Catalyzes the oxidation of glycolate to glyoxylate, with a reduction of O2 to H2O2 (PubMed:16595582). Is a key enzyme in photorespiration in plants (By similarity). To a lesser extent, is also able to oxidize glyoxylate to oxalate in vitro (PubMed:16595582). Can exert a strong regulation over photosynthesis, possibly through a feed-back inhibition on Rubisco activase (PubMed:19264754). Does not seem to play a role in oxalate accumulation (PubMed:16595582). glycolate + O2 = glyoxylate + H2O2 Competitively inhibited by oxalate. Photosynthesis; photorespiration; glycine from 2-phosphoglycolate: step 2/3. Homotetramer (By similarity). Interacts with rice dwarf virus (RDV) P8. This interaction promotes viral P8 relocation to virus factories peripheral to peroxisomes. Binds to CATB and CATC; these interactions are disturbed by alpha-hydroxy-2-pyridinemethanesulfonic acid (HPMS) and salicylic acid (SA) (PubMed:26900141). Expressed constitutively in leaves (at protein level). Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.