
Hexose

In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2....
 sugar
Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
 transporters and H+-ATPases for the active transport
Active transport

Active transport is the mediated process of moving particles across a biological membrane against a Concentration_gradient#In_biology . If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport....
 of nutrients from the plant cell.

The rust fungus will then continue to grow and invade the plant until it is ready for sporulation.

Gallery


See also

  • soybean rust
    Soybean rust

    Soybean rust, also known as Asian soybean rust, is a disease that affects soybeans and other legumes. It is caused by two types of fungus, Phakopsora pachyrhizi and Phakopsora meibomiae....
  • Wheat leaf rust
    Wheat leaf rust

    Wheat leaf rust, also known as brown rust, is a serious fungal disease affecting wheat and rye caused by the Rust Puccinia triticina....
  • Robigus
    Robigus

    In Roman mythology, Robigus was a fertility god who protected crops against diseases. He was worshipped alongside his sister Robigo for over 1700 years....
  • Common groundsel, a species susceptible to rust


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