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IAA or Indole-3-acetic acid is considered as the principal natural auxin. Synthesis of IAA from the amino acid tryptophan. Synthesis of IAA occurs in three steps, first tryptophan is converted to indole-3-pyruvic acid or IPA. This reaction is catalyzed by the multispecific enzyme tryptophan amino transferase. In the second step after decarboxylation IPA is converted into Indole -3-acetaldehyde or IAAld. This reaction is catalyzed by the enzyme Indole -3-pyruvate decarboxylase. The last step is the oxidation of IAAld to IAA by NAD dependent indole-3-acetaldehyde oxygenase. Here biosynthesis of IAA takes place using tryptophan, but certain plant does not require tryptophan for synthesis of IAA.

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