2-Cysteine Peroxiredoxins and Thylakoid Ascorbate Peroxidase Create aWater-Water Cycle That Is Essential to Protect the Photosynthetic Apparatus under High Light Stress Conditions

Different peroxidases, including 2-cysteine (2-Cys) peroxiredoxins (PRXs) and thylakoid ascorbate peroxidase (tAPX), have been proposed to be involved in the water-water cycle (WWC) and hydrogen peroxide (H₂O₂)-mediated signaling in plastids. We generated an Arabidopsis (Arabidopsis thaliana) double-mutant line deficient in the two plastid 2-Cys PRXs (2-Cys PRX A and B,2cpa 2cpb) and a triple mutant deficient in 2-Cys PRXs and tAPX (2cpa 2cpbtapx). In contrast to wild-type andtapxsingle-knockout plants,2cpa 2cpbdouble-knockout plants showed an impairment of photosynthetic efficiency and became photobleached under high light (HL) growth conditions. In addition, double-mutant plants also generated elevated levels of superoxide anion radicals,H₂O₂, and carbonylated proteins but lacked anthocyanin accumulation under HL stress conditions. Under HL conditions, 2-Cys PRXs seem to be essential in maintaining the WWC, whereas tAPX is dispensable. By comparison, this HL-sensitive phenotype was more severe in2cpa 2cpb tapxtriple-mutant plants, indicating that tAPX partially compensates for the loss of functional 2-Cys PRXs by mutation or inactivation by overoxidation. In response to HL,H₂O₂- and photooxidative stress-responsive marker genes were found to be dramatically up-regulated in2cpa 2cpb tapxbut not2cpa 2cpbmutant plants, suggesting that HL-induced plastid to nucleus retrograde photooxidative stress signaling takes place after loss or inactivation of the WWC enzymes 2-Cys PRX A, 2-Cys PRX B, and tAPX.