## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Control of Lipid Metabolism  
#### 4.4.1 Light Intensity-Associated Alterations in Lipid Metabolism  
In plants, de novo fatty acid synthesis takes place mostly in the chloroplasts of green tissues or in plastids of non-photosynthetic organs and limitedly in the mitochondria, and most enzymes are affected by light conditions (Table 16). Accordingly, fatty acid and membrane lipid synthesis were significantly increased in LL compared to control conditions, while the opposite relationship was observed between HL and control-condition-grown _Arabidopsis_[172]. In addition, it was shown that light intensity-dependent changes in fatty acid synthesis derived from the post-translational regulation of the plastidic acetyl-CoA carboxylase activity. The effect of light intensity on lipid synthesis is mediated by the redox system. Thus, the formation of fatty acids from 3-phosphoglycerate (a component of CBC, a link to carbon assimilation) is an NADPH-consuming process [152]. In addition, members of lipid/fatty acid biosynthesis (Malonyl CoA:ACP transacyase, MCAT; Acetyl-CoA biotin carboxyl carrier) are targets of modification by Trx, Grx, and GSH [173]. Among the lipid biosynthetic enzymes, the light and redox control of ACCase, the Trx regulation of monogalacosyl diacylglycerol synthase (MGD), and the Prx Q requirement of fatty acid desaturase 4 (FAD4) were shown in _Arabidopsis_[174]. Light control of _AtFAD7_ was demonstrated in _Arabidopsis_ since its promoter contains a light-responsive element [175]. Further evidence for the light regulation of FADs has been reported in cotton (_Gossyrium hirsutum_) [176] and soybean cell suspension [177]. In this process, the light-responsive HY5 transcription factor participates since it targets many genes involved in lipid biosynthesis, for example, digalacosyldiacyl glycerol synthase 1 (DGD1), FAD3, or FAD6 [123]. The light-dependent transcriptional control of triacylglycereeds (main components of plant oils) was shown in olive plants since the _phospholipid: diacylglycerol acyltransferase_ gene was downregulated under darkness [178]. Further evidence for the light control of lipid formation was obtained by the study of its diurnal changes [179]. Thus, polar lipid fatty acid biosynthesis occurs in the light period of the diurnal cycle, whereas fatty acid desaturation takes place in the dark period in _Arabidopsis_. Furthermore, membrane galactolipid, phospholipids, and lysophospholipid profiling showed interesting diurnal regulation of the plant lipidome, highlighting the role of phospholipase D [179]. Similar to _Arabidopsis_, phospholipid levels were also affected by light in barley since the expression of genes encoding the components of the phospholipid signalling pathway follows a circadian rhythm [180]. The results obtained in dicot and monocot species indicate the general role of light in the regulation of phospholipid levels in plants. Changes in light intensity are useful for the modification of lipid composition in order to increase stress tolerance of crops cultivated in glasshouses or on vertical farms. Both HL and LL proved to be appropriate for this purpose in _Arabidopsis_[55; 181] and olive callus cultures at the control growth temperature [182]. In both species, differences were found in lipid composition between HL, ambient light, and/or LL. In _Arabidopsis_, ACCase activity increased in LL compared to normal light and HL, whereas total lipid content was also higher under LL than in normal light and HL caused a decrease in total lipid level. However, the authors detected no consistent changes in the protein abundance of ACCase subunits, which indicates a post-translational regulation of the enzyme [55]. Similar to the control growth temperature, light conditions also affected the lipid composition at high temperatures based on the differences between the amounts of several lipid species in light and darkness in _Arabidopsis_[181].