An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect 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 equilibri ...

pressure capable of punching through even Mylar. Following spore attachment and germination on the host surface, the emerging germ tube perceives physical cues such as surface hardness and hydrophobicity, 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 ...

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 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 into the underlying epidermal 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, ...

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

, mucilaginous substances continue to be extruded at the tips of the germ tube, 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, 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 glyco ...

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. left, ''Uromyces appendiculatus'', germ tube and appressorium In response to surface signals, the germ tube tip undergoes a

cell differentiation Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...

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 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 ...

, the host's ripening hormone and the plant cutin monomer hexadecanoic acid. Long chain fatty acids and the tripeptide sequence Arg- Gly- Asp inhibit appressorium induction. Rust fungi 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 b ...

, since they can only infect plants through these pores. Other fungi tend to form appressoria over

anticlinal Anticlinal may refer to: *Anticline, in structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. *Anticlinal, in stereochemistry, a torsion angle between 90° to 150°, and –90° to –150°; see Alkane_st ...

cell walls, and some form them at any location.

References

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