Life on Earth thrives with solar energy. Photosynthetic organisms such as vascular plants, microscopic and macroscopic algae and certain group of bacteria harvest the energy of sunlight and store it as chemical bond energy by synthesizing carbon-rich polymers (sugar molecules) using carbon dioxide and water molecules [1]. This energy conversion is critical for the sustainability of life on our planet as these energy-rich molecules are consumed by all heterotrophic organisms such as animals, bacteria, protozoa and fungi.

Light is one of the key signals which organisms use to orient themselves in order to survive. Generally, the visible light spectrum represents beams of light which the human eye can view or sense. Visible solar light contains seven different wavelengths from violet (350–400 nm), indigo (400–450 nm), blue (450–495 nm), green (490–570 nm), yellow (570–600 nm), orange (600–620 nm) to red light (620–750 nm). In addition to these, the far ends of visible light contain ultraviolet (UVC 200–280 nm, UVB 280–315 nm, UVA 315–400 nm) and infrared (IR 780 nm–1000 µm) light spectra, which can also be sensed by some organisms using special forms of photoreceptors. These extreme spectra, particularly UVC and UVB light, pose great risk to all kinds of life forms, as they cause production of reactive oxygen species and damage the genetic material DNA by forming pyrimidine dimers [2]. Therefore, seeing the light is necessary not only for orientation, but also for preparation for stress conditions.

Fungi represent one of the most diverse life forms, ranging from unicellular yeast to multicellular filamentous and mushroom fungi. These versatile organisms are friends and foes of mankind since they are used as food sources, food additives, fermentation agents, producers of enzymes and sources of bioactive compounds and they are plant, animal and human pathogens [3]. Fungi play important roles in sustainability of global resources by contributing to nutrient recycling and forming mycorrhiza with forest components.

Light signals help fungi to regulate their morphology and physiology. For example, growth, metabolism, development, germination, sexual fruit body formation and production of secondary metabolites and even pathogenicity can be controlled by illumination conditions ^{[4][5][6][7][8][9][10][11][12]}[4,5,6,7,8,9,10,11,12]. Some fungi such as Phycomyces blakesleeanus even show a clear phototropism towards light, which is very characteristic of light response of plants [13]. Certain fungal light responses such as phototropism or fruit body formation are very overt. However, some responses such as changing metabolism require more investigation to distinguish slight effects of light on these processes. The fungal eye is much more sensitive to low light intensity and even the light intensities which a human eye cannot detect (10⁻⁹ mol of photons m⁻² can be sensed by fungal eye). Since early studies of fungal light responses, the main phenomena researchers investigated was the perception of light signals by fungi and their translation into morphological, physiological, and metabolic responses. Photoreceptors are protein molecules that capture photons using their light-absorbing organic chemical groups named chromophores [14]. Absorption of photons by the chromophore generates conformational changes in the photoreceptor protein, turning it into a signalling molecule which initiates a signal transduction towards the nucleus [15].

Although the studies on physiological, physical, molecular biology and biochemical aspects of fungal light reception and signalling shed light on tremendous amount of information over the last century, light sensing and light signalling are multidimensional processes, which require further mechanistic studies. Many genetic studies, morphological and physiological observations were carried out in different plant and human pathogenic fungi. Most mechanistic knowledge comes from a few filamentous fungi such as Aspergillus nidulans and Neurospora crassa.

2. Fungal Light Responses

Fungi establish different life forms throughout their life cycle. It often starts with germination of a spore which can be either an asexual or a sexual spore depending on the fungal species. Spores swell by taking up nutrients and lead to formation of germlings which further utilize nutrients and develop web-like mycelia which represent fungal vegetative life forms (Figure 1). These highly branched mycelia can differentiate into asexual or sexual organs, which finally lead to asexual or sexual spores. Along with these developmental processes, fungi also coordinate their primary and secondary metabolism ^[11][16][11,20] whereby production of certain pigments and bioactive metabolites is also associated with development ^[17][18][21,22]. Undoubtedly, light has a big impact on all these developmental and metabolic processes. For example, light often has a negative impact on germination of fungal spores which causes delays in the germination process. Spore germinations of most Aspergilli, such as the model organism A. nidulans, human pathogen Aspergillus fumigatus, and even some industrial strains of Aspergillus oryzae, are delayed or reduced by extended illumination (Figure 1) ^{[19][20][21][22]}[17,23,24,25]. This phenomenon is not only specific to Aspergilli, but can also be seen in other genera including Fusarium ^[20][23], not only at the germination stage; elongation and extension of hyphae are also highly responsive to illumination (Figure 1). While light suppresses hyphal growth in N. crassa, on the other hand, it promotes carotenogenesis [5]. Likewise, hyphal growth of biocontrol agent Trichoderma atroviride is also negatively influenced by light exposure ^[23][26]. Fungi produce different pigments such as carotenoids and melanin whose biosynthesis is triggered by a light signal. These pigments are a part of an important protection mechanism against the harmful effects of UV light. They often absorb excessive energy of light and quench reactive oxygen species formed under light. Promotion of carotenoid and melanin formation by light have been shown in a range of organisms, including Neurospora, Fusarium, Aspergillus and Phycomyces ^[24][25][27,28]. The switch from vegetative growth to asexual or sexual development is also highly responsive to light and controlled by illumination regimes (Figure 1). Although light inhibits initial germination of spores, light often promotes asexual spores (conidiogenesis) in most fungi such as Aspergillus, Neurospora, Trichoderma, and Fusarium, whereas production of sexual fruit bodies of some Mucorales and Basidiomycota members also requires light ^[13][26][13,29]. Some other fungi such as A. nidulans need darkness for fruit body formation (Figure 1) ^[11][27][11,30]. In addition to developmental decisions, light also controls production of secondary metabolites. For example, production of the carcinogenic polyketide mycotoxin sterigmatocystin is inhibited by light exposure in A. nidulans, suggesting that from morphogenesis to metabolism, light signalling plays important roles in fungal world ^[17][21]. Why have all these fungal species adopted light-dependent development? Although light effects on development seem to be complicated as light inhibits spore germination while it promotes production of spores, there is a fundamental logic behind these phenomena. One of the fundamental functions of fungal photoreception is self-defense against the harmful effects of UV light. Fungal spores are densely packed with protective pigments and osmolytes which protect the next generation of fungi from stressors such as UV light and oxidative stress ^[28][29][30][31,32,33]. The second fundamental function of photoreception is possibly to find optimum conditions or time for spore dispersal ^[31][32][34,35]. Asexual sporulation is a default mode of sporulation in fungi, which is important for the survival of a species among competitors, which requires less energy than sexual sporulation ^[33][36]. Fungi often live in soil which is covered by organic matter preventing spore dispersal. Prior to sporulation, fungal mycelia show a positive phototropism by growing its spore-carrying structures towards the air, and light induces production of sporulation structures, which allows the fungus to initiate its default reproduction mode in order to facilitate the dispersal of spores under good weather ^[32][35]. However, this is only one side of the story since there are other responses which cannot be explained in simple ecological and evolutionary models. Therefore, these fungal responses require more detailed studies with their ecological explanations.