Photorespiration: origins and metabolic integration in interacting compartments

Preface Photorespiration: origins and metabolic integration in interacting compartments This special issue on photorespiration focuses on recent advances in this topic. The majority of the papers summarizes and extends contributions given at the 2nd workshop, 'Photorespiration–key to better crops', held in Warnemuende, Germany in June 2015. origins and metabolic integration in interacting compartments' (FOR 1186–Promics). The term photorespiration (PR) describes a light-induced biochemical process that converts 2-phosphoglycolate (2PG) into 3-phosphoglycerate (3PGA) and is accompanied by O 2 uptake and CO 2 release. It is closely associated with photosynthetic CO 2 assimilation and represents one of the major highways of carbon metabolism in most plants. By mass flow, surpassed only by photosynthesis, PR actually constitutes the second most important process in the land-based biosphere. Plants using the most widespread C 3 type of photosynthesis for CO 2 assimilation display particularly massive photorespiratory CO 2 production. PR is initiated by competition of O 2 with CO 2 at the active site of the universal carboxylating enzyme Ribulose 1,5-bisphosphate Carboxylase/Oxygenase (Rubisco) (Smith, 1976), which produces large amounts of 2PG during the day. Hence, PR essentially acts as a salvage or metabolic repair process that converts the toxic by-product 2PG into the useful Calvin–Benson cycle intermediate 3PGA. It is supposedly the most important ancient ancillary metabolic process that enables plants to thrive in an O 2-containing atmosphere (Osmond, 1981). To convert 2PG into 3PGA, the concerted action of many plastidial, peroxisomal, mitochondrial, and also cytosolic enzymes is necessary, which makes this pathway the most prominent example of subcellular metabolic integration in higher plants. However, PR also leads to the loss of a considerable fraction of freshly assimilated C and N as photorespiratory CO 2 and NH 3. Quantitatively, PR can decrease photosynthesis by up to 30% under current atmospheric concentrations of CO 2 and O 2 and even more at elevated temperature (e.g. Sharkey, 1988; Zhu et al., 2004). This substantial decrease of net photosynthe-sis led to the somewhat misleading view of PR as a 'wasteful' process limiting photosynthetic productivity in C 3 plants (e.g. Garrett, 1978; Siedow and Day, 2001). However, genetic analysis showed that PR is essential for all organisms performing oxygenic photosynthesis, since mutations of genes encoding for key photorespiratory enzymes always resulted in the pho-torespiratory phenotype and frequently in lethality (Somerville, 2001), i.e. corresponding mutants of cyanobacteria, red algae (Rademacher et al., this issue), chlorophytes, C 3 and C 4 …