Photorespiratory mutants of the mitochondrial conversion of glycine to serine

Two mutants of barley (Hordeum vulgare L.), LaPr 85/55 and LaPr 87/30, have been isolated that accumulate glycine, with a concomitant reduction in the aminodonors glutamate and alanine, when transferred to air. Studies have shown that these plants have wild-type levels of serine transhydroxymethylase (EC 2.1.2.1) activity. When supplied ¹⁴CO₂, 48 and 66% of the supplied carbon was retained as glycine in LaPr 85/55 and LaPr 87/30, respectively, compared with a value of 11% for the wild type. In the short-term, both mutant plants are unable to metabolize ¹⁴C] glycine, but when fed the isotope for 2 hours, LaPr 85/55 was able to metabolize most (70%) of the supplied carbon into sugars with only 15% remaining in glycine. LaPr 87/30, however, was unable to metabolize more than 4% of the supplied carbon into sugars even after 2 hours. Measurement of glycine decarboxylase (EC 2.1.2.10) activity via the glycine-bicarbonate exchange reaction showed LaPr 85/55 to have approximately 70% wild-type activity with LaPr 87/30 having only 14% wild-type activity. The approximation of LaPr 85/55 to wild-type activities was maintained for ¹⁴CO₂ release from ¹⁴C]glycine feeding and ammonia accumulation in the presence of methionine sulphoximine with the equivalent rates for LaPr 87/30 being less than 40% and 10%, respectively. CO₂ fixation rates for the mutants fell to between 35 and 40% of wild-type rates within 10 min of transfer to air. This was shown to be partly due to a run down of aminodonors, because when 40 millimolar serine was supplied through the xylem stream these rates recovered for both mutants to 70% of the wild-type rate. These data suggested a mutation in a glycine transport system for LaPr 85/55 and in the proteins of glycine decarboxylase for LaPr 87/30. Western blotting with antisera to the P, H, T, and L proteins of glycine decarboxylase showed cross-reaction against all four proteins for LaPr 85/55 but little cross-reaction against P or H protein for LaPr 87/30, reaffirming the possibility of a transport mutation in LaPr 85/55. We also suggest that genes for P and H proteins could be either coordinately regulated or that one protein is undetectable or unstable in the absence of the other.