Indole-3-acetic acid (IAA) occurs in all plants, 

Peas contain 4-chloroindole-3-acetic acid. 

Mustards and corn contain indole-3- butyric acid (IBA).

TLS Online TPP Program

#Id: 7515

#Unit 6. System Physiology – Plant

The highest concentrations of free auxin in the living plant are in the apical meristems of shoots and in young leaves because these are the primary sites of auxin synthesis. 

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#Id: 7516

#Unit 6. System Physiology – Plant

Auxin transport takes place polarly

Main axes of shoots and roots have apex-base structural polarity

This structural polarity is dependent on the polarity of auxin transport

Went discovered that IAA moves mainly from the apical to basal end

This type of unidirectional transport is termed polar transport

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#Id: 7517

#Unit 6. System Physiology – Plant

Auxin is the only plant growth hormone known to be transported polarly.  Auxin efflux takes place by PIN proteins

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#Id: 7518

#Unit 6. System Physiology – Plant

Auxin influx 

Auxin can enter plant cells from any direction by either of two mechanisms:

Passive diffusion of the protonated (IAAH) form across the phospholipid bilayer

Secondary active transport of the dissociated (IAA–) form via a 2H+–IAA– symporter

The dual pathway of auxin uptake arises because the passive permeability of the membrane to auxin depends strongly on the apoplastic pH.

TLS Online TPP Program

#Id: 7519

#Unit 6. System Physiology – Plant

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#Id: 7520

#Unit 6. System Physiology – Plant

Polar transport require energy and is gravity independent