Studies on the roles and regulation of two glyoxylate reductases in plants
Succinic semialdehyde (SSA) is a mitochondrial intermediate in the metabolism of GABA, whereas glyoxylate is a peroxisomal metabolite generated from photorespiratory glycolate. Recent evidence indicates that distinct cytosolic and plastidial glyoxylate reductase isoforms from ' Arabidopsis thaliana' (L.) Heynh ('At'GLYR1 and ' At'GLYR2, respectively) catalyse the 'in vitro' NADPH-dependent conversion of SSA to gamma-hydroxybutyrate (GHB) and glyoxylate to glycolate. In this thesis, recombinant 'Arabidopsis' GLYR1 was demonstrated to simultaneously catalyze both reactions 'in vitro'. Time-course experiments revealed that GHB accumulated in leaves of 'Arabidopsis ' subjected to salinity, drought, submergence, cold or heat stress, and tobacco ('Nicotiana tabacum' L.) subjected to submergence. This was generally accompanied by higher GABA and Ala levels, higher NADPH/NADP + ratios, and lower Glu levels. Furthermore, expression analysis of ' Arabidopsis' revealed that the relative abundance of GLYR1 transcript was enhanced under salinity, drought, submergence, cold and heat, as was the GLYR2 transcript under cold and heat. Other time-course experiments revealed that 'Arabidopsis glyr1' and 'glyr2' knockout mutants, unlike wild-type (wt) plants, did not have elevated levels of GLYR1 or GLYR2 transcript, GHB and NADPH in rosette leaves of plants subjected to submergence or low CO2 conditions (75 versus 380 [mu]mol mol -1 at 21% O2). Glycolate, glyoxylate and succinate also accumulated in 'glyr1' and 'glyr2' under low CO2, whereas Gly did not. Manipulation of redox balance via the use of 'Arabidopsis' NAD kinase (NADK) mutants ('nadk1 ' and 'nadk3' knockout mutants, NADK1 overexpressor, and NADK2 underexpressor) altered the plant response to submergence. Thus, it can be concluded that: (1) GHB accumulation is a general response to abiotic stress and is regulated by both biochemical and transcriptional processes; and (2) GLYR activity is involved in the detoxification of both SSA and glyoxylate, and the recycling of phosphorylated pyridine nucleotides, thereby linking GABA metabolism and photorespiration to redox homeostasis.