## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Control of Secondary Metabolism
#### 4.6.1 Effect of Light Intensity on Secondary Metabolism  
Basically, terpenes (and their derivatives terpenoids, containing oxygen besides carbon and hydrogen) are the largest class of secondary products, and they are formed by the fusion of isoprene units [122]. Several enzymes of their synthesis are regulated by light conditions, the redox system, and the HY5 transcription factor (Table 10). Among terpenoids, quinones and chromatol have a role in the response to HL stress [219]. Light control of the biosynthesis of monoterpene precursors was shown in the Moth orchid (_Phalanopsis bellina_) since the promoter of the gene encoding geranyl diphosphate synthase (GPPS, catalysing the mentioned process) was reported to have a light-responsive element [220]. However, it did not have HY5-interactive elements; therefore, its control is independent of this central regulator of light response. Similar to monoterpene, the synthesis of triterpenes seems to be light-associated too, since the promoter of the _KcMS_ gene, encoding a triterpene synthase, contains light-responsive elements in the mangrove plant _Kandelia candel_[221]. Light control of farnesyl diphosphate synthase (FPS), which is the key enzyme in isoprenoid (which consists of one or more isoprene units) biosynthesis, was shown in plants, too. The overexpression of its mitochondrial isoform (FPS1L) in _Arabidopsis_, for instance, caused leaf chlorosis under HL, which might be triggered by H\({}_{2}\)O\({}_{2}\) accumulation [222]. Interestingly, shading enhanced FPS expression in _Aralia elata_ (Miq.) Seem, in contrast to squalene synthase (SQS) and squalene epoxidase (SQE), two other enzymes of the isoprenoid metabolic pathway. However, more excessive shading downregulated the expression of all three enzymes [223]. The effect of light on the isoprenoid pathway may be connected to the redox system since the SQE-deficient _dry2/sqe1-5 Arabidopsis_ mutant showed deficient ROS production and NADPH oxidase function [224]. Phenolics form a chemically diverse group of secondary metabolites with almost 10,000 individual substances, and one of their largest group consists of flavonoids [122]. The biosynthesis of flavonoids (flavones, flavonoids, isoflav, and anthocyanins) is affected by light and HY5 (Table 10) [225]. The connection of flavonoids with the redox system is based on their good ROS scavenging capacity [226]. However, there is little information available on the redox and light dependency of the biosynthesis of non-anthocyanin flavonoids. While flavonoid biosynthesis received more attention in the past 30 years, little is known about their catabolic processes, and the available information is mostly about anthocyanin degradation [227]. Enzymes catalysing the immediate formation of anthocyanins, flavones, and flavonoids (UDP-sugar glucosyltransferase, UGT; anthocyanin acyltransferase, AAT; O-methyltransferaseerase, OMT) are shared between the biosynthetic pathways of the three main types of these compounds [225]. _UGT_ and _AAT_ gene expression showed strong upregulation under HL treatment [228]. However, as was demonstrated in the work of [229], the changes in the activity of early or shared flavonoid biosynthetic enzymes might not result in a change in every flavonoid type compound [229]. The effect of light conditions on phenylalanine ammonia lyase (PAL), catalysing one of the earliest steps of flavonoid biosynthesis, may be mediated by the Trxs [50; 204]. The connection between Trxs and flavonoid biosynthesis was proven in _Arabidopsis_, where the NADPH-dependent thioredoxin reductase (NTR) deficient double mutants _ntra ntrb_ (NTRA: nuclear, NTRB: mitochondrial isoform) showed increased expression of the flavonoid biosynthesis-related transcription factor _MYB111/PFG3_ (_MYB FAMILY TRANSCRIPTION FACTOR 111 /PRODUCTION OF FLAVONOL GLYCOSIDES 3_), the genes _F3H/TT7_ (_FLAVONOL 3'-HYDROXYLASE/TRANSPARENT TESTA 7 PROTEIN_), _F3H/TT6_ (_FLAVANONE 3-HYDROXYLASE/TRANSPARENT TESTA 6 PROTEIN_), and _flavonol synthase 1_ (_FLS1_) [230].