# Production of Fungal Pigments: Molecular Processes and Their Applications
## 2 Pigmented Fungi and Biosynthesis of Pigments
### Polykeptides  
Polykite-based pigments of fungal origins are abundantly produced by many fungi, including most filamentous ascomycete genera. Anthraquinones and naphthoquinones are representative classes of polyktetide-based pigments in fungi, displaying various colors [1]. The pigment class of anthraquinones is characterized by a polycyclic aromatic hydrocarbon structure stemming from the merger of three benzene rings. The diversity of anthraquinones results from the presence of different substituents, such as -OH, -CH3, -OCH3, and -CH2 OH, and from the reduction of carbonyl groups as well as double bonds in the benzene ring, allowing the formation of hydroxyanthraquinones. Anthraquinones can generate a broad scope of hues, from yellows to reds and even blue shades. Their wide spectrum of colors is attributed to their relatively short conjugated chromophores. They give weak yellows in simple unsubstituted forms, while allowing the attachment of substituents to generate extensive bathochromic shifts of the absorption maximum and, as a result, give reds and even blues. Anthraquinones including hydroxyanthraquinones (HAQNs) are commonly produced by fungi, such as _Aspergillus_ spp., _Eurotium_ spp., _Fusarium_ spp., _Dreschlera_ spp., _Penicillium_ spp., _Emericella purpurea, Culvularia lunata, Mycosphaerella rubella_, and _Microsporum_ spp. [1; 54]. Of note is Arpink Red(tm), the polyketide anthraquinone-based natural food colorant initially described in 2004 [55]. This colorant is commercially produced using the fungus _Penicillium_ _axilum_ var. _Armenia_. Like anthraquinones, naphthoquinone pigments are produced by many fungal species. Most naphthoquinones are colored, usually varying between yellow, orange, and brown [56]. It has been reported that the biosynthesis of naphthoquinone pigments in some _Fusarium_ species is triggered by environmental stresses, manifested under the conditions of growth inhibition or arrest [57]. Indeed, many fungal secondary metabolites, such as naphthoquinone pigments, are induced by factors of biotic and abiotic origin, including competition from other organisms, the presence of toxic compounds, etc. [58]. _Penicillium_ and the related genus _Talaromyces_ can produce naphthoquinone pigments and are widespread, requiring minimal conditions to grow. These fungi are aerobic, require minimal nutrients, and can grow quickly at room temperature. Thus, these genera are promising candidates for the biosynthesis of pigments in laboratory and industrial settings and are particularly relevant to the modulation of biomass or pigment production [9]. The synthesis of fungal polyketides requires polyketide synthase (PKS) and uses precursors such as acetyl-CoA and malonyl-CoA [59]. Characteristic polyketide pigments produced by fungi are known to be anthraquinones, hydroxyanthraquinones, and naphthoquinones [59]. Bikaverin, belonging to the naphthoquinone class, is a kind of bright red pigment present predominantly in the genus _Fusarium_, among which _Fusarium fujikuroi_ is a representative species [60]. In _F. fujikuroi_, bikaverin (viz. 6, 11-dihydroxy-3, 8-dimethoxy-1-methyl-benzo-xanthesin-7, 10, 12-trion) is produced via a polyketide biosynthetic pathway (Figure 3a). Genetic analyses with _F. fujikuroi_ have demonstrated that a gene cluster composed of six _bik_ genes, encoding a multifunctional polyketide synthase, is responsible for bikaverin biosynthesis [61]. Within the _bik_ gene cluster situated on chromosome 5 of _F. fujikuroi_, _bik_ 1-3 are recognized to be biosynthetic genes, _bik_ 4 and _bik_ 5 are the regulatory genes, and _bik_ 6 encodes a transporter of bikaverin (Figure 3b). Figure 3: Schematic representation of bikaverin biosynthetic route in _Fusarium fujikuroi_. (**a**) The preferred biosynthetic steps are indicated by bold arrows, and some possible conversions are indicated by dashed arrows. (**b**) Organization of bikaverin (bik) gene cluster in _F. fujikuroi_. Direction of transcription for each predicted gene is indicted by the arrows [62; 63].