The Genetic Dissection of a Short-Term Response to Low CO2 Supports the Possibility for Peroxide- Mediated Decarboxylation of Photorespiratory Intermediates in the Peroxisome

Dear Editor, Photorespiration reduces plant carbon assimilation by an estimated 25%-50% of net CO2 assimilation in typical C3 plants （Roussel and Igamberdiev, 2011）. It results from the oxygenase activity of Rubisco giving rise to phosphogly- colate. Subsequent photorespiratory metabolism involves decarboxylation of glycine in the mitochondrion and reduc- tion of hydroxypyruvate, usually assumed to occur in the peroxisome. Recent research in Arabidopsis has highlighted possible alternative sites of hydroxypyruvate reductase （HPR） activity （Figure 1A）. In addition to the accepted reduction by peroxisomal HPR1, Timm et al. （2008） identified cytosolic HPR2 and a putative chloroplast HPR3 （Timm et al., 2011）. For HPR2 or HPR3 to function in photorespiration, hydroxypyru- vate would first need to pass from the peroxisome into the cytosol. Another potential fate for hydroxypyruvate is its non-enzymic （NE） decarboxylation by reaction with hydrogen peroxide as previously suggested by experiments performed on isolated peroxisomes （Walton and Butt, 1981）.