L-Tryptophan Aminotransferase and L-Tryptophan Dehydrogenase, Enzymes of IAA Synthesis, in Normal and Tumorous Tobacco Tissue Cultures

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Summary The normal BV-N tobacco tissue culture as well as tumor tissue cultures T-24 (octopine positive) and C-58 (nopaline positive) contain active L-tryptophan aminotransferase (TAT) and L-tryptophan dehydrogenase (TDH), which are the primary enzymes of the indolylpyruvate (IPyA) pathway leading to the IAA synthesis in plants. In both cultures the activity of TAT gradually increased during the whole subcultivation period, whereas TDH activity slowly decreased. The activity changes of TAT and TDH neither coincided with changes of the content of L-trp ( El Bahr et al. 1984), nor of IAA. The regulatory mechanism in the IAA synthesis is thought to be based on the low affinity of L-trp to the enzymes (KM are of millimolar range), limiting the endogenous conversion of L-trp to IPyA. The tobacco tumor tissue cultures differed from the normal ones in their higher IAA level, steeper growth rate and in the kinetics of the investigated enzymes. In crown-gall tissues the pH optimum of TAT was 9.2 as compared to 8.5 of normal tissue, the dissociation of the coenzyme PRP is lowered in TAT of tumor tissue, and finally, the stronger activation of TDH by the coenzyme NADPH in comparison to NADH, which was shown by normal tissue, could not be registered in the crown-gall tissue. Normal tissue cultures as well as crown-gall tissues preserve their ability to synthesize auxin by the IPyA pathway characteristic of eukaryotes. Even if the properties of the enzymes TAT and TDH in crown-gall tissues are modified, it does not seem probable that TAT and TDH represent the key enzymes responsible for the strong increase of the IAA content in the tumor strains.