Plant Physiology - Musienko M. M. 2001

General principles of plant growth and morphogenesis regulation
Auxins

Auxins were the first phytohormones to be investigated. Natural auxin was identified in 1934 in F. Kögl's laboratory as indole-3-acetic acid (IAA) in the tips of cereal coleoptiles. Auxins are substances of an indolic nature. To detect auxins, the well-known Went oat test is used, in which Went cut off the tips of oat seedling coleoptiles and placed them on Agar blocks for an hour so that the cut surfaces were in contact. Then Went cut the agar into small blocks and placed them on one side of decapitated coleoptiles, which were kept in the dark during the experiment. Over the next hour, a curvature was observed in the direction opposite to where the agar cube was located (Fig. 175).

With this experiment, Went demonstrated that the Influence of the apex is associated with the release of some chemical compound. Initially, it

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Fig. 175. The "oat" test for auxin detection

was absorbed by the agar cube and then diffused into the decapitated part of the coleoptile, where it caused a growth effect manifested by the bending of the seedling. Went named this compound auxin (from the Greek auxein — to increase). It is also called heteroauxin. Today, many synthetic preparations are known that stimulate growth similarly to indoleacetic acid. These include indolepyruvic, chlorophenoxyacetic, 2,4-dichlorophenoxyacetic acids, and Other Compounds. They are cheaper to produce compared to IAA and are often more active, since plants lack Enzymes to break them down.

The precursor of IAA in the plant is Tryptophan, and possibly tryptamine, with two known pathways of auxin Biosynthesis:

Different plant species use different pathways for its biosynthesis from tryptophan, while maize is characterized by the fact that different pathways are observed in it even at individual stages of ontogenesis.

Various indole derivatives are precursors of auxin; they can be converted into IAA in the plant and act as auxin.

Auxins are found in higher and lower plants, and they are also present in the saliva and urine of humans and animals. In higher plants, their synthesis is most intensive in young leaves and buds, active cambium, pollen, and developing seeds. A small amount of auxin has been detected in ROOT apex Meristems, but it is likely transported there via conducting tissue from the root base.

The transport of auxin by parenchyma and cambial Cells is carried out polarly at a speed of 10–15 mm/h from the SHOOT apex to the root. In the stem, movement occurs through vascular bundles. From the leaves, auxin moves via the phloem. This movement is not polar, but it coincides with the transport of CARBOHYDRATES.

Auxin transport requires Energy Expenditure. IAA penetrates into the apical part of The Cell passively along with H⁺ ions, while at the basal end, carriers may be involved that export auxin out of the cell even against a concentration gradient.

Despite the clearly expressed polarity, it is still unclear how auxin stored in storage tissue reaches the shoot apex. It may arrive in an inactive form and then be converted into an active one in the coleoptile. Alternatively, it may be completely degraded in the endosperm,

to be synthesized anew in the seedling. In the root, the polarity of movement is weakly expressed; as a rule, this movement is acropetal, i.e., toward the root tip.

Physiological Action of Auxin

Auxin exerts a diverse influence on the plant depending on the stage of ontogenesis, plant species, and, particularly, the type of tissue. At high concentrations, it is toxic. The herbicide 2,4-D is a synthetic auxin used for weed control. The most pronounced effect of auxin is manifested in the stimulation of growth by elongation, as well as in The formation of cambium, vascular bundles, and roots.

The first Structure/182.html">Practical Application of auxins was based on their ability to stimulate root formation in cuttings (Fig. 177).

Fig. 177. Stimulation of root formation under METABOLISM/18.html">The Influence of auxin: a — cuttings treated with auxin 21 days before the photograph was taken, b — untreated cuttings. Pay attention to the formation of adventitious roots in the treated plants

In agricultural practice, indole-butyric acid (IBA) is also used for rooting cuttings:

Tissues containing auxins are characterized by an attracting capacity. Auxin influences the Differentiation of the conducting tissue of growing shoots, and a similar reaction has been noted in callus. Auxins control fruit growth and cause The phenomenon of apical dominance, where the apical bud inhibits the growth of lateral (axillary) buds (Fig. 178).

Auxin is able to delay the opening of lateral buds and the shedding of leaves and fruits. During leaf senescence, a process of reutilization of certain ions, Amino Acids, and sugars occurs, which flow back into the stem. Then, enzymes destroy the cell walls of the abscission zone at the Base of the petiole by dissolving the middle lamella and hydrolyzing Cellulose microfibril fibers. Beneath the abscission layer, a protective layer of suberin-impregnated cells forms, which isolates the leaf from the shoot prior to leaf fall (see Fig. 183). Along with other factors, leaf fall correlates with a decrease in auxin content in the leaves.

Fig. 178. Apical dominance (A): a — intact plant (pronounced apical dominance), b — decapitated plant, c — auxin replaces the apical bud; B — effect of auxin Treatment (left — auxin-treated plant, right — control)

Application of auxins allows for regulating leaf fall, fruit drop, and flower abscission in agricultural production. They affect protoplasmic viscosity, enhance Water uptake by various tissues (e.g., potato tuber parenchyma), and induce cytoplasmic streaming. Their primary role lies in growth movements — Tropisms and nasties.

Upon entering the cell, IAA binds to specific receptors, affecting the functional activity of membranes, polyribosomes, and the nuclear apparatus.

In The Plasma Membrane, auxin induces the operation of the H⁺-pump, resulting in the acidification of The Cell wall matrix. Consequently, acid Hydrolases are activated and the cell wall softens, which is a prerequisite for cell enlargement by elongation.

The complex of the phytohormone and receptor enters The Nucleus and activates the synthesis of all RNA forms, including mRNAs, leading to the formation of new polyribosomes and Protein Synthesis in the Cytoplasm. Although the "acid growth hypothesis" accounts for the initial auxin-induced cell elongation, it does not explain its subsequent effects on the plant.

There are likely Two Types of auxin effects during the elongation phase. The first is short-term, driven by "acid growth," followed by a long-term effect associated with The regulation of expression of at least 10 specific genes responsible for growth processes.

Auxins increase the Coupling of oxidation and phosphorylation as well as the ATP content. They have a positive effect on the cellular energy charge (The ratio of the sum of ATP and ADP to AMP). It is well established that even minor shifts in the cellular energy potential alter the rates of enzymatic reactions.

Almost all tissues can degrade auxin via IAA oxidase. It breaks down IAA where it is no longer needed, such as at the base of the elongation zone. Along with the enzymatic oxidation of IAA, its light-induced Cleavage (photooxidation) is of great significance. Ultraviolet rays with a wavelength of 280 nm exert a particularly strong destructive effect.

Control over GROWTH AND DEVELOPMENT processes via auxins is only possible if their intracellular concentration can be regulated. The balance between auxin synthesis, IAA oxidation, and IAA conjugation can serve as a sensitive regulatory system for indole-3-acetic acid concentration, and consequently, for growth processes.