Regulation of nitrate reductase by various nitrogen sources in cultured Ipomoea sp.

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Abstract
The nitrate reductase of Ipomoea sp. (morning glory) grown in suspension culture on a chemically defined medium is induced by nitrate. The developmental pattern of the enzyme shows 3 peaks of enzyme activity: peak I during the initial stages of growth, peak II during the exponential growth, and peak III just before the onset of the stationary phase of growth. The full induction of nitrate reductase by nitrate is dependent on the availability of a source of energy, N, P and S. After 24 h of growth high levels of nitrate reductase are detected in the presence of casein hydrolysate and ammonium pyrrolidine carboxylate and an appreciable level in the presence of nitrite and ammonium as N sources. Glutamine, asparagine and amino acids are also regulators of nitrate reductase synthesis whether they are used as sole N sources or as additives in nitrate media. When used as sole nitrogen source the amino acids generally fall into 3 distinct classes. Amino acids in the 1st class promote an increase in the level of nitrate reductase during the initial stages of growth. Those in the 2nd class promote a rapid decay of the enzyme while those in the 3rd class initially cause enzyme decay but then promote the synthesis of the enzyme on further growth. Interactions between amino acids as N sources occurred. The possible reasons for the 3 peaks of nitrate reductase activity when Ipomoea is grown on nitrate are discussed.