Induction of nitrate reductase in detached corn leaves: the effect of the age of the leaves

Abstract

It is generally accepted that young leaves have higher nitrate reductase (NR) activity than older ones. To determine the physiological basis of this observation and gain a better understanding of the regulation of NR activity, we investigated NR activity and protein and NR mRNA in corn (Zea mays L.) leaves of different age induced with ammonium nitrate. Using detached leaves to facilitate control over nitrate availability and evaluation of protein synthesis, we found that when exposed to ammonium nitrate, the younger (3‐day‐old) leaves developed higher levels of NR activity and protein (assayed with an enzyme‐linked immunosorbent assay) than the older (5‐day‐old) leaves. This difference in response appeared not to be due to a higher decay rate of enzyme activity in older leaves. Evaluation of soluble protein synthesis using labelled methionine indicated that the rates of synthesis were significantly higher in the younger than in the older tissue, but rates of protein turnover (assayed with an unlabelled methionine chase) were also higher. NR protein was also synthesized and degraded more rapidly in younger leaves. The higher steady‐state level of nitrate reductase activity and protein in 3‐day‐old leaves apparently resulted from a greater net synthesis of enzyme protein, which reflected the higher total rate of synthesis of soluble protein in younger leaves. RNA blots of NR mRNA showed that the transcript levels were similar in 3‐ and 5‐day‐old leaves. This indicates that the NR gene was equally responsive to ammonium nitrate induction in the two ages of leaves. Our results show a poor correlation between the level of NR transcript and the amount of NR protein synthesized from it in leaves of different age. The data suggest that the NR transcript is used much less efficiently for production of NR protein in the older than in the younger leaves. We conclude that this low efficiency can account for the decrease in NR activity and protein found in older leaves.