Stress Metabolism. IX. The Significance of End-Product Inhibition of Proline Biosynthesis and of Compartmentation in Relation to Stress-Induced Proline Accumulation

Abstract

The rapid and extensive accumulation of proline during water stress in barley (cv. Prior) and tobacco (cv. White Burley) indicates that feedback inhibition of proline synthesis under these circumstances is unlikely. The incorporation into proline of radioactivity from administered L-[14C]glutamic acid was inhibited by the presence of proline in turgid but not in stressed tissues, suggesting that feedback inhibition of proline synthesis occurs in turgid but not in stressed tissues. An alternative explanation of the data, that the radioactivity in the L-glutamate pool of turgid tissue was diluted by non-labelled glutamate derived from the oxidation of the added proline, was not supported. Although rapid oxidation of added proline occurred, this did not appear to enter, to any significant extent, the glutamate pool concerned with proline synthesis. Neither aqueous nor non-aqueous isolation techniques demonstrated any specific enrichment with proline of specific cell fractions during stress. Moreover, in plants which had accumulated radioactivity in free proline during stress from administered L-[14C]glutamate, radioactivity accumul- ated in the proline incorporated in protein to an extent which suggested that the whole proline pool was equally available for protein synthesis. This does not favour the hypothesis that proline is accumulated exclusively in specific cellular organelles during stress.