Photorespiratory metabolism of glyoxylate and formate in glycine-accumulating mutants of barley and Amaranthus edulis

Glycine-accumulating mutants of barley (Hordeum vulgare L.) and Amaranthus edulis (Speg.), which lack the ability to decarboxylate glycine by glycine decarboxylase (GDC; EC 2.1.2.10, were used to study the significance of an alternative photorespiratory pathway of serine formation. In the normal pho torespiratory pathway, 5,10-methylenetetrahydrofolate is formed in the reac tion catalysed by GDC and transferred to serine by serine hydroxymethyltran sferase. In an alternative pathway, glyoxylate could be decarboxylated to f ormate and formate could be converted into 5,10-methylenetetrahydrofolate i n the Cl-tetrahydrofolate synthase pathway. In contrast to wild-type plants , the mutants showed a light-dependent accumulation of glyoxylate and forma te, which was suppressed by elevated (0.7%) CO2 concentrations. After growt h in air, the activity and amount of 10-formyltetrahydrofolate synthetase ( FTHF synthetase; EC 6.3.4.4), the first enzyme of the conversion of formate into 5,10-methylenetetrahydrofolate, were increased in the mutants compare d to the wild types. A similar increase in FTHF synthetase could be induced by incubating leaves of wild-type plants with glycine under illumination, but not in the dark. Experiments with C-14 showed that the barley mutants i ncorporated [C-14]formate and [2-C-14]glycollate into serine. Together, the accumulation of glyoxylate and formate under photorespiratory conditions, the increase in FTHF synthetase and the ability to utilise formate and glyc ollate for the formation of serine indicate that the mutants are able parti ally to compensate for the lack of GDC activity by bypassing the normal pho torespiratory pathway.