THEDARKSIDE OF THEMYCELIUM: Melanins of Phytopathogenic Fungi

Abstract

Melanins are darkly pigmented polymers that protect organisms against environmental stress. Even when not directly involved in pathogenesis, fungal melanin is likely required by melanizing phytopathogens for survival in the environment. However, some phytopathogenic fungi that produce melanized appressoria for host invasion require appressorial melanogenesis for pathogenicity. Much less is known about the role melanins play in pathogenesis during infection by other phytopathogens that do not rely on appressoria for host penetration. Here we focus on one such phytopathogenic fungus, Gaeumannomyces graminis var. tritici, the etiologic agent of the devastating root disease of cereals, take-all. This fungus is lightly pigmented in culture, but requires melanin biosynthesis for pathogenesis, perhaps to produce melanized, ectotrophic macrohyphae on roots. However, the constitutively melanized, asexual Phialophora anamorph of G. graminis var. tritici is nonpathogenic. In addition, melanization of G. graminis var. graminis is not required to produce root disease on its rice host. Explanations for these apparent contradictions are suggested, as are other functions for the melanins of phytopathogenic fungi.

Microscopic Investigation on Fungal Pigment Formation and its Morphology in Wood Substrates

“…Silver also precipitates in hyphal septa, which suggests that septa also melanize, and silver precipitation does not occur near hyphal tips or in tricyclazole-treated hyphae or their septa. These findings are in agreement with previous research by Henson et al [21] and Butler and Lachance [17], who also observed the binding of Azure A dye to the septa of melanized hyphae of G. graminis. A copper sulfide-silver staining technique for fungal melanin detection in electron microscopy was further developed by Butler et al [3] and proved to be more efficient in labeling melanin produced in the black yeast Phaeococcomyces, in the G. graminis var.…”

“…The morphological variation of melanin assembly in different wood substrates reported in this research might explain the various chemical structures previously reported for fungal melanin [33,[39][40][41] as well as for the multiple genes identified in melanin biogenesis [21,[42][43][44][45]. Melanin deposits produced in vitro by T. versicolor in sugar maple tended to be attached to the wood cell walls, while in the same substrate pretreated with catechol, those bindings seemed sparse (Fig.…”

Antibiosis and dark-pigments secretion by the phytopathogenic and environmental fungal species after interaction in vitro with a Bacillus subtilis isolate

“…However, future studies are needed to investigate this hypothesis. In many instances, microbial resistance to hostile environment is directly associated with dark pigmented polymers (Henson et al, 1999, Jacobson, 2000. Melanized cells are more resistant to hydrolytic enzymes, oxidative radicals, high temperatures, antifungals, and host defenses (Nosanchuck andCasadevall, 2003, Henson et al, 1999).…”

Purification and characterisation of a sulphur rich melanin from edible mushroom Termitomyces albuminosus Heim

“…Melanisation in fungi mostly seen in hyaline hyphae, sclerotia, appressoria, reproductive structures or conidia (Cordero and Casadevall 2017). Hyphal melanin is often found to be deposited as the outermost layer or internal layer in cell wall only with age or other stress (Bell and Wheeler 1986; Henson et al 1999; Butler et al 2001). Melanogenesis in pathogenic fungi plays a key role in pathogenesis in species such as Cryptococcus neoformans (Polacheck and Kwon-Chung 1988), Gaeumannomyces graminis var.…”

“…Melanogenesis in pathogenic fungi plays a key role in pathogenesis in species such as Cryptococcus neoformans (Polacheck and Kwon-Chung 1988), Gaeumannomyces graminis var. tritici, Magnaporthe grisea, Alternaria alternata, Colletotrichum lagenarium, Cochliobolus heterostrophus (Henson et al 1999), Paecilomyces variotti (Babitskaya et al 2000a), Rhizoctonia solani (Chen et al 2015) and Aspergillus spp. (Babitskaya et al 2000a; Schmaler-Ripcke et al 2009; Gonçalves et al 2012; Pal et al 2013).…”