1.1.1.29 malfunction enzyme knockdown leads to an inhibition of ascorbic acid uptake 1.1.1.29 metabolism HPR1 phosphomimetic variant T335D exhibits reduced NADH-dependent hydroxypyruvate reductase activity but improved NADPH-dependent activity. Complementation of the Arabidopsis HPR1 mutant by either wild-type HPR1 or HPR1 nonphosphorylatable mutant T335A fully complements the photorespiratory growth phenotype of the HPR1 mutant in ambient air, whereas HPR1 T335D-containing HPR1 mutant plants remain smaller and have lower photosynthetic CO2 assimilation rates. These phenotypes were associated with subtle perturbations in the photorespiratory cycle of HPR1 T335D-complemented HPR1 mutant rosettes compared to all other HPR1-containing lines 1.1.1.29 physiological function rates of photosynthesis under photorespiratory conditions are slightly reduced in mutant plants lacking isoform hpr1 expression or additionally lacking both of the peroxisomal malate dehydrogenase genes, mutant pmdh1pmdh2hpr1. The CO2 compensation points increase under photorespiratory conditions in both mutants. The increase in CO2 compensation points in the hpr1 and pmdh1pmdh2hpr1 plants is not associated with changes in mitochondrial respiration but with an increase in the non-respiratory CO2 released per Rubisco oxygenation reaction 1.1.1.29 physiological function HPR1 mutant plants show a stunted phenotype when grown in air. In addition to being smaller, the mutants have significantly less chlorophyll and carotenoid than the wild-type Columbia-0. The total extractable activity of Rubisco is the same as in wild-type. The 2-phosphoglycerate content is increased in the HPR1 mutant plants compared with the wild-type plants