Thermal adaptation and acclimation of higher plants at the enzyme level: kinetic properties of NAD malate dehydrogenase and glutamate oxaloacetate transaminase in two genotypes of Arabidopsis thaliana (Brassicaceae)

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Abstract
Kinetic properties of NAD malate dehydrogenase (MDH) and glutamate oxaloacetate transaminase (GOT) were analyzed in two genotypes of Arabidopsis thaliana collected in two sites of contrasting climates. Plants from each genotype were acclimated under controlled conditions at four different thermoperiods: 5–10° C, 7–15° C, 15–25° C and 25–28° C. Apparent energy of activation for MDH of the cold adapted genotype were significantly lower at low temperatures of acclimation, while for GOT, significant differences were found but no clear patterns emerge from the data. No differences of significance between the two genotypes were observed for apparent Km and Kcat of both enzymes. For MDH, apparent Kms for oxaloacetic acid increased as a positive function of assay temperature but for GOT, Kms for α-oxoglutaric acid did not vary significantly over the 10–35° C assay temperature range. Kcat values for both enzymes increased about 2 fold for every 10° C raise in assay temperature. Concentrations of both enzymes significantly increased in plants of both genotypes acclimated to the coldest thermoperiod. The concentration of GOT was signficantly higher in plants of the cold adapted genotype acclimated to 5–10° C and 7–15° C. Results suggest that MDH and GOT from the cold adapted genotype are more efficient in the modulation of catalysis at low temperatures, while the opposite is found for plants of the warm-adapted genotype through enhanced thermostability of the mitochondrial fraction of MDH.