Growth and Development of Horticultural Crops (1+1)

Biosynthesis of Auxin (IAA) in plants

Tryptophan dependent pathways

• In 1935, Thimann demonstrated that a fungus Rhizopus suinus could convert an amino acid tryptophan (trp) into indole-3 acetic acid (IAA). Since then, it is generally held that tryptophan is primary precursor of IAA in plants.
• The indole-3-acetic acid (IAA) can be formed from tryptophan by 3 different pathways.
(a) TAM (Tryptamine) pathway

• Tryptophan is decarboxylated to form tryptamine (TAM) followed by deamination of the latter resulting in the formation of indole-3-acetaldehyde (IAld). The enzymes involved are tryptophan decarboxylase and tryptamine oxidase respectively. IAId is readily oxidised to indole-3-acetic acid (IAA) by the enzyme IAId dehydogenase.
(b) IPA (Indole-3-pyruvic acid) pathway

• Tryptophan is deaminated to form indole-3-pyruvic acid (IPA) followed by decarboxylation of the latter resulting in the formation of indole-3-acetaldehyde (IAId). The enzymes involved are tryptophan transminase and indole pyruvate decarboxylase.
• One of the above two methods (sometimes both) is most common pathway of formation of IAA in plants.
(c)IAN (Indole-3-acetonitrile) pathway

• It occurs in some plants especially those belonging to families Brassicacease, Poacceae and Musaceae. Tryptophan is converted into IAA in the presence of the enzyme nitrilase. Indole-3-acetaldoxime and indole-3-acetaonitrile (IAN) are the intermediates


Pathways of tryptophan-dependent indole-3-acetic acid biosynthesis.
http://www.ruhr-uni-bochum.de/sfb480/Bilder%20Teilprojekte/A%2010/Figure%201.jpg

• The abbreviations are: AAO1: indole-3-acetaldehyde oxidase; AMI1: amidase 1 (indole-3-acetamide hydrolase); IAOx-N-oxide: indole-3-acetaldoxime-N-oxide; IPAD: indole-3-pyruvic acid decarboxylase; MYR: myrosinase; NIT1-3: nitrilases isogenes 1–3; TDC: tryptophan decarboxylase; TMO: tryptophan-2-monooxygenase; TRPA: tryptophan aminotransferase; YUCCA: flavin monooxygenase-like protein.