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 ( T r x ) 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 ( O 2 ) 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.