Aspartic-acid synthesis in C3 plants.

In a previous study (Melzer and O'Leary, 1987, Plant Physiol. 84, 58-60), we used isotopic methods to show that a substantial fraction of protein-bound aspartic acid in tobacco is derived from anaplerotic synthesis via phosphoenolpyruvate (PEP) carboxylase. Similar studies in soybean (Glycine max L.) and spinach (Spinacia oleracea L.) showed a similar pattern, and this pattern persists with age because of slow protein turnover. A more quantitative analysis indicates that about 40% of protein-bound aspartate is derived in this manner. Analyses of free aspartic and malic acids show that contribution of PEP carboxylase to the synthesis of these acids decreases with increasing age. The C4 plant Zea mays L. did not show this pattern.