Auxin biosynthesis pathway (2 types)

Auxin biosynthesis pathway has different types. There are two types of pathway- one is Tryptophan-dependent pathway and another one is Tryptophan-independent pathway.

The precursor of auxin hormone:

Tryptophan (aromatic amino acid)

The key enzyme of tryptophan-dependent auxin biosynthesis:

YUCCA–> a gene product, Tryptophan aminotransferase.

Auxin biosynthesis pathway: Tryptophan-dependent pathway:

There are four tryptophan-dependent pathway. Those are-

1. IPA (Indole-3-pyruvate) pathway
2. Tryptamine pathway
3. IAN (Indole-3-acetonitrile) pathway
4. IAM (Indole-3-acetamide) pathway

IPA (Indole-3-pyruvate) pathway

1. Deamination of the tryptophan occurs in the first step of the IPA pathway by the enzyme tryptophan transaminase.
2. Deamination of the tryptophan produces indole-3-pyruvate (IPA).
3. The first product of the pathway is IPA (indole-3-pyruvate). Hence, the name is IPA pathway.
4. Indole-3-Acetaldehyde is produced by the decarboxylation of the IPA (indole-3-pyruvate) by the enzyme decarboxylase.
5. Finally, Indole-3-acetice acid (IAA) is produced by the dehydrogenation of Indole-3-Acetaldehyde.

Example:

Tomato

The schematic representation of IPA (Indole-3-pyruvate) pathway:

Tryptamine pathway

1. Tryptamine is produced by the decarboxylation of tryptophan.
2. Tryptamine is the first product of the pathway. Hence, the name of the pathway is tryptamine pathway.
3. Tryptamine is deaminated to form Indole-3-Acetaldehyde.
4. Finally, Indole-3-acetice acid (IAA) is produced by the dehydrogenation of Indole-3-Acetaldehyde.

Example:

Tomato

The schematic representation of tryptamine pathway:

IAN (Indole-3-acetonitrile) pathway

1. Indole-3-acetaldoxime is produced from the tryptophan by the enzyme oxidoreductase.
2. Then Indole-3-acetaldoxime is converted to IAN (Indole-3-acetonitrile) by the enzyme acetaldoxime dehydratase.
3. Finally, Indole-3-acetice acid (IAA) is produced from Indole-3-Acetonitrile (IAN) by the enzyme nitrilase.

Example:

Brassicaceae, grass family, banana family.

The schematic representation of IAN (Indole-3-acetonitrile) pathway:

IAM (Indole-3-acetamide) pathway

1. IAM (Indole-3-acetamide) is produced as the first product of this pathway from tryptophan by the enzyme tryptophan monooxygenase.
2. Finally, Indole-3-acetice acid (IAA) is produced from IAM (Indole-3-acetamide) by the enzyme Indole acetamide hydrolase.

Example:

Agrobacterium.

The schematic representation of IAM (Indole-3-acetamide) pathway:

Auxin biosynthesis pathway: Tryptophan-independent pathway:

1. This pathway is mutants for both maize and Arabidopsis.
2. Instead of tryptophan, the pathway is started from indole.
3. The precise pathway is not know properly.
4. This pathway is observed in several plant species.

Other related notes:

1. Phototropin: https://thebiologyislove.com/phototropin/
2. Dark reaction: https://thebiologyislove.com/calvin-benson-cycle-or-dark-reaction/

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