Response of Cytosolic-Isozyme and Plastid-Isozyme Levels of 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase to Physiological State of Nicotiana silvestris in Suspension Culture

Two isozymes of 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase are partitioned into plastid (DS-Mn) and cytosolic (DS-Co) compartments of at least several higher plants (RA Jensen 1986 Rec Adv Phytochem 20: 257-258). Differential variation of isozyme levels and in the timing of their expression was observed during growth of Nicotiana silvestris in suspension culture. The ratio of DS-Co to DS-Mn varied about fivefold in comparison of the different physiological stages of growth. Cultures maintained in exponential phase for >10 generations (EE cells) possessed balanced-growth properties and did not exhibit the considerable variation of isozyme levels found during the initial 2 to 3 generations of exponential growth (E cells) that followed subculture of stationary-phase cultures. The plastid isozyme level declined substantially in stationary phase, responded immediately to subculture, and reached a peak in early exponential growth similar to the steady-state level of DS-Mn in EE cells. In contrast, the cytosolic isozyme level peaked in late exponential growth. A recent history of stationary-phase physiology appeared to foster elevated synthesis of DS-Co since the steady-state level of DS-Co in EE cells was much lower than in E cells.