Modulation by weak bases or weak acids of the pH of cell sap and phosphoenolpyruvate carboxylase activity in leaf discs of C₄ plants

Abstract

When leaf discs of a C₄ species, Alternanthera pungens (L.) H.B. and K. or Amaranthus hypochondriacus L., were preincubated in 7.5 mM NH₄Cl, the pH of the cell sap increased by nearly 0.3 unit, while the activity of phosphoenolpyruvate carboxylase (PEPC) about doubled compared to the cell sap from control leaf discs (preincubated in 5 mM Tricine-KOH, pH 8.5). The sensitivity of PEPC to L-malate (a feedback inhibitor) decreased marginally as a result of cytosolic alkalization. The pH of the cell sap and PEPC activity decreased by nearly 0.4 unit and 50%, respectively, when leaf discs were incubated in weak organic acids such as propionic, butyric or salicylic acid. Thus, our results demonstrate a marked modulation in vivo of cell sap pH and PEPC activity in leaf discs from C₄ plants by external alkalizing or acidifying reagents. The rise in PEPC activity due to alkalization of leaf discs was not sensitive to cycloheximide, implying that cytosolic protein synthesis was not involved in the activation of PEPC. Despite the marked increase in the PEPC activity due to the base-loading of leaf discs, the change in malate sensitivity of the enzyme was only marginal, indicating that there was no significant increase in the extent of PEPC-phosphorylation. Besides the physiological significance, the technique of acid/ base-loading may be an important tool for studying the regulation of PEPC in leaf discs of C₄ species, since the activity of PEPC could be enhanced apparently without phosphorylation of the enzyme.