# Production of Fungal Pigments: Molecular Processes and Their Applications
## 3 Stress Factors and Fungal Pigment Production  
Pigments are often produced to perform ecological roles in response to environmental stresses, such as ultraviolet (UV) light, ionizing radiation, oxidizing agents, nutrient deprivation, hypersaline, and host immunoreactivity. Rather than producing new compounds to build up fungal cells (primary metabolism), these fungi start to biosynthesize secondary metabolites manifested as pigmentation. The yield of the pigments might have originated from the defense demands of the fungi to fulfill the protective role, preventing their mycelia from being hydrolyzed by enzymes produced by other microbes [82]. It could also be a by-product of entering dormancy. In addition, in view of known antioxidant characteristics of these lipophilic pigments, carotenoid production in fungi has been suggested as a natural mechanism to protect against photo-oxidative damage in light-intensive habitats [83; 84]. In _Fusarium_ spp., light has been shown to stimulate the production of carotenoids. In both _Fusarium aquaductuum_ and _F. fujikuroi_, when illumination was supplied in the initially dark-grown culture, carotenoids accumulated for several hours upon light onset, exhibiting a typical orange pigmentation [85; 86]. _F. fujikuroi_ can produce carotenoids in addition to the phytohormone gibberellin [87] and the polyketide bikaverin [61; 88]. Indeed, light induction of carotenoid biosynthesis has been commonly found in the genus _Fusarium_, being first reported in _F. aquaductuum_ in 1969 [89]. Investigation concerning the effects of light on the photoinduction in _F. aquaductuum_[89] has unveiled that the levels of produced carotenoids depend on the logarithm of the incident light over a 100-fold range, in which the reciprocity law holds true over a broad spectrum of light intensity and time. In the case of _F. aquaductuum_, the accumulation of carotenoid is induced only by light with a wavelength shorter than 520 nm [89]. Early studies with _F. aquaductuum_ regarding photo-regulated carotenogenesis were subsequently extended to _F. fujikuroi_ and _F. oxysporum_. The major carotenoid produced by _Fusarium_ spp. is neurosporaxanthin, an acidic apocartenoid converted from geranylger-anyl pyrophosphate (GGPP) by the sequential activity of four enzymes, encoded by the genes _carRA_, _carB_, _carT_, and _carD_. Upon exposure to light, the mycelia rapidly displayed pigmentation due to the formation of carotenoids, which started to accumulate 1 h after illumination and reached a maximum about 16 h afterwards [86], exhibiting the pattern of photo-induction of carotenoids similar to that of _N. crassa_[90]. It is widely recognized that photoinduction of carotenogenesis in _Fusarium_ species is attributed to a transcriptional elevation of most of the structural genes involved in the carotenoid synthesis [91]. Fungi often utilize light to perceive high temperature, UV radiation (genotoxic stressor), and the soil/air interface for their survival and proliferation (i.e., spore dispersal). Fungi respond to light by a sophisticated photosensory system consisting mainly of photoreceptors. Exposure to light, especially blue light, is regarded as the major triggering factor for carotenogenesis [92; 93]. The blue light receptors in fungi are composed of VIVID (VVD) protein, the white collar complex (WCC), and the cryptochrome (CRY) [94]. The Vvd-null mutant of _N. crassa_ was observed to have enhanced carotenoid accumulation as manifested by intensely pigmented mycelia under constant light. In contrast to _N. crassa_, a mutation of VvdA, an orthologue of _N. crassa_ Vvd, in _F. fujikuroi_, might result in paler pigmentation. Further studies have unveiled that the VvdA-regulated carotenogenesis in _F. fujikuroi_ is a biphasic photo-response, which bursts in the early phase but decays in the subsequent phase [95].