Auxin-Biosynthesis,Transport,Mechanism of action

Natural auxins (indole-3-acetic acid = IAA) were detected in different organs of actively growing plants—buds, young leaves, roots and stem apices, cotyledons, etc. and are the only phytohormones that have analogues in the animal world.


The auxin precursor in plants is tryptophan or substances derived from its degradation. It is formed by following three steps involving three enzymes: transaminase, which catalyzes the conversion of tryptophan into tryptamine, decarboxylase—from tryptamine to indole pyruvic acid, which transforms into β-indole acetaldehyde and aldehyde dehydrogenase, which catalyzes the formation of
β-indole acetic acid (Fig.).

The presence of two active groups—carboxyl and amine and the properties conferred by the nucleus of the indole molecule, transform IAA in a very active substrate for biochemical reactions which leads to its rapid inactivation, both in vivo and in vitro.

The quantity of auxins in plants is determined by the action of the enzyme auxin oxydase that interferes in controlling the levels of endogenous auxins in the root tip and can interfere with auxin metabolism.
Indole acetic acid is found in plants in two forms:

• bound (70 %) to other macromolecules, less mobile, which in most cases, don’t have phytohormone activity and lack toxicity. Auxins linked with proteins bind to the active cell centers and suppress their activity. Under the action of proteolytic hydrolases auxins are released from their protein substrates and exert again their phytohormone activity;
• free (30 %)—available auxins are mobile and are easily transported to different organs of growth.

Fig. Indole acetic acid biosynthesis


Chromatography investigations and the use of labeled atoms showed that auxins are circulating in a polar manner at a speed of 10–20 mm/h, basipetally through the phloem and parenchyma. Transport requires energy and can be blocked by the presence of alkaloids and lack of oxygen. Acropetal movement is less intense.

Auxins circulate in plants usually during the period of activation of morpho-genetic processes of growth and development, when some regions are involved in the synthesis of phytohormones while others are involved in various growth processes.

Migration of auxin molecules are based on the electrical charge of its carboxylic groups. The mechanism of polar auxin transport consist in the fact that, in apical cone cells, IAA enters passively with hydrogen ions while on the basal part it is actively secreted through the cell membrane and is based on the difference in the electrical potential between the top of the plant, with negative charge, and its
base with a positive electric charge (Fig.).

Mechanism of action

Auxins act on gene expression by activating the cytosolic protein ARF (auxin response factor).

Fig. Polar auxin transport

ARF is a transcription factor that can enter the nucleus, where it binds the promoter sequences of various genes and alters gene expression levels. IAA influences polyribosomes and the activity of the nuclear apparatus including:

RNA polymerase (RNA polymerase-1) due to the increasing content of the transcription initiation factor ϒ

cellulose synthases by inducing de novo synthesis of certain types of proteins like citrate synthase, invertase, peroxidase, of co-enzymes, vitamins, etc. (Fig. ). In the membrane, auxins interact with specific receptors ABP1 (auxin-binding protein 1) (low concentrations of the phytohormone trigger cell division). Biological significance.

The functional range of auxin is very broad. Low doses accelerate growth having a catalytic role while larger doses halt the growth of roots and shoots. IAA is the main hormone of cell division and cambium activity, one of the histogenetic factors and, in particular, of root formation by inducing true
secondary meristems.

It contributes to root differentiation, stimulating the formation of lateral roots and determining cell elongation. Physiologically, auxins trigger particular metabolic reactions, stimulating plant growth, and seed germination and inhibiting aging processes in tissues.

Tissues rich in auxin attract nutrients conditioning apical dominance and inhibiting lateral bud growth, adjust leaf falling, callus formation in tissue culture, fruit growth, slows down the aging process. IAA plays an important role in the phenomenon of tropisms and nasties. Ensures integration of plant organ activity.

Auxins stimulate photosynthesis by increasing CO2 conversion into photosynthesis products and their subsequent translocation. Practical applications. Synthetic auxins are used as herbicides, they function as inhibitors at high concentrations. They are used for rooting, obtaining parthenocarpy, switching sex in plants, etc.

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