## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Control of Secondary Metabolism
#### 4.6.2 Regulation of Secondary Metabolism by Light Spectrum  
The effect of the light spectrum on secondary metabolism was not studied as intensively as that of the light intensity (Table 10). Considering the metabolism of terpen, the regulation of FPS (isoprenoid synthesis) by light spectrum was shown in rice since BL enhanced the expression of the _FPS_ gene [241]. This effect is probably mediated by [PERSON] based on the enhanced _FPS_ transcript abundance of transgenic _Artemisia annua_ overexpressing the _AtCRY1_ gene [242]. The metabolism of another group of terpen, phytoenes, which are intermediates in the biosynthesis of carotenoids (tetraterpene derivatives containing oxygen), are synthesized from two molecules of geranylgeranyl pyrophosphate by phytoene synthase (PSY), was also affected by light spectrum [243]. PSY activity was enhanced by RL and WL, but not by FRL compared to darkness during mustard photomor-phogenesis [243]. Interestingly, PSY protein content was correlated negatively with PSY activity which indicates a negative feedback regulation [243]. The phytochrome-related control of the _PSY_ gene was demonstrated in _Arabidopsis_ since PIFI and PIF3 repressed it under RL illumination [244].  
The biosynthesis of flavonoids is generally activated under UV exposure [225]. Besides UV, BL induced FLS1, which catalyses the oxidation of flavanones to flavones, in parsley [245]. Interestingly, the effect of UV-B on the level of a flavonoid, quercetin 3-galactosidase was differently modified by RL and FRL in silver birch (_Betula pendula_ Roth) since it was higher after UV-B + RL treatment than after UV-B + FRL treatment [246]. In addition, the amount of two other phenolics, chlorogenic acid and a cinnamic acid derivative, was influenced by the FRL: RL ratio in this species. These observations indicate that the ratio of the various spectral ranges is important in the control of the metabolism of phenolics.  
The control of anthocyanin synthesis by light spectrum was shown in apple (_Malus domestica_) peel, where anthocyanin synthesis was increased by the UV range of sunlight through upregulation of the MdMYB10 transcription factor and key enzymes of anthocyanin biosynthesis: PAL, chalcone synthase (CHS), flavanone-3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), and anthocyanin synthase (ANS) [247]. Lettuce plants grown under a 3:1 ratio of RL to BL accumulated more anthocyanin, which was derived from the transcriptional regulation of the related biosynthesis genes [248].  
The level of indole alkaloids was also influenced by the light spectrum since RL increased the level of two of them, vindoline and catharanthine, in Madagascar periwnlike (_Catharanthus roseus_ L.) [249]. This RL control is mediated by the photoreceptor-responsive HY5 transcription factor, which can also coordinate the process with other metabolic pathways and the redox regulation through its corresponding target genes [250]. In addition, the amount of GPV was increased by red, blue, or FR monochromic lights in photomorphogenic _P. leiocarpa_ plants [239]. Although there are some studies available discussing the effect of various spectral ranges on the accumulation of alkaloids and the other discussed secondary metabolites, the underlying molecular regulatory mechanisms need further investigation.