An auxin, indole-3-acetic acid (IAA) was the first plant hormone identified. It is manufactured primarily in the shoot tips i.e. in leaf primordia and young leaves, in embryos and in parts of developing flowers and seeds. It is transported from cell to cell through the parenchyma surrounding the vascular tissues by the expenditure of energy as ATP. IAA moves in one direction only i.e. the movement is polar and downward. Such downward movement in shoots is said to be basipetal movement and in roots it is acropetal.

Biosynthesis and metabolism

IAA is chemically similar to the amino acid tryptophan, which is generally the molecule from which IAA is derived. Three mechanisms have been suggested to explain this conversion. Tryptophan is converted to indolepyruvic acid through a transamination reaction. Indolepyruvic acid is then converted to indoleacetaldehyde by a decarboxylation reaction. The finalnal step involves oxidation of indoleacetaldehyde resulting in indoleacetic acid. Tryptophan undergoes decarboxylation resulting in tryptamine. Tryptamine is then oxidized and deaminated to produce indoleacetaldehyde. This molecule is further oxidized to produce indoleacetic acid. The enzymes responsible for the biosynthesis of IAA are most active in young tissues such as shoot apical meristems and growing leaves and fruits. The same tissues are the locations where the highest concentrations of IAA are found. One way plants can control the amount of IAA present in tissues at a particular time is by controlling the biosynthesis of the hormone. Another control mechanism involves the production of conjugates, which are, in simple terms, molecules that resemble the hormone but are inactive. The formation of conjugates may be a mechanism of storing and transporting the active hormone. Conjugates can be formed from IAA via hydrolase enzymes. Conjugates can be rapidly activated by environmental stimuli signaling a quick hormonal response. Degradation of auxin is the final method of controlling auxin levels.

Role of auxins
• Activates the differentiation of vascular tissue in the shoot apex and in calluses.
• Initiates division of the vascular cambium in the spring.
• Promotes growth of vascular tissue in healing of wounds.
• Activates cellular elongation by increasing the plasticity of the cell wall.
• Maintains apical dominance indirectly by stimulating the production of ethylene, which directly inhibits lateral bud growth.
• Activates a gene required for making a protein necessary for growth and other genes for the synthesis of wall materials made and secreted by dictyosomes.
• Promotes initiation and growth of adventitious roots in cuttings.
• Promotes the growth of many fruits (from auxin produced by the developing seeds).
• Suppresses the abscission (separation from the plant) of fruits and leaves (lowered production of auxin in the leaf is correlated with formation of the abscission layer).
• Inhibits most flowering (but promotes flowering of pineapples).
• Activates tropic responses.
• Controls aging and senescence, dormancy of seeds.

Transport of auxin occurs unidirectionally, infact auxin is the only plant hormone that is known to be transported polarly or unidirectionally. The direction of auxin transport is basipetal i.e. from shoot apex to the base direction. This basipetal polar transport in the stem mostly occurs through the vascular parenchyma tissue. The polar transport does not occur symplastically; i.e. auxin exits the cell through the plasma membrane that is known as auxin efflux. It diffuses across the middle lamella and enters the cell below through the plasma membrane. The polar transport of auxin is completely energy dependent and a gravity independent process. Synthetic compounds TIBA (2, 3, 5 – triidobenzoic acid) and NPA (Naphthylphthalamic acid) act as the auxin efflux inhibitors and thus block the polar transport.

The Auxin (IAA) influx occurs by two different mechanisms
i. Simple passive diffusion of the protonated (IAA) form, which is across the plasma membrane.
ii. Secondary active transport of the deprotonated (IAA-) anionic form, which is through a 2H+ – IAA- symporter.

The protonated (IAA) is lipophilic and it readily diffuses across the plasma membrane. In contrast, deprotonated (IAA-) anionic form is negatively charged and thus does not cross membrane by simple diffusion. Since the plasma membrane H+-ATPase normally maintains the pH of the cell wall at 5 (approx.), which is about half of the auxin in the apoplast; it will be in the protonated form and will diffuse passively across the plasma membrane down the concentration gradient. Once IAA enters the cytosol, which has a neutral pH of ~7.2, nearly all of it will get dissociated to the anionic form. The deprotonated (IAA-) anionic form enters the cell through a 2H+ -IAA- symporter known as the influx carrier, which is uniformaly distributed around the cell plasma membrane. A 2H+ -IAA- symporter-mediated secondary active uptake mechanism is responsible for the accumulation of auxin. The molecular genetic studies in Arabidopsis thaliana have identified a permease type IAA influx carrier that has been termed as AUX1.
Auxin carriers PIN proteins mediate the auxin efflux. PIN proteins, which are named after the pin-shaped inflorescences formed by the pin1 mutant of Arabidopsis, are the secondary transporters. They are asymmetrically localized on the plasma membrane of the cells (normally concentrated at the basal ends of the cell in the longitudinal pathway) and their intercellular auxin flow directionality is determined by their polarity. System physiology – Plant 382 Ace The Race
In phloem, auxin transport is non-polar in nature. Mostly auxins, which are synthesized in mature leaves, appear as transported to the rest of plant via non-polarity. In the phloem, non-polar transport of auxin is passive in nature. On the other hand, there appear to be two transport streams in root. First is an acropetal stream which is arriving from the shoot and flows through the xylem parenchyma cells in the central cylinder of the root which directs the auxin toward the root tip. Another is basipetal stream which reverses the direction of flow and moving auxin away from the root tip, through the outer epidermal and cortical cell files.

 Physiological effects
• Cell elongation expansion is stimulated by auxins in stream and coleoptiles. Auxins promote elongation growth by increasing the cell wall extensibility according to the acid growth hypothesis. By activating (directly or indirectly) proton pump, H+-ATPase, auxin stimulates proton pumping, which is present on the plasma membrane. As a result, the pH of the cell wall falls as low as 4.5. A class of cell wall protein is activated by the low pH, which is termed as expansins, that disrupts the hydrogen bonding between cellulose microfibrils, which cause the laminate structure of the cell wall to loosen. With the reduced rigidity of the cell wall, cell can elongate. The pH is raised back to normal; then expansin-triggered loosening of the wall is reversed which shows that expansin cellulose does not break covalent bonds. A fungal compound fusicoccin induces rapid cell elongation and triggers proton pumping out of sensitive cells, accompanied by wall loosening like an auxin. Permeation of the cell wall with buffers prevents the lowering of extracellular pH, which can block the action of fusicoccin or auxin.
• Cell Differentiation Auxin concentration regulates the differentiation of relative amounts of xylem and phloem. In general, the differentiation of xylem is induced by high auxin concentrations and phloem differentiation is induced by low auxin concentrations.
• Rooting Elongation of the primary root is inhibited by auxins, but auxin stimulates initiation of root on stem cuttings (adventitious roots) and lateral development of root. The root hair zone, the lateral roots are commonly found and originate from cells present in the pericycle. Auxin stimulates these pericycle cells to divide. The dividing cells gradually give rise to the lateral root.
• Apical Dominance In most plants, the growth of lateral buds is inhibited by the growing apical bud and this phenomenon is called apical dominance. Removal of the shoot apex usually results in the transported basipetally from the apical bud.
• Tropic Response The differential growth in plant organs under the influence of directional stimuli (i.e. light, gravity) induced auxin, is termed as tropic response. Tropic stimuli induce the lateral redistribution of auxin according to the Cholodny-Went theory which results unequal accumulation between opposing regions of a responding organ and thus differential growth.
• Fruit Set The transition of a quiescent ovary to a rapidly growing young fruit is known as fruit set. Auxin promotes development of fruit. Fruit growth may depend on auxin produced in developing seeds. After fertilization, auxin is produced in the endosperm and the embryo of developing seeds. Treatment of the unpollinated flowers with auxin may induce seedless fruits in some plant species. The production of such seedless fruits is known as parthenocarpy.

 Auxins Signaling Pathway

Many aspects of plant growth and development are controlled by a pivotal plant hormone that is auxin. It has receptor termed ABP1 (Auxin Binding Protein-1) located in the lumen of endoplasmic reticulum as well as plasma membrane. Physiological effects mediate through transcriptional regulation. A family of transcriptional factor activates hundreds of genes called Auxin Response Factor (ARFs) in response to auxin, which recognizes specific promoter elements characteristic of auxin response genes. The protein products of a family of genes named AUX/IAAs (auxin/indole-3-acetic acids) directly interact with AFRs and inhibit their transcriptional activity in the absence of auxin. In the presence of auxin, AUX/IAAs are rapidly removed, abrogating the inhibition of ARFs and the expression of auxins response genes. This process regulates by auxin through Transport Inhibitor Response 1 (TIR1). TIR1 is a soluble and nuclear-located auxin receptor protein. AUX/IAAs interact directly with TIR1. AUX/IAA-TIR1 binding and ubiquitination and degradation of AUX/IAAs is stimulated by auxin.