An appressorium is a specialized cell typical of many fungal

plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

s that is used to infect

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

plants. It is a flattened,

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

l "pressing" organ, from which a minute infection peg grows and enters the host, using

turgor Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibriu ...

pressure capable of punching through even

Mylar BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aro ...

. Following spore attachment and germination on the host surface, the emerging

germ tube A germ tube is an outgrowth produced by spores of spore-releasing fungi during germination. The germ tube differentiates, grows, and develops by mitosis to create somatic hyphae.C.J. Alexopolous, Charles W. Mims, M. Blackwell, ''Introductory Myc ...

perceives physical cues such as surface hardness and

hydrophobicity In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

, as well as chemical signals including wax

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

s that trigger appressorium formation. Appressorium formation begins when the tip of the germ tube ceases polar growth, hooks, and begins to swell. The contents of the spore are then mobilized into the developing appressorium, a

septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interatri ...

develops at the neck of the appressorium, and the germ tube and spore collapse and die. As the appressorium matures, it becomes firmly attached to the plant surface and a dense layer of

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

is laid down in the appressorium wall, except across a pore at the plant interface. Turgor pressure increases inside the appressorium and a penetration

emerges at the pore, which is driven through the

plant cuticle A plant cuticle is a protecting film covering the outermost skin layer (epidermis) of leaves, young shoots and other aerial plant organs (aerial here meaning all plant parts not embedded in soil or other substrate) that have no ''periderm''. The ...

into the underlying

epidermal The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...

cells. The osmotic pressure exerted by the appressorium can reach to even 8 MPa, what enables them to puncture the plant cuticle. This pressure could be achieved solely by the presence of melanin-pigmented cell wall, that is impermeable to compounds larger than water molecules, therefore the highly concentrated ions cannot leak out from it.

Formation

The attachment of a fungal

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

on the surface of the host plant is the first critical step of infection. Once the spore is hydrated, an adhesive

mucilage Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of m ...

is released from its tip. During

germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...

, mucilaginous substances continue to be extruded at the tips of the

, which are essential for germ tube attachment and appressorium formation. Spore adhesion and appressorium formation is inhibited by

hydrolytic enzyme Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some common examples of hydrolase enzymes are este ...

s such as α-

mannosidase Mannosidase is an enzyme which hydrolyses mannose. There are two types: * alpha-Mannosidase * beta-Mannosidase A deficiency is associated with mannosidosis Mannosidosis is a deficiency in mannosidase, an enzyme. There are two types: * Alpha-manno ...

, α-

glucosidase Glucosidases are the glycoside hydrolase enzymes categorized under the EC number 3.2.1. Function Alpha-glucosidases are enzymes involved in breaking down complex carbohydrates such as starch and glycogen into their monomers. They catalyze t ...

, and

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

, suggesting that the adhesive materials are composed of

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

s. Germination is also inhibited at high spore concentrations, which might be due to a lipophilic self inhibitor. Self inhibition can be overcome by hydrophobic wax from rice leaf. image:Uromyces appendiculatus, germ tube and appressorium, 5363967.jpg, left, ''Uromyces appendiculatus'',

and appressorium In response to surface signals, the germ tube tip undergoes a cellular differentiation, cell differentiation process to form a specialized infection structure, the appressorium. Frank B. (1883), in 'Ueber einige neue und weniger bekannte Pflanzenkrankheiten', coined the name "appressorium" for the adhesion body formed by the bean pathogen ''

Gloeosporium lindemuthianum ''Colletotrichum lindemuthianum'' is a fungus which causes anthracnose, or black spot disease, of the common bean plant (''Phaseolus vulgaris''). It is considered a hemibiotrophic pathogen because it spends part of its infection cycle as a biotro ...

'' on the host surface. Appressorium development involves a number of steps: nuclear division, first septum formation, germling emergence, tip swelling and second septum formation. Mitosis first occurs soon after surface attachment, and a nucleus from the second round of mitosis during tip swelling migrates into the hooked cell before septum formation. A mature appressorium normally contains a single nucleus. The outside plasma membrane of the mature appressorium is covered by a melanin layer except at the region in contact with the substratum, where the penetration peg, a specialized hypha that penetrates the tissue surface, develops. Cellular glycerol concentration sharply increases during spore germination, but it rapidly decreases at the point of appressorium initiation, and then gradually increases again during appressorium maturation. This glycerol accumulation generates high turgor pressure in the appressorium, and melanin is necessary for maintaining the glycerol gradient across the appressorium cell wall.

Initiation

Appressoria are induced in response to physical cues including surface hardness and hydrophobicity, as well as chemical signals of aldehydesZhu, M., et al. (2017).
Very-long-chain aldehydes induce appressorium formation in ascospores of the wheat powdery mildew fungus ''Blumeria graminis''.
Fungal biology 121(8): 716-728. https://doi.org/10.1016/j.funbio.2017.05.003 exogenous

cAMP Camp may refer to: Outdoor accommodation and recreation * Campsite or campground, a recreational outdoor sleeping and eating site * a temporary settlement for nomads * Camp, a term used in New England, Northern Ontario and New Brunswick to descri ...

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...

, the host's

ripening Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make the ...

hormone and the plant

cutin Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, which assist in cuticle stru ...

monomer

hexadecanoic acid Palmitic acid (hexadecanoic acid in IUPAC nomenclature) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms.Gunstone, F. D., John L. Harwood, and Albert J. Dijkstra. The Li ...

Long chain fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...

s and the

tripeptide A tripeptide is a peptide derived from three amino acids joined by two or sometimes three peptide bonds. As for proteins, the function of peptides is determined by the constituent amino acids and their sequence. The simplest tripeptide is glycine ...

sequence

Arg Arg or ARG may refer to: Places *''Arg'' () means "citadel" in Persian, and may refer to: **Arg, Iran, a village in Fars Province, Iran **Arg (Kabul), presidential palace in Kabul, Afghanistan **Arg, South Khorasan, a village in South Khorasan P ...

Gly 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 ...

- Asp inhibit appressorium induction.

Rust fungi Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales (previously known as Uredinales). An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''Puccinia'', are currently a ...

only form appressoria at

stomata In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...

, since they can only infect plants through these pores. Other fungi tend to form appressoria over anticlinal cell walls, and some form them at any location.

References

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Mycologia ''Mycologia'' is a peer-reviewed scientific journal that publishes papers on all aspects of the fungi, including lichens. It first appeared as a bimonthly journal in January 1909, published by the New York Botanical Garden under the editorship of W ...

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