Enzymes of auxin biosynthesis and their regulation I. Tryptophan and phenylalanine aminotransferase in pea plants

Abstract

The transaminations of L-tryptophan (L-trp) and of L-phenylalanine (L-phe) are catalysed in vitro by the same non-spec!fic aminotransferase. The transaminations procceed at the same pH (pH 8.5) and temperature (45 C) optima, have parallel increases in activity with addition of the coenzyme pyridoxal phosphate (PRP) and have identical elution characteristics in gel chromatography. The enzyme from pea seedlings has a relatively weak affinity for both amino acids (KM L-trp = 4.16 • 10 -I mmol 1-1; KM L-phe = 2.10 • 10 -1 mmol 1-1). Differences in affinity for a series of keto acids in the pea enzyme were observed, with pyruvate having the strongest and glyoxylate the weakest affinity. Transamination of L-trp and L-phe was demonstrated by enzyme extracts from pea, maize and tomato, but was not detected in kohlrabi. The amino acids L-asparagine (L-asn), L-phe, L-lysine (L-lys), L-methionine (L-met) have distinct inhibitory effects on the transamination of L-trp. Indolylacetylaspartate and tryptophol were shown to be competitive inhibitors. The regulation at the molecular level of L-trp transaminase activity is discussed.The deamination of L-trp or L-phe to indolylpyruvate (IPyA) and phenylpyruvate (PPyA) is usually regarded as the first step of auxin biosynthesis in higher plants. It may be catalysed by several systems among which L-tryptophan aminotransferase (TAT) has been most frequently investigated (

Biosynthesis of Auxin by the Gram-Positive PhytopathogenRhodococcus fasciansIs Controlled by Compounds Specific to Infected Plant Tissues

“…(35) and in Rhizobium lupini (21). This step in the IPyA pathway is indeed considered a regulatory reaction for IAA synthesis in bacteria (5,21) and also in plants (55). Since strain D188-5 lacks the virulence-associated linear plasmid pFiD188 and the cadmium resistance-associated plasmid pD188 (17), plasmid-encoded proteins are likely contributing to the production of IAA through the conversion of IEt.…”

Cloning and characterization of a locus encoding an indolepyruvate decarboxylase involved in indole-3-acetic acid synthesis in Erwinia herbicola

“…Alternatively, this phenomenon may reflect posttranscriptional regulation of IAA synthesis. It has been suggested that the control of IAA synthesis in plants occurs at least partially via regulation of the last reaction in the IPyA pathway, the conversion of IAAld to IAA by an IAAld oxidase, in conjunction with the formation and oxidation of TOL (45).…”

Partial purification and characterisation of an aromatic amino acid aminotransferase from mung bean (Vigna radiata L. Wilczek)

“…Enzymes showing tryptophan or aromatic amino acid aminotransferase activity have been assayed and partially purified from a number of plant species (see, for example, Truelsen 1972Truelsen , 1973Noguchi and Hayashi 1980;Vackova et al 1983;Kutacek 1985;E1 Bahr et al 1987;Taylor and Wightman 1987;Terziivanova-Dimova and Kutacek 1991;Koshiba et al 1993). These enzymes have been implicated in the biosynthesis of the plant hormone indole-3-acetic acid (IAA) from tryptophan (see, for example, Truelsen 1972;Kutacek 1985).…”