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Characterization of Glyoxylate/Succinic Semialdehyde Reductases in Plants and Impact of Elevated CO2 on γ-Aminobutyrate Metabolism in ‘Empire’ Apple Fruit Stored Under Controlled Atmosphere Conditions

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Title: Characterization of Glyoxylate/Succinic Semialdehyde Reductases in Plants and Impact of Elevated CO2 on γ-Aminobutyrate Metabolism in ‘Empire’ Apple Fruit Stored Under Controlled Atmosphere Conditions
Author: Brikis, Carolyne
Department: Department of Plant Agriculture
Program: Plant Agriculture
Advisor: Shelp, Barry
Abstract: Plant NADPH-dependent glyoxylate/succinic semialdehyde reductases 1 and 2 (GLYR1 and GLYR2) are hypothesized to detoxify the reactive aldehydes, photorespiratory-derived glyoxylate and γ-aminobutyrate (GABA)-derived succinic semialdehyde (SSA), into glycolate and γ-hydroxybutyrate (GHB), respectively. Here, recombinant GLYR1 and/or GLYR2 proteins from apple, rice and Arabidopsis thaliana were produced and used for kinetic analyses of enzyme activity. Plant GLYRs used NADPH more efficiently than NADH, preferred glyoxylate to SSA, and were feedback inhibited by a high NADP+/NADPH ratio, suggesting that GLYRs are redox regulated in planta. Furthermore, green fluorescent protein-GLYR fusions were generated to investigate their subcellular localizations using various transient and stable expression systems. GLYR1 was cytosolic, whereas GLYR2 was dual localized to mitochondria and plastids, which was corroborated by the isolation and recovery of glyoxylate reductase activity in Percoll gradient-purified mitochondria from an atglyr1 mutant. Plant GLYRs are proposed to have specialized functions related to GABA- and photorespiratory stress-derived aldehydes within their respective cellular compartments. Abiotic stress causes GABA accumulation in plants and plant organs such as apple fruit, notably under controlled atmosphere (CA) storage used to preserve apple quality. CA is comprised of multiple abiotic stresses (i.e., elevated CO2, low O2 and chilling) and can cause physiological disorders, which may be associated with GABA. GABA can originate from the decarboxylation of glutamate or the terminal oxidation of the polyamines putrescine and spermidine. Here, we determined the incidence of CO2 injury and profiled the metabolites and transcripts associated with GABA metabolism in ‘Empire’ apples stored with 0.03 or 5 kPa CO2 and 2.5 kPa O2 at 0 or 3oC, and sampled over a 16-week period. Fruit stored at elevated CO2, particularly at 0oC, developed a higher incidence of external CO2 injury, which was more associated with polyamines than GABA. Discrepancy between metabolite and gene expression patterns suggests that GABA is primarily derived from glutamate and that it is catabolized to GHB, rather than succinate, due to a highly reduced redox environment within the fruit. These findings are interpreted as evidence for the involvement of both gene-dependent and -independent mechanisms for the metabolism of GABA in apples stored under CA conditions.
URI: http://hdl.handle.net/10214/9105
Date: 2015-05


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