Effects of Heat Shock on Amino Acid Metabolism of Cowpea Cells

Abstract

When cowpea (Vigna unguiculata) cells maintained at 260C are transferred to 420C, rapid accumulation of y-aminobutyrate (>10-fold) is induced. Several other amino acids (including jB-alanine, alanine, and proline) are also accumulated, but less extensively than y-aminobutyrate. Total free amino acid levels are increased approximately 1.5-fold after 24 hours at 420C. Heat shock also leads to release of amino acids into the medium, indicating heat shock damage to the integrity of the plasmalemma. Some of the changes in metabolic rates associated with heat shock were estimated by monitoring the 15N labeling kinefics of free intracellular, extracellular and protein-bound amino acids of cultures supplied with 15NH4 , and analyzing the labeling data by computer simulation. Preliminary computer simulation models of nitrogen flux suggest that heat shock induces an increase in the yaminobutyrate synthesis rate from 12.5 nanomoles per hour per gram fresh weight in control cells maintained at 260C, to as high as 800 nanomoles per hour per gram fresh weight within the first 2 hours of heat shock. This 64-fold increase in the -y-aminobutyrate synthesis rate greatly exceeds the expected (Q10) change of metabolic rate of 2.5-to 3-fold due to a 160C increase in temperature. We suggest that this metabolic response may in part involve an activation of glutamate decarboxylase in vivo, perhaps mediated by a transient cytoplasmic acidification. Proline appears to be synthesized from glutamate and not from omithine in cowpea cells. Proline became severalfold more heavily labeled than omithine, citrulline and arginine in both control and heatshocked cultures. Proline synthesis rate was increased 2.7-fold by heat shock. Alanine, #B-alanine, valine, leucine, and isoleucine synthesis rates were increased 1.6-, 3.5-, 2.0-, 5.0-, and 6.0-fold, respectively, by heat shock. In contrast, the phenylalanine synthesis rate was decreased by 50% in response to heat shock.The differential effects of heat stress on metabolic rates lead to flux and pool size redistributions throughout the entire network of amino acid metabolism. where t, and t2 are the temperatures for velocities V, and V2 (21). For most enzyme-catalyzed reactions, Q,0 is between 1.5 and 2.5, and remains relatively constant over the physiological temperature range (0-50°C) (21). Thus, in general, rates of an enzyme reaction are expected to approximately double with every 10°C increase in temperature, assuming substrates in the reaction are nonlimiting, and assuming no irreversible thermal inactivation of the enzyme.Temperature response curves for photosynthesis show a Qio of approximately 2 in the native desert species Tidestromia oblongifolia over the temperature range 15 to 40°C, but a Q,o of close to 0 over this same temperature range in the coastal species Atriplex glabriuscula, with extensive irreversible deactivation of photosynthesis at temperatures between 40°C and 50°C in the latter, but not the former species (2). Clearly, plant species exhibit striking variation fo...

Metabolic fingerprinting of Tomato Mosaic Virus infected Solanum lycopersicum

“…GABA induction in plants in response to an abiotic stress treatment is a widely observed phenomenon (Wallace et al, 1984;Mayer et al, 1990;Allan et al, 2008). Our results agree with a previously published study of the metabolic changes in vitis plants 360 affected by the Esca disease.…”

Characterization of Two Glutamate Decarboxylase cDNA Clones from Arabidopsis

“…These findings suggest that GAD may be stimulated in vivo by Ca 2ϩ signal pathways. This hypothesis is consistent with data collected from studies demonstrating the rapid increase in cytoplasmic Ca 2ϩ concentrations (Knight et al, 1991(Knight et al, , 1992Price et al, 1994;Cholewa et al, 1997) and GABA titers (Wallace et al, 1984;Mayer et al, 1990;Cholewa et al, 1997) in plant cells upon exposure to various environmental stimuli.…”

[gamma]-Aminobutyric Acid Stimulates Ethylene Biosynthesis in Sunflower

“…In addition to these normally observed levels, plants can rapidly accumulate considerable amounts of GABA under several stress conditions such as mechanical stress (Wallace et al, 1984;Ramputh and Bown, 1996), hypoxia (Streeter and Thompson, 1972;Roberts et al, 1992;Shelp et al, 1995), cold shock (Cholewa et al, 1997), darkness (Wallace et al, 1984), heat shock (Mayer et al, 1990), and water stress (Rhodes et al, 1986). For instance, mechanical damage in soybean leaves increases GABA levels by 10-to 25-fold within 1 to 4 min of the start of the stimulus (Ramputh and Bown, 1996).…”