Photorespiration involves three types of organelles: chloroplasts, peroxisomes and mitochondria. The main features of this pathway include: (i) the conversion of two-carbon molecule, 2-phosphoglycolate, into glycine; (ii) the decarboxylation of glycine to generate serine, and (iii) the conversion of serine into 3-phosphoglycerate. The first step of the pathway is oxygenation of RuBP (D-ribulose-1,5- bisphosphate) leads to the production of one molecule of 3-PG (3-Glycerol phosphate) and one of 2-phosphoglycolate catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase). Rubisco is a bifunctional enzyme that catalyzes both the carboxylation and oxygenation (Suzuki and Makino, 2012). The next step of the pathway involves the dephosphorylation of 2-phosphoglycolate to form Glycolate, which is exported to the cytoplasm where it is oxidized in the peroxisomes to Glyoxlate. Glyoxylate is then converted into Glycine by two different enzymes: serine:glyoxylate aminotransferase (SGAT) and glutamate:glyoxylate aminotransferase (GGAT) (Zhang et al., 2015). Glycine is in fact converted into serine in the mitochondrion by Glycine decarboxylase (GDC). GDC has four different subunits (P, H, T and L), which catalyze the transfer of a methylene group from glycine to Tetrahydro folate (THF) with the concomitant release of NH3, CO2 and NADH. The methylene group is then transferred to another glycine molecule to form serine by a serine hydroxylmetyltransferase (SHMT). Back in the peroxisome, serine is used to convert glyoxylate into Hydroxypyruvate via SGAT. The last step in peroxisome is reduction of hydroxypyruvate into Glycerate by an NADH-dependent Hydroxypyruvate reductase (HPR). Glycerate is then redirected to the chloroplast where it is phosphorylated to 3-PG and reenters the Calvin cycle. Photorespiration is enhanced by high temperature and the outcome of photorespiration is loss of CO2 and energy in photosynthetic cells. Photorespiration is necessary under conditions of high light intensity and low CO2 concentration (i.e. when stomata is closed under water stress) to dissipate excess ATP and reducing power from the photosynthesis light reactions, thus, to prevent damage to the photosynthetic apparatus. Sørhagen et al., 2013 proposed a crosstalk pathway between photorespiration and pathogen defence through meta-expression analysis of photorespiratory genes during pathogen attack.
