Mechanism of Auxin Action in Plants

Let us make an in-depth study of the mechanism of auxin action in plants.

Auxin (IAA) causes wide range of physiological effects in plants. Some of these effects such as cell elongation in shoot occur in minutes in response to auxin while others such as abscission occur in days in response to auxin treatment. Among these, the most important physiological effects of auxin is to stimulate cell elongation in stems and coleoptile and extensive research work has been done on it.

Most studies of auxin induced growth in plants have been carried out on excised sec­tions of dicot stems (such as soybean hypocotyls and pea epicotyls) and coleoptile (such as oat (Avena) and maize coleoptile). But, in recent years auxin-induced growth has also been demonstrated in intact auxin-deficient mutants of pea. The target sites of auxin action in dicot stems are the outer tissues including epidermis and outer cortex. In coleoptile, all non-vascular tissues i.e., epidermis and mesophyll are re­sponsive to auxin treatment.

The sequence of events following addition of auxin (IAA) to plant system is usually envisaged on the following lines:

(i) The initial event would involve adsorption of auxin to an auxin-specific binding site in target cells.

(ii) The auxin-binding site complex would then initiate a cascade of reactions involving:

(a) Membrane phenomena leading to media acidification and

(b) Nucleic acid related phenomena leading to longer range changes in enzymes involved in growth. One of the concurrent of (a) and (b) would be changes in plasticity of cell wall so that growth may occur.

1. Presence of Auxin-Binding Receptor:

A possible auxin-binding receptor protein has been identified in plants which is called as auxin-binding protein 1 (ABP1). This protein appears to be a dimer made of two polypeptides of about 22 kD each. ABP1 is located in lumen of endoplasmic reticulum (ER), but it is believed to be active on surface of cell.

2. Minimum Lag Time for Auxin Induced Growth is 10 Minutes:

When coleoptile or stem sections are excised and placed in sensitive growth measuring device, the growth response to auxin can be measured with high degree of precision. If auxin is absent in the medium, the growth rate declines rapidly.

Addition of auxin on the other hand, markedly stimulates growth rate (measured in terms of elongation or % increase in length) after a lag period of 10-12 minutes only. In both types of tissues the maximum growth (5 to 10 fold over the control rate) is attained after half an hour to one hour of auxin treatment.

In case of Avena coleoptile section, the maximum growth rate is maintained for 18 – 20 hours if sucrose or KCl is present in growth medium. Sucrose or KCl prolong the growth response to auxin by providing osmotically active solutes which can be taken up for maintaining turgor pressure during cell elongation. Although minimum lag time of 10 minutes for auxin-induced growth can be increased by using suboptimal conc. of auxin or by lowering the temperature, but it cannot be shortened by reversal of these conditions.

3. Auxin Causes Rapid Increase in Cell Wall Extensibility:

Cell-wall enlargement in plants involves two steps, (i) osmotic uptake of water across the plasma membrane resulting in increased turgor pressure of the cell and (ii) extension of cell wall in response to increased turgor pressure.

It is generally believed that auxin causes an increase in plastic (i.e., irreversible) exten­sibility of the cell wall by wall loosening events that require continuous input of metabolic energy. Wall loosening involves rearrangement of load-bearing bonds of cell wall that reduces wall stress – a phenomenon called as stress relaxation.

4. Auxin-Induced Proton (H⁺) Extrusion Acidifies the Cell Wall, Resulting in Wall Loosen­ing-The Acid Growth Hypothesis:

Rayle and Cleland (1970) in U.S.A. and Hager, Menzel and Cross (1971) in Germany first proposed independently that protons (H⁺) may be involved in auxin-induced cell wall loosen­ing. According to their acid growth hypothesis,

(i) Auxin causes responsive cells to extrude protons (H⁺ ions) actively from cytoplasm to cell wall resulting in decrease of cell wall pH (apoplastic pH) i.e., acidification of the cell wall,

(ii) The low apoplastic pH activates cell wall loosening enzymes which break the load- bearing bonds, thus increasing extensibility of the cell wall and

(iii) The extrusion of protons is facilitated by H⁺-ATPases located in plasma-membrane.

The acid growths hypothesis made three predictions, (i) auxin should increase the rate of proton extrusion and its kinetics should closely match to that of auxin-induced growth, (ii) neutral buffers should inhibit auxin-induced growth and (iii) compounds (other than auxin) which promote extrusion of protons (H⁺ ions) should also stimulate growth.

All the above predictions have since been confirmed. A fungal phytotoxin called fusicoccin (which is not an auxin or hormone) is known to stimulate both proton extrusion and growth in excised stem and coleoptile sections.

Two models are available to explain the mechanism of auxin-induced proton extrusion:

(a) According to one model, auxin initiates a signal transduction pathway resulting in production of secondary messengers which directly activate pre-existing H⁺-ATPases located in the plasma-membrane.

(b) According to another model, auxin-induced secondary messengers activate the expression of genes which encode the plasma-membrane H⁺-ATPases, The latter are synthesised on rough endoplasmic reticulum and targeted to plasma-membrane via Golgi-bodies.

It is believed that auxin may stimulate proton extrusion by both activation of pre-existing H⁺-ATPases and stimulation of synthesis of new H⁺-ATPases.

The fungal phytotoxin fusicoccin (obtained from the fungus Fusicoccum amygdali) is known to stimulate proton extrusion and growth only by activating the pre-existing H⁺-ATPases in plasma-membrane. Fusicoccin is sometimes called as ‘super-auxin’ because it causes much lower apoplastic pH than auxin. However, this phytotoxin is not an auxin or hormone and it does not mimic other physiological effects of auxin such as cell division. It does not stimulate expres­sion of any of the auxin-induced genes.

5. Acid Induced Wall Loosening is Mediated by Specific Proteins Called Expansins:

Previously, hydrolytic enzymes such as celluloses, hemi-celluloses and pectinases were considered as wall loosening enzymes which were activated by low pH during auxin-induced growth. But, while these enzymes loosen cell walls irreversibly, the auxin-induced growth is reversible with metabolic inhibitors. Therefore, these hydrolytic enzymes are not involved in wall-loosening during auxin-induced growth.

There are now compelling evidences to suggest that a group of cell wall proteins called expansions causes cell wall loosening in response to acidic pH. Expansions appear to loosen the cell walls by breaking the hydrogen bonds between polysaccharide components of the cell wall allowing the latter to stretch more easily.

6. Auxin Maintains the Capacity for Acid Induced Wall Loosening by Synthesis of New Wall Polysaccharides:

Auxin-induced acidification and loosening of cell walls is accompanied by biosynthesis of new wall polysaccharides so that growth may continue for longer period especially in coleop­tile. Auxin is known to increase activities of certain enzymes which are involved in biosyn­thesis of cell wall polysaccharides.

A constant supply of new wall materials maintains the capacity for acid induced wall loos­ening (CAWL) in response to auxin. Cell walls of sections treated with auxin have greater capacity for acid stimulated growth than those of control sections or sections treated with fungal phytotoxin fusicoccin. Polysaccharides synthesis and CAWL do not correlate well with rapid kinetics of growth and are therefore, parts of long term growth responses to auxin.