Figure 4: Thiol-disulphide, plastoquinone-plastoquinol interactions in chloroplast leading to activation of enzymes and redox signaling. In chloroplasts, reduced ferridoxin (Fdred) reduces the regulatory protein thioredoxin via enzyme ferridoxin-thioredoxin reductase (FTR). This, in turn, activates chloroplastic enzymes. Molecular oxygen is a natural oxidant that reverses this activation via formation of superoxide () and H2O2·H2O2 is diffusive and moderately stable oxidant and inactivates these enzymes. H2O2 production particularly from the electron flow of Z-scheme of photosynthesis is exacerbated during stress. Ascorbate-glutathione cycle, on the other hand, destroys H2O2. Turning over of GSH pool is central to the processes. The accumulation of H2O2 together with changes in thiodisulfide, plastoquinone-plastoquinol ratio of cell, provides the redox signals leading to changes in gene expression.