TLS Online TPP Program

#Id: 7513

#Unit 6. System Physiology – Plant

Auxin biosynthesis from tryptophan (Trp). 

In the first step, Trp is converted to indole-3-pyruvate (IPyA) by the TAA family of tryptophan amino transferases. Subsequently, IAA is produced from IPyA by the YUCCA family of flavin monooxygenases.

TLS Online TPP Program

#Id: 7514

#Unit 6. System Physiology – Plant

Although free IAA is the biologically active form of the hormone but toxic in high concentration, the vast majority of auxin in plants is found in a covalently bound state are considered hormonally inactive.

IAA has been found to be conjugated to both high- and low-molecular-weight compounds.

Low-molecular-weight conjugated auxins include esters of IAA with glucose or myo-inositol and amide conjugates such as IAA-N-aspartate.

High-molecular-weight IAA conjugates include IAAglucan

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. 

TLS Online TPP Program

#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

TLS Online TPP Program

#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

TLS Online TPP Program

#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.