# Caroenoid Biosynthesis in _Fusarium_
## 4 Regulation by Light
As supporting evidence, the accumulation of phytoene in the albino _carB_ mutant SG43 is still dependent on light [22], and the mutants of gene _carX_[38], involved in retinal formation, or those of the rhodopsin genes _carO_[59] and _opsA_[40] exhibit a full carotenoid photoinduction. In _N. crassa_, a fungus with a similar action spectrum for light-induced carotenoid biosynthesis [60], the photoinduction is totally dependent on the White Collar complex, consisting of the photoreceptor WC-1 and its partner WC-2 (reviewed by [61]). However, the mutants of the _wc-I_ orthologous genes of _F. fuijikuroi_ (_uvcoA_[62]) and _F. oxysporum_ (_wc1_[63]) conserve to different extents a detectable carotenoid photoinduction under continuous illumination. The targeted mutation of the orthologous _wc-1_ and _wc-2_ genes in _F. graminearum, fgwc-1_ and _fgwc-2_, result in a paler pigmentation of the surface colonies under light, but the levels of carotenoids either in the wild type and the mutants were not chemically determined [64]. The analysis of carotenoid accumulation on surface cultures revealed a two-stage response in _F. fuijikuroi_, with a first rapid increase dependent on WcoA and a slower subsequent carotenoid accumulation depending on another flavin photoreceptor, the DASH cryptochrome CryD [65]. The photoactivity of this flavin photoreceptor has been experimentally demonstrated [66], and its participation as a second photoreceptor explains the maintenance of photoinduced carotenoid accumulation in the _wcoA_ mutants under constant illumination. The photoreceptors WcoA and CryD are not only involved in the regulation of carotenogenesis, as indicated by the alteration in the production of other pigments and secondary metabolites in their corresponding mutants [62; 67], even in the dark in the case of WcoA. The photoinduction of _Fusarium_ carotenogenesis results from a rapid increase in the transcript levels of most of the structural genes. _Northern_ blot experiments in _F. fujikuroi_ showed similar induction kinetics for the four genes of the _car_ cluster, _carRA_, _carB_, _carO_[59], and _carX_[38], as well as _carT_[42]. The transcripts reach maximal levels after one hour of exposure to light and decay afterwards, a down-regulating phenomenon known as photoadaptation. This photoinduction pattern has been confirmed by RT-qPCR approaches (Figure 2), and similar results have been obtained in _F. oxysporum_[68] and _F. verticilloides_[69]. The transcriptional photoresponse is similar to the one exhibited by the orthologous genes in _N. crassa_ (reviewed by [70]), with the exception of the GGPP synthase gene _al-3_, that exhibits in this fungus a strong photoinduction while its _F. fujikuroi_ ortholog _ggs1_ is hardly affected by light [26]. Our recent RNA-seq data have revealed however a significant photoinduction of _ggs1_ mRNA (Figure 3). A minor photoresponse was exhibited by the gene _carD_ from _F. fujikuroi_[47], corroborated by the RNA-seq data (Figure 3), while no photoinduction was detected in the case of _ylo-1_ from _N. crassa_[48], both genes encoding the ALDH enzymes responsible for the last reaction for NX production (Section 3). Therefore, in _F. fujikuroi_, the whole NX biosynthetic pathway is regulated by light. Figure 2: Photoinduction of carotenogenesis in _F. fujikuroi_. (**A**) Aspect of 7-day old colonies of the wild type IMI58289 grown on minimal medium in the dark or under continuous illumination; (**B**) Example of the kinetics of mRNA accumulation for the genes _carRA_, _carB_, and _carT_ in the wild type FKMC1995. Transcript levels were determined by RT-qPCR (real time quantitative PCR) and referred to those of \(\beta\)-tubulin gene. Levels in the dark for each gene were taken as 1 (adapted from [65]). Figure 3: Effect of light and _carS_ mutation on the expression levels for the _Fusarium_ genes involved in carotenoid metabolism.