The role of auxin

Do you know what auxin is? Do you know some of its functions? The most important function of auxin is to promote cell growth, especially cell elongation, which plays a great role in plant growth. Auxin can change the distribution of some nutrients inside the plant, so that the plant can better absorb some nutrients and allow the plant to develop better. Let's take a look at the role of auxin.

Auxin has many functions. Do you all know that it has a dual function. It can promote the growth of plants, but it can also inhibit the growth of plants. It can make plants germinate well, but it can also inhibit the germination of plants. It can prevent fruit and flower drop, and it can also thin out flowers and fruits.

Physiological effects

The most obvious effect of auxin is to promote growth, but the effect on the growth of stems, buds and roots varies depending on the concentration. The optimum concentrations of the three are stems>buds>roots, which are approximately 10E-5 mol, 10E-8 mol, and 10E-10 mol per liter, respectively. The direction of indoleacetic acid movement in plants shows obvious polarity, mainly from top to bottom. The apical dominance that inhibits the growth of axillary buds in plant growth is closely related to the polar transport and distribution of indoleacetic acid. Auxin also has the function of promoting callus formation and inducing rooting.

Auxin acts in multiple locations and is primarily involved in cell wall formation and nucleic acid metabolism. Experiments using radioactive amino acids to feed isolated tissues have demonstrated that auxin promotes protein biosynthesis while promoting growth. Auxin promotes RNA biosynthesis particularly significantly, thereby increasing the RNA/DNA and RNA/protein ratios. Among all RNAs, rRNA is the one whose synthesis is most promoted. In terms of its effect on the cell wall, auxin activates the hydrogen ion pump, lowers the pH value outside the plasma membrane, and greatly increases the elasticity and plasticity of the cell wall, thereby loosening the cell wall and increasing its water absorption capacity. Considering that the time threshold for auxin to affect protoplasm flow is 2 minutes, and that for causing coleoptile elongation is 15 minutes, which is an extremely short time, it is believed that its effect is not through affecting gene regulation, but may occur through affecting the translation process in protein synthesis (especially proteins in the cell wall or plasma membrane).

Because auxin is easily destroyed through metabolism in the body, its effect is short-lived when applied externally. Its analogues have similar physiological effects and are not easily destroyed, so they are widely used in agricultural production (see plant growth regulators). Auxin is synthesized in expanding young leaves and apical meristem, transported over long distances through the phloem, and accumulated from top to bottom toward the base. Roots can also produce auxins and transport them from bottom to top. Auxin in plants is formed from tryptophan through a series of intermediates. The main pathway is through indoleacetaldehyde. Indoleacetaldehyde can be formed by oxidative deamination of tryptophan to indolepyruvic acid followed by decarboxylation, or by decarboxylation of tryptophan to tryptamine followed by oxidative deamination. Indoleacetaldehyde is then oxidized to indoleacetic acid. Another possible synthetic pathway is the conversion of tryptophan to indoleacetic acid via indoleacetonitrile, found in cruciferous plants.

In plants, indoleacetic acid can combine with other substances and lose its activity, such as combining with aspartic acid to form indoleacetylaspartic acid, combining with inositol to form indoleacetic acid inositol, combining with glucose to form glucoside, and combining with protein to form indoleacetic acid-protein complex. Bound indoleacetic acid often accounts for 50-90% of the indoleacetic acid in plants and may be a storage form of auxin in plant tissues. They can produce free indoleacetic acid after hydrolysis.

Indoleacetic acid oxidase, which is ubiquitous in plant tissues, can oxidatively decompose indoleacetic acid.

Auxin has many physiological effects, which are related to its concentration. At low concentrations it can promote growth, but at high concentrations it will inhibit growth or even cause plant death. This inhibitory effect is related to whether it can induce the formation of ethylene. The physiological effects of auxin are manifested at two levels.

At the cellular level, auxin can stimulate the division of cambium cells; stimulate the elongation of branch cells and inhibit the growth of root cells; promote the differentiation of xylem and phloem cells, promote the rooting of cuttings, and regulate the morphological construction of callus tissue.

Auxin acts from seedling to fruit ripening at both the organ and whole plant levels. Auxin controls the reversible red light inhibition of mesocotyl elongation in seedlings; when indoleacetic acid is transferred to the underside of the branch, it produces geotropism of the branch; when indoleacetic acid is transferred to the backlit side of the branch, it produces phototropism of the branch; indoleacetic acid causes apical dominance; leaf senescence is delayed; auxin applied to leaves inhibits abscission, while auxin applied to the adaxial end of the abscission layer promotes abscission; auxin promotes flowering, induces the development of parthenocarpic fruits, and delays fruit ripening.

Now everyone should have a better understanding of the role of auxin, and know what auxin is. Auxin is indispensable in the growth process of plants. Therefore, auxin has now been well utilized in the development of agriculture and plays a great role in promoting cell growth.