Light regulation of metabolic pathways in fungi

Abstract

Light represents a major carrier of information in nature. The molecular machineries translating its electromagnetic energy (photons) into the chemical language of cells transmit vital signals for adjustment of virtually every living organism to its habitat. Fungi react to illumination in various ways, and we found that they initiate considerable adaptations in their metabolic pathways upon growth in light or after perception of a light pulse. Alterations in response to light have predominantly been observed in carotenoid metabolism, polysaccharide and carbohydrate metabolism, fatty acid metabolism, nucleotide and nucleoside metabolism, and in regulation of production of secondary metabolites. Transcription of genes is initiated within minutes, abundance and activity of metabolic enzymes are adjusted, and subsequently, levels of metabolites are altered to cope with the harmful effects of light or to prepare for reproduction, which is dependent on light in many cases. This review aims to give an overview on metabolic pathways impacted by light and to illustrate the physiological significance of light for fungi. We provide a basis for assessment whether a given metabolic pathway might be subject to regulation by light and how these properties can be exploited for improvement of biotechnological processes.

Fig. 1

Model for the function of the white collar complex (WCC) in N. crassa (adapted from Dunlap and Loros 2004). Light (yellow sun) influences the transcription of frequency (frq), vivid (vvd), and white collar 1 (wc-1). Through an as yet unknown posttranscriptional mechanism, FRQ promotes synthesis or accumulation of WC-1 from existing wc-1 message. The components of the clock build a circadian feedback loop, in which WCC plays the central role. Arrows imply a positive regulation; lines ending with bars imply a negative one

Fig. 2

Model for the function and localization of VeA (adapted from Bayram et al. 2008b). a In light, VeA is predominantly localized in the cytoplasm and VelB supports asexual sporulation. LaeA shows low activity. b In darkness, an increased amount of VeA is imported into the nucleus by KapA

Fig. 3

Model for the VeA protein complex (adapted from Calvo 2008). VeA interacts with the phytochrome-like red light sensor FphA, where the chromophore-binding region in this protein is essential for maintenance of this interaction. FphA in turn interacts with the photoreceptor homolog LreB and hence connects VeA to the A. nidulans equivalent of the N. crassa WCC

Fig. 4

Biosynthetic pathway for β-carotene in the Mucorales and neurosporaxanthin in Neurospora. Conversion of β-carotene to abscisic acid (empty arrows) has only been shown in Cercospora rosicola, but in no other fungus (Assante et al. 1977)

Fig. 5

Schematic representation of the influence of light on glycolysis. Green diamonds indicate increased levels of the respective metabolite upon cultivation in light, and red diamonds reflect lower levels. For fructose-1,6-bisphosphate (yellow diamond), no alteration was found, and in case of white diamonds, no data on the levels of these metabolites in light were available. Red balloons indicate lower enzymatic activities of the respective enzyme