Reversal of Glyphosate Inhibition of Carrot Cell Culture Growth by Glycolytic Intermediates and Organic and Amino Acids

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Abstract
Various cytokinins and purines were ineffective in reversing glyphosate (0.25 mM)-induced growth inhibition of carrot (D. carota L.) cell suspension cultures. Aspartate was particularly effective in reversing glyphosate inhibition, but asparagine and various combinations of lysine, methionine, threonine and homoserine (eventual products of aspartate metabolism) were not effective. When organic acids of the tricarboxylic acid cycle were added to the medium, particularly good reversal of inhibition could be obtained with .alpha.-ketoglutarate, succinate and malate. Citrate provided only moderate reversal but the reversal given by glutamate was comparable to that of aspartate and the more effective tricarboxylic acid cycle intermediates. Pyruvate was somewhat toxic to cells when added early in the cell cycle but was most effective at reversing glyphosate inhibition when added at this time. If pyruvate addition was delayed, it was less toxic but was also a less effective reversing agent for glyphosate inhibition. All of the effective reversing agents for glyphosate inhibition found in this study can serve either directly or indirectly as C skeletons for respiration and ammonia assimilation and have previously been shown to be effective detoxifying agents for ammonia in cell culture systems. The results of this study suggest that glyphosate inhibition of growth in this system may be due to depletion of respiratory substrate which may eventually result in ammonia accumulation.