## 4 Light Intensity- and Spectrum-Associated Adjustment of Metabolism
### Light Regulation of Sulphur Assimilation and Glutathione Metabolism  
#### 4.3.1 Light Intensity-Dependent Control of Sulphur Assimilation and Glutathione Metabolism  
Sulphate reduction, having a diurnal rhythm, exhibits a great sensitivity to light, as shown by the 5-10-fold increase in the activity of its key enzyme, adenosine phospho-sulphate reductase (APR), during the light period compared to the dark in leaves of 2 maize varieties at seedling stage (Figure 5) [148]. The light activates this enzyme at the transcriptional level, as shown in _Arabidopsis_, resulting in the increased incorporation of \({}^{35}\)S from sulphate into cysteine and GSH [163]. A light-dependent control of sulphate reduction was also observed in wheat at both transcript and metabolite levels [67]. Interestingly, the increase in light intensity had an activating effect on these parameters. The bZIP transcription factor HY5 participates in the transcriptional regulation of APR through its direct binding to the APR1 and APR2 promoters, which was confirmed by chromatin immunoprecipitation in _Arabidopsis_ (Figure 5) [164]. In addition, a similar regulation of a sulphate transporter (AtSULTR1;2), another key gene of sulphate assimilation regulated by HY5, was observed in _Arabidopsis_[164]. The PHYTOCHROME AND FLOWERING TIME 1 (PFT1) protein may be involved in this process by bridging HY5 with other transcription factors during the light-dependent control of APR [165].  
The induction of sulphate reduction by HL allows the accumulation of its end product, cysteine, which is a precursor of GSH and has a central role in the redox-dependent regulation of metabolic pathways. Accordingly, an increase in cysteine content resulted in a simultaneous accumulation of GSH with increasing light intensity in wheat (Figure 5) [67]. This change derived from transcriptional activation of GSH synthesis, as indicated by an increase in _glutathione synthase_ expression. Interestingly, the larger glutathione pool became more reduced at increased light intensities due to the unchanged level of GSSG and the increased transcriptional activation of the enzymes of the ascorbate glutathione cycle (GR, APX). These changes, in turn, alter the redox environment of tissues and consequently many metabolic pathways [152].  
HL can induce the GSH-dependent protective mechanisms against environmental stresses, as was observed in wheat subjected to drought [72]. With increasing light intensity, the amount of GSSG and its ratio compared to GSH increased, which resulted in a better recovery of the seedlings grown under HL. These results show that moderate oxidative stress induced by HL could reduce the negative effect of drought. The GSSG/GSH ratio strongly depends on the activity of GR and APX (reducing GSSG in the ascorbate-GSH cycle), enzymes which were induced by HL at the transcriptional level in wheat [67]. The activation of APX activity by HL was observed in the leaves of apple trees, too [166]. Besides GR, GSTs also have a strong influence on the redox state of the glutathione pool since they reduce the amount of GSH by catalysing GSH conjugation to electrophilic compounds [167]. The more soluble derivatives can more easily enter the vacuoles, where they can be stored or converted to other metabolites. The expression and activity of GSTs are reduced by LL and darkness, and they are increased by HL [72; 167]. The light-induced activation of GSTs may reduce the harmful effect of abiotic stresses through the detoxification of lipid peroxides by their conjugation with GSH.