Plant Hormone Auxin: Major Functions, Biosynthesis, Transport and Mode of Action

• First plant hormone discovered was auxin.
• By Frits Went
• The chemical itself was first isolated from urine, it is indole acetic acid
• Auxin a- C₁₈ H₃₃ O₅ & Auxin b C₁₈ H₃₀O₄ (IAA) exist in plants

Naturally occurring auxins

• IAA(Indole3- Acetic Acid)
• Indole-3-acetaldehyde,
• Indole-3-pyruvic acid
• Indole-3-acetonitrile
• Indole-3-ethanol
• Phenyl acetic acid

Synthetic Auxins

• Indole 3- Butyric Acid (IBA)
• Indole 3-propionic acid
• Indole Pyruvic acid
• NAA- Napthelene Acetic acid
• 2,4,D- 2,4, Dichlorophenoxy acetic acid

Synthetic auxins are more stable
Major effect of auxin? cell elongation

• Cell division and differentiation
• Prevention of lodging: Naphthyl acetamide is commonly used

What is wound hormone?
Healing of injury in plants by the application of auxins is due to the secretion of traumatic acid (wound hormone)

Root initiation: using NAA & IBA
Maintenance of apical dominance by IAA
Prevention of abscission layer
Promote flowering by NAA and 2,4-D
Delay ripening
As weedicide

2, 4-D : high concentration toxic to dicots (broad leaved plants) but monocots are resistant.

Mode of action of 2,4-D as a weedicide?

Increased metabolic activity and ethylene production leading to early senescence and disordered morphogenesis
2, 4-D +2, 4, 5-T more effective

What is ‘agent orange’?

Agent orange used by US military to defoliate forests in parts of South Vietnam was 2,4-D +2,4,5-T

Auxin synthesis :Auxin Precursor: Amino acid Tryptophan. The essential element required for the synthesis for tryptophan is zinc.

4 pathways

•     IPA indole-3- pyruvic acid pathway
•     TAM (tryptamine pathway)
•     IAN (indole-3-acetonitrile pathway)
•     IAM (indole-3-acetamide pathway)

Auxin destruction

•      photo oxidation
•      enzymic oxidation by IAA oxidase
•      Polar transport of auxin is an active process that involves carriers
•      Basipetal transport in the shoot apex (from top to base)
•      Acropetal transport in the root (from base to top).

Mechanism of auxin transport: Chemiosmotic hypothesis based on two propositions

•    Proton Motive Force(PMF) : a pH gradient is created along the plasma membrane that facilitate the transport of auxin.
•    IAA efflux carrier: auxin transporting cells have IAA efflux carrier in their basal part.
Auxin can enter cells through by diffusion or influx transporters like AUX I in the plasma membrane and moves out through efflux transporters called PIN proteins: transmembrane proteins inserted in localized portions of the PM.

What are PIN proteins? Are auxin efflux transporters.

What are Phytotropins? Are inhibitors of polar transport (auxin transport inhibitors).
Phytotropins: 3 well known examples

a) Morphactins (9-hydroxyflourine-9-carboxylic acid)
b) TIBA (2,3,5-triiodo benzoic acid)
c) NPA (Naphthylphtalamic acid)
        - Mode of action: preventing auxin efflux
        - Natural phytotropin : flavonoids

 Auxin > increased proton pumping > H⁺ to acidify cell wall > cell wall loosens (expansin protein catalyze pH dependent wall extension growth > turgor pressure pushes loosened wall and growth occurs.

Proteins involved in auxin action 

•     ABPI (Auxin binding protein I)
•     Transport inhibitor response protein