Trichothecium roseum

Trichothecium roseum is a fungus in the division Ascomycota first reported in 1809.^[1] It is characterized by its flat and granular colonies which are initially white and develop to be light pink in color.^[1] This fungus reproduces asexually through the formation of conidia with no known sexual state.^[1] Trichothecium roseum is distinctive from other species of the genus Trichothecium in its characteristic zigzag patterned chained conidia.^[2] It is found in various countries worldwide and can grow in a variety of habitats ranging from leaf litter to fruit crops.^[2] Trichothecium roseum produces a wide variety of secondary metabolites including mycotoxins, such as roseotoxins and trichothecenes, which can infect and spoil a variety of fruit crops.^[1] It can act as both a secondary and opportunistic pathogen by causing pink rot on various fruits and vegetables and thus has an economical impact on the farming industry.^[1] Secondary metabolites of T. roseum, specifically Trichothecinol A, are being investigated as potential anti-metastatic drugs. Several agents including harpin, silicon oxide, and sodium silicate are potential inhibitors of T. roseum growth on fruit crops.^[3][4][5] Trichothecium roseum is mainly a plant pathogen and has yet to show a significant impact on human health.^[1]

History and classification

The genus Trichothecium is small and heterogeneous comprising seventy-three recorded species.^[1] This genus was first reported in 1809.^[1] The main members of the genus include Trichothecium polybrochum, Trichothecium cystosporium, Trichothecium pravicovi, and Trichothecium roseum.^[1] Trichothecium roseum has morphologically different conidiophores and conidia than the other three main species, which made development of these features the center of extensive study throughout the years.^[1] Since Trichothecium fungi lack a sexual phase, systematic classification was not uniform following their discovery.^[1] These fungi were initially grouped into Fungi imperfecti under the form classification Deuteromycetes.^[1] In 1958, Tubaki expanded Hughes’ classification of soil Hyphomycetes, part of the form class of Fungi imperfecti, by adding a ninth section in order to accommodate T. roseum and its unique conidial apparatus.^[6][7] Trichothecium has now been classified under the class Sordariomycetes.^[1] A recent classification has placed Trichothecium under the phylum Ascomycota since they produce conidial stages that are similar to the perfect fungi.^[1]

Trichothecium roseum conidiophore depicted in line drawing from Popular Science Monthly Vol 9 (1876).

Morphology

Trichothecium roseum colonies are flat, granular, and powdery in appearance.^[1][2] The color of the colonies appears to be white initially and develop into a light pink to peach color.^[1] The genus Trichothecium is characterized by its pinkish colored colonies.^[8]

Conidiophores of T. roseum are usually erect and are 200-300μm in length.^[9] They arise singly or in loose groups.^[1] Conidiophores are simple hyphae,^[10] which are septate in their lower half,^[6] and bear clusters of conidia at the tip.^[2] These conidiophores are indistinguishable from vegetative hyphae until production of the first conidium.^[1] Conidium development is distinctive^[2] and was first described by Ingold in 1956.^[6] Conidia arise as blowouts from the side of the conidiophore apex which is thus incorporated into the base of each spore.^[6] After the first conidium is blown out, before it matures, the apex of the conidiophore directly below blows out a second conidium from the opposite side.^[6] Conidia are pinched out from the conidiophore one after another in alternating directions in order to form the characteristic zigzag patterned chain.^[1] Conidia of T.roseum (15-20 × 7.5-10 μm)^[9] are smooth and clavate.^[1] Each conidium is two celled with the apical cell being larger than the curved basal cell.^[1] Conidia are light pink and appear translucent under the microscope.^[1] They appear a more saturated pink colour when grown in masses in culture or on the host surface.^[1]

Growth and physiology

Trichothecium roseum reproduces asexually by the formation of conidia with no known sexual stage.^[1] Trichothecium roseum is relatively fast-growing as it can form colonies reaching 9 cm (4 in) in diameter in ten days at 20 °C (68 °F) on malt extract agar.^[8] This fungus grows optimally at 25 °C (77 °F) with a minimum and maximum growing temperature of 15 °C (59 °F) and 35 °C (95 °F) respectively.^[8] Trichothecium roseum can tolerate a wide pH range but grows optimally at a pH of 6.0. Sporulation occurs rapidly at pH 4.0-6.5 and a combination of low temperature (15 °C (59 °F)) and high glucose concentration can increase the size of conidia.^[8] Treatment of T. roseum with colchicine increases the number of nuclei in conidia, growth rate, and biosynthetic activities.^[8] There are a variety of sugars that T. roseum can utilize including D-fructose, sucrose, maltose, lactose, raffinose, D-galactose, D-glucose, arabinose, and D-mannitol.^[8] Good growth also occurs in the presence of various amino acids including L-methionine, L-isoleucine, L-tryptophan, L-alanine, L-norvaline, and L-norleucine.^[8]

Secondary metabolites

Trichothecium roseum can produce numerous secondary metabolites that include toxins, antibiotics, and other biologically active compounds.^[1] Diterpenoids produced include rosolactone, rosolactone acetate, rosenonolactone, desoxyrosenonolactone, hydroxyrosenonolactones, and acetoxy-rosenonolactone. Several sesquiterpenoids are also produced by T. roseum including crotocin, trichothecolone, trichothecin, trichodiol A, trichothecinol A/B/C, trichodiene, and roseotoxin.^[1][8][11]

Biomedical applications

Trichothecium roseum was found to antagonize pathogenic fungi, such as Phyricularia oryzae and Phytophthora infestans, in vitro.^[12] It was suggested that the antifungal compound trichothecin was the main contributor to this action.^[12] In other studies trichothecinol B isolated from T. roseum displayed modest antifungal activity against Cryptococcus albidus and Saccharomyces cerevisiae.^[13]

Various studies have indicated that Trichothecinol A isolated from T. roseum strongly inhibited TPA-induced tumour promotion on mouse skin in carcinogenesis tests and therefore may be valuable for further investigation as cancer preventive agent.^[13][14][15] Anti-cancer studies have also shown that Trichothecinol A significantly inhibits cancer cell migration and therefore can be developed as a potential new anti-metastatic drug.^[15]

Habitat and ecology

Trichothecium roseum is a saprophyte ^[10] and is found worldwide.^[8] It has been found in soils in various countries including Poland, Denmark, France, Russia, Turkey, Israel, Egypt, the Sahara, Chad, Zaïre, central Africa, Australia, Polynesia, India, China, and Panama.^[8] Known habitats of T. roseum include uncultivated soils, forest nurseries, forest soils under beech trees, teak, cultivated soils with legumes, citrus plantations, heathland, dunes, salt-marshes, and garden compost.^[8] Commonly, this fungus can be isolated from the tree leaf litter of various trees including birch, pine, fir, cotton, and palm.^[8] It has also been isolated from several food sources such as barley, wheat, oats, maize, apples, grapes, meat products, cheese, beans, hazelnuts, pecans, pistachios, peanuts, and coffee.^[10] Levels of T. roseum in foods other than fruits are generally low.^[10]

Plant pathology

There are approximately two hundred twenty-two different plant hosts of T. roseum found worldwide.^[1] Trichothecium roseum causes pink rot on various fruits and vegetables.^[1] It is considered both a secondary and opportunistic pathogen since it tends to enter the fruit/vegetable host through lesions that were caused by a primary pathogen.^[1] Disease caused by this fungus is characterized by the development of white powdery mold that eventually turns pink.^[1] Antagonistic behaviours of T. roseum with certain plant pathogenic fungi was reported by Koch in 1934.^[16] He started that T. roseum actively parasitized stroma of Dibotryon morbosum which causes black knot disease in cherry, plum, and apricot trees.^[16]

Apple disease

Trichothecium roseum is known to produce pink rot on apples particularly following an apple scab infection caused by Venturia inaequalis.^[1] Studies have shown that roseotoxin B, a secondary metabolite of T. roseum, can penetrate apple peels and cause lesions.^[17] Trichothecium roseum also causes apple core rot which is a serious problem in China.^[18] Core rot not only causes economic loss but it is also associated with high levels of mycotoxin production.^[18] There have been reports of the presence of trichothecenes, specifically T-2 toxin, in infected apples in China.^[18] T-2 toxin has the highest toxicity of the trichothecenes and poses a threat to individuals who consume these infected apples due to its carcinogenicity, neurotoxicity, and immunotoxicity.^[18]

Grape disease

Trichothecium roseum was identified, along with Acremonium acutatum, as the two strains of pathogenic fungi which caused white stains on harvested grapes in Korea.^[19] The presence of mycelia on the surface of the grapes resulted in a white stained, powdery mildew appearance.^[19] Trichothecium roseum was identified using fungal morphology and nucleotide sequencing by PCR.^[19] It appears as though the fungus covers the surface of the grape only and does not penetrate into the tissue.^[19] This stain lowers the quality of the grapes and causes serious economic losses.^[19]

Trichothecin, trichothecolone, and rosenonolactone, which are secondary metabolites of T. roseum, were detected in wines.^[20] Presence of small quantities of trichothecin can inhibit alcohol fermentation.^[20] Trichothecium roseum rot has been reported to be increasing in wineries in Portugal.^[20] In this case, T. roseum appeared to grow over rotten grapes that were infected with gray rot.^[20] Mycotoxins were only detected in wines that were made with grapes that had gray rot and thus these toxins may be indicators of poor quality grapes.^[20] Grape contamination by T. roseum appears to be prominent in temperate climates.^[20]

Other fruit disease

Cases of T. roseum pink rot have been reported on numerous other fruits, however detailed studies have yet to be pursued.^[1] Pink T. roseum rot has been reported on tomatoes in Korea and Pakistan.^[21][22] It also causes pink rot in muskmelons and watermelons in Japan, the United States, South America, India, and the United Kingdom.^[1] Trichothecium roseum is reported to grow also on bananas and peaches.^[1]

Prevention of plant disease

Preventative measures can be taken to avoid growth of T. roseum in fruit crops.^[23] These include ensuring adequate ventilation in the storage facility, avoiding injuring and bruising the fruit, and ensuring adequate storage temperatures.^[23] Pre- and postharvest applications have been suggested as measures to control T. roseum production on fruit crops.^[1] In particular, studies have been done on testing various compounds to prevent T. roseum growth on several melon types.^[3][4][5] Harpin was inoculated on harvested Hami melons and caused significantly reduced lesion diameter and thus decreased T. roseum growth.^[3] Silicon oxide and sodium silicate also reduced the severity of pink rot and lesion diameter in harvested Hami melons.^[4] Pre-harvest inoculation of harpin on muskmelons decreased the amount of pink rot caused by T. roseum on harvested melons.^[5]

References

External links

• Trichothecium roseum in MycoBank.

• Trichothecium roseum in Index Fungorum

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