Peroxide processing in photosynthesis: antioxidant coupling and redox signalling

Photosynthesis has a high capacity for production of hydrogen peroxide (H2O 2), but the intracellular levels of this relatively weak oxidant are contro lled by the antioxidant system, comprising a network of enzymatic and non-e nzymatic components that notably includes reactions linked to the intracell ular ascorbate and glutathione pools, Mutants and transformed plants with s pecific decreases in key components offer the opportunity to dissect the co mplex system that maintains redox homeostasis. Since H2O2 is a signal-trans ducing molecule relaying information on intracellular redox state, the pool size must be rigorously controlled within each compartment of the cell. Th is review focuses on compartment-specific differences in the stringency of redox coupling between ascorbate and glutathione, and the significance this may have for the flexibility of the control of gene expression that is lin ked to photosynthetic H2O2 production.