## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Control of Carbohydrate Metabolism  
#### 4.1.1 Light Intensity-Associated Regulation of Carbohydrate Metabolism  
Plants have the ability to produce carbohydrates by using light energy and water to convert CO2 to different types of carbohydrates via photosynthesis [122]. NADPH ensures the reducing power for this process; therefore, it depends on the cellular redox environment (Table A1). There are 5 important enzymes in the Calvin-Benson cycle (CBC), which are strongly regulated by light, namely: ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), fructose-1,6-bisphosphatase (FBP), sedoheptulose-1,7-bisphosphatase (SBP), and ribulose-5-phosphate kinase (RPK) (Figure 4). These enzymes are also controlled by the transcription factor HY5, coordinating carbon assimilation with other metabolic processes at the transcriptional level [123]. In the shoot, HY5 promotes carbon assimilation and translocation, whereas, in the root, it activates NRT2.1 expression and nitrate uptake. It has been shown in _Arabidopsis_ mutants that HY5 is a shoot-root mobile signal that coordinates shoot growth and carbon assimilation with root growth and nitrogen uptake in different light environments [124; 125; 126; 127]. HY5 is a master regulator of many genes, including those that affect the redox state in tissues. Thus, it can mediate the light regulation of CBC through the ferredoxin/th thioredoxin  
Figure 4: The effect of light intensity and spectrum on carbohydrate metabolism. Monochromic red or blue lights may inhibit or enhance the enzymes of the Calvin–Benson cycle. Most of these enzymes are HY-5 and/or Trx-regulated. Interestingly, several enzymes of carbohydrate catabolism are light repressed, while others are light enhanced. Light-associated redox regulation of the Calvin–Benson cycle is better known than in the case of tricarboxylic acid (TCA) cycle. Rubisco: ribulose-1,5-bisphosphate carboxylase oxygenase; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; FBP: fructose-1,6-bisphosphatase; Fd: ferredoxin; SBP: sedoheptulose-1,7-bisphosphatase; RPK: ribulose-5-phosphate kinase; Trx: thioredoxin; HY5: elongated hypocotyl 5; HL: high light; GAPC: cytosolic GAPDH; GAPC-SOH: oxidised GAPC; GAPC-SSG: glutathionedAPC; Grx: glutaredoxine; PFK: phosphofractokinase; TrxL2: Trx-like2; MDH: malate dehydrogenase; NAD-ME: NAD-dependent malic enzyme; FUM: fumarase; CS: citrate synthase; IDH: isocitrate dehydrogenase; ACO: aconitase; PDC: pyruvate dehydrogenase complex; 2-OGDH: 2-oxoglutarate dehydrogenase; OAA: oxaloacetate; 2 OG: 2-oxoglutarate; PhyA: phytochrome A; AcCoA: acetyl-CoA; Fru-6-P: fructose-6-phosphate; Fru-1,6-P\({}_{2}\): fructose-1,6-bisphosphate; G3P: glyceraldehyde-3-phosphate; 1,3 DPGA: 1,3-diphosphoglycerate. (Fd/Trx) system to which the enzymes of CBC are linked, except for Rubisco (Figure 4). Ferredoxin (Fd) determines the distribution of electrons to distinct processes, including the production of NADPH and redox signalling for the activation of CBC enzymes via the ferredoxin-dependent thioredoxin reducing system. The Fd/Trx system transfers reducing equivalents to various proteins containing redox-active cysteine residues, thereby modulating their activity [128; 129; 130]. As it has been already discussed in Section 2.1, the HL illumination of a distant leaf could launch an ROS wave across the plant, which prepares CO\({}_{2}\) assimilation in a non-illuminated leaf for the upcoming illumination. During this process, H\({}_{2}\)O\({}_{2}\) as an oxidant may control the activity of CBC through the oxidation of thioredoxins. The HL sensitivity of the enzyme activities of CBC was shown in the _Arabidopsis_ chloroplast glucose-6-phosphate/phosphate translocator mutant, _gpt2_, which does not acclimate to HL [131].