An appressorium is a specialized cell typical of many fungal

plant pathogen Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like orga ...

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 * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

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 o ...

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 equilib ...

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 stability, dimensional stability, transparency reflectivity, an ...

. 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 My ...

perceives physical cues such as surface hardness and

hydrophobicity In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly intermolecular force, repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to b ...

, as well as chemical signals including wax

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

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 language, Latin for ''something that encloses''; septa) is a wall, dividing a Body cavity, cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Hum ...

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 (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

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 cells. The osmotic pressure exerted by the appressorium can reach up to 8 MPa, which allows it to puncture the plant cuticle. This pressure is achievable due to a melanin-pigmented cell wall which is impermeable to compounds larger than water molecules, so the highly-concentrated ions cannot escape from it.

Formation

The attachment of a fungal

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

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, with the direction of their movement always opposite to that of the secretion of ...

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, ...

, 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 enzymes such as α- mannosidase, 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 the ...

, and

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

, suggesting that the adhesive materials are composed of

glycoprotein Glycoproteins are proteins which contain oligosaccharide (sugar) 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 a ...

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'' 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,

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 bond, carbon–carbon doub ...

, 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 t ...

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, the other being cutan. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, ...

monomer hexadecanoic acid. Long chain fatty acids 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. In terms of scientific investigati ...

sequence Arg- Gly- Asp inhibit appressorium induction.

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

only form appressoria at

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

, 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

{{Reflist, 30em, refs= {{cite journal , vauthors=Bourett TM, Howard RJ, year=1990 , title=''In vitro'' development of penetration structures in the rice blast fungus ''Magnaporthe grisea'' , journal=Canadian Journal of Botany , volume=68 , issue=2 , pages=329–42 , doi=10.1139/b90-044 {{cite journal , vauthors=Braun EJ, Howard RJ , s2cid=35667834 , year=1994 , title=Adhesion of fungal spores and germlings to host-plant surfaces , journal=Protoplasma , volume=181 , issue=1–4 , pages=202–12 , doi=10.1007/BF01666396 {{cite journal , vauthors=Correa A, Staples RC, Hoch HC , s2cid=8417737 , year=1996 , title=Inhibition of thigmostimulated cell differentiation with RGD-peptides in ''Uromyces'' germlings , journal=Protoplasma , volume=194 , issue=1–2 , pages=91–102 , doi=10.1007/BF01273171 {{cite journal , vauthors=Deising HB, Werner S, Wernitz M , title=The role of fungal appressoria in plant infection , journal=Microbes and Infection / Institut Pasteur , volume=2 , issue=13 , pages=1631–41 , year=2000 , pmid=11113382 , doi=10.1016/S1286-4579(00)01319-8, doi-access=free {{cite journal , vauthors=deJong JC, McCormack BJ, Smirnoff N, Talbot NJ, s2cid=205026525, year=1997, title=Glycerol generates turgor in rice blast , journal=Nature , volume=389 , issue=6648 , pages=244–5 , doi=10.1038/38418, bibcode=1997Natur.389..244D, doi-access=free {{cite journal , vauthors=Flaishman MA, Kolattukudy PE , title=Timing of fungal invasion using host's ripening hormone as a signal , journal=Proceedings of the National Academy of Sciences of the United States of America , volume=91 , issue=14 , pages=6579–83 , year=1994 , pmid=11607484 , pmc=44246 , doi=10.1073/pnas.91.14.6579, bibcode=1994PNAS...91.6579F , doi-access=free {{cite journal , vauthors=Gilbert RD, Johnson AM, Dean RA , year=1996 , title=Chemical signals responsible for appressorium formation in the rice blast fungus ''Magnaporthe grisea'' , journal= Physiological and Molecular Plant Pathology, volume=48 , issue=5 , pages=335–46, doi=10.1006/pmpp.1996.0027 {{cite journal , author1=Hegde Y , author2=Kolattukudy PE , year=1997 , title=Cuticular waxes relieve self-inhibition of germination and appressorium formation by the conidia of ''Magnaporthe grisea'' , journal= Physiological and Molecular Plant Pathology, volume=51 , issue=2 , pages=75–84 , doi=10.1006/pmpp.1997.0105 {{cite journal , title=Penetration of hard substrates by a fungus employing enormous turgor pressures , journal=Proceedings of the National Academy of Sciences , vauthors=Howard RJ, Ferrari MA, Roach DH, Money NP , year=1991 , volume=88 , pages=11281–84 , doi=10.1073/pnas.88.24.11281 , pmid=1837147 , issue=24 , pmc=53118, bibcode=1991PNAS...8811281H , doi-access=free {{cite journal , vauthors=Howard RJ, Valent B , title=Breaking and entering: host penetration by the fungal rice blast pathogen ''Magnaporthe grisea'' , journal=Annual Review of Microbiology , volume=50 , pages=491–512 , year=1996 , pmid=8905089 , doi=10.1146/annurev.micro.50.1.491 {{cite journal , vauthors=Lee YH, Dean RA , year=1993 , title=cAMP regulates infection structure formation in the plant-pathogenic fungus ''Magnaporthe grisea'' , journal=Plant Cell , volume=5 , issue=6 , pages=693–700 , url=http://www.plantcell.org/cgi/reprint/5/6/693 , format=PDF , doi=10.2307/3869811, pmid=12271080 , pmc=160306 , jstor=3869811 {{cite journal , vauthors=Ohtake M, Yamamoto H, Uchiyama T , year=1999 , title=Influences of metabolic inhibitors and hydrolytic enzymes on the adhesion of appressoria of ''Pyricularia oryzae'' to wax-coated cover-glasses , journal=Bioscience, Biotechnology, and Biochemistry , volume=63 , issue=6 , pages=978–82 , url=http://www.jstage.jst.go.jp/article/bbb/63/6/978/_pdf , format=PDF , doi=10.1271/bbb.63.978, pmid=27389332 , doi-access=free {{cite journal , vauthors=Shaw BD, Kuo KC, Hoch HC , year=1998 , title=Germination and appressorium development of ''Phyllosticta ampelicida'' pycnidiospores , journal=

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 ...

, volume=90 , issue=2 , pages=258–68 , url=http://www.cybertruffle.org.uk/cyberliber/59350/0090/002/0258.htm , doi=10.2307/3761301, jstor=3761301 , url-access=subscription {{cite journal , vauthors=Xiao JZ, Ohsima A, Kamakura T, Ishiyama T, Yamaguchi I , year=1994 , title=Extracellular glycoprotein(s) associated with cellular differentiation in ''Magnaporthe grisea'' , journal=Molecular Plant-Microbe Interactions , volume=7 , issue=5 , pages=639–44 , doi=10.1094/MPMI-7-0639 , url=http://www.apsnet.org/publications/mpmi/BackIssues/Documents/1994Articles/Microbe07-639.pdf Fungal morphology and anatomy