Penicillium digitatum

Penicillium digitatum (/ˌpɛnɪˈsɪlɪəm/digitatum/) is a mesophilic fungus found in the soil of citrus-producing areas. It is a major source of post-harvest decay in fruits and is responsible for the widespread post-harvest disease in Citrus fruit known as green rot or green mould. In nature, this necrotrophic wound pathogen grows in filaments and reproduces asexually through the production of conidiophores. However, P. digitatum can also be cultivated in the laboratory setting. Alongside its pathogenic life cycle, P. digitatum is also involved in other human, animal and plant interactions and is currently being used in the production of immunologically based mycological detection assays for the food industry. Penicillium digitatum is a species within the Ascomycota division of Fungi. The genus name Penicillium comes from the word 'penicillus' which means brush, referring to the branching appearance of the asexual reproductive structures found within this genus. As a species, P. digitatum was first noted as Aspergillus digitatus by Christiaan Hendrik Persoon in 1794 who later adopted the name Monilia digitata in Synopsis methodica fungorum (1801). The synonym M. digitata can also be found in the writings of Elias Magnus Fries in Systema mycologicum (1832). However, the current binomial name comes from the writings of Pier Andrea Saccardo, particularly Fungi italici autographie delineati et colorati (1881). In nature, P. digitatum adopts a filamentous vegetative growth form, producing narrow, septate hyphae. The hyphal cells are haploid, although individual hyphal compartments may contain many genetically identical nuclei. During the reproductive stages of its life cycle, P. digitatum reproduces asexually via the production of asexual spores or conidia. Conidia are borne on a stalk called a conidiophore that can emerge either from a piece of aerial hyphae or from a soil-embedded network of hyphae. The conidiophore is usually an asymmetrical, delicate structure with smooth, thin walls. Sizes can range from 70–150 µm in length. During development, the conidiophore can branch into three rami to produce a terverticillate structure although biverticillate and other irregular structures are often observed. At the end of each rami, another set of branches called metulae are found. The number of metulae varies with their sizes ranging from 15–30 × 4–6 µm. At the distal end of each metula, conidium-bearing structures called phialides form. Phialides can range in shape from flask-shaped to cylindrical and can be 10–20 µm long. The conidia produced, in turn, are smooth with a shape that can range from spherical to cylindrical although an oval shape is frequently seen. They are 6–15 µm long and are produced in chains, with the youngest at the base of each chain. Each conidium is haploid and bears only one nucleus. Sexual reproduction in P. digitatum has not been observed. Penicillium digitatum can also grow on a variety of laboratory media. On Czapek Yeast Extract Agar medium at 25 °C, white colonies grow in a plane, attaining a velvety to deeply floccose texture with colony sizes that are 33–35 mm in diameter. On this medium, olive conidia are produced. The reverse of the plate can be pale or slightly tinted brown. On Malt Extract Agar medium at 25 °C, growth is rapid yet rare, forming a velvety surface. At first, colonies are yellow-green but ultimately turn olive due to conidial production. Colony diameter can range in size from 35 mm to 70 mm. The reverse of the plate is similar to that observed for Czapek Yeast Extract Agar medium. On 25% Glycerol Nitrate Agar at 25 °C, colony growth is planar yet develops into a think gel with colony size diameter ranging from 6–12 mm. The back of the plate is described as pale or olive. At 5 °C, 25% Glycerol Nitrate Agar supports germination and a colonial growth of up to 3 mm in diameter. This species fails to grow at 37 °C. On Creatine Sucrose Agar at 25 °C, colony size diameter ranges from 4 to 10 mm. Growth is restricted and medium pH remains around 7. No change on the back of the plate is noted. Growth on media containing orange fruit pieces for seven days at room temperature results in fruit decay accompanied by a characteristic odour. After 14 days at room temperature, the reverse is colourless to light brown. Penicillium digitatum is found in the soil of areas cultivating citrus fruit, predominating in high temperature regions. In nature, it is often found alongside the fruits it infects, making species within the genus Citrus its main ecosystem. It is only within these species that P. digitatum can complete its life cycle as a necrotroph. However, P. digitatum has also been isolated from other food sources. These include hazelnuts, pistachio nuts, kola nuts, black olives, rice, maize and meats. Low levels have also been noted in Southeast Asian peanuts, soybeans and sorghum. Penicillium digitatum is a mesophilic fungus, growing from 6–7 °C (43–45 °F) to a maximum of 37 °C (99 °F), with an optimal growth temperature at 24 °C (75 °F). With respect to water activity, P. digitatum has a relatively low tolerance for osmotic stress. The minimum water activity required for growth at 25 °C (77 °F) is 0.90, at 37 °C (99 °F) is 0.95 and at 5 °C (41 °F) is 0.99. Germination does not occur at a water activity of 0.87. In terms of chemicals that influence fungal growth, the minimum growth inhibitory concentration of sorbic acid is 0.02–0.025% at a pH of 4.7 and 0.06–0.08% at a pH of 5.5. Thiamine, on the other hand, has been observed to accelerate fungal growth with the effect being co-metabolically enhanced in the presence of tyrosine, casein or zinc metal. In terms of carbon nutrition, maltose, acetic acid, oxalic acid and tartaric acid support little, if any, growth. However, glucose, fructose, sucrose, galactose, citric acid and malic acid all maintain fungal growth. Production of ethylene via the Citric acid cycle has been observed in static cultures and is suggested to be connected to mycelial development. Addition of methionine inhibits such cultures but can be utilized for the production of ethylene following a lag phase in shake cultures. The production observed in shake cultures can be inhibited by actinomycin D and cycloheximide and modulated by inorganic phosphate. In addition, aminoethoxyvinyl glycine and methoxyvinyl glycine have been shown to inhibit both shake and static cultures. Production of mycotoxins or secondary metabolites by P. digitatum has not been observed although this species has been shown to be toxic to both shrimp and chicken embryos. With respect to fungicidal tolerance, there are known strains of P. digitatum resistant to various commonly used fungicides. Reports have been made concerning fungicides thiabendazole, benomyl, imazalil, sodium-o-phenylphenate as well as fungistatic agent, biphenyl, with no prior treatment required in the case of biphenyl. The mechanism of P. digitatum resistance to imazalil is suggested to lie in the over-expression of the sterol 14α-demethylase (CYP51) protein caused by a 199 base-pair insertion into the promoter region of the CYP51 gene and/or by duplications of the CYP51 gene.