STRUCTURE AND PROPERTIES OF BIOMOLECULES - A. E. Zemlyakov - 2017

08. BIOREGULATORS: PHYTOHORMONES

A number of low-molecular-weight secondary metabolites play a crucial regulatory role in plant vital activity. Among these endogenous bioregulators, a distinct group is formed by phytohormones, which are characterized by a generalized effect on all plants and extremely low active concentrations (down to 10^-11 M). Phytohormones are produced by the Cells of growing Organs, such as ROOT tips, young leaves, and shoots. Subsequently, these substances are transported to other plant organs, where they trigger various biochemical processes. Plant growth responses are typically regulated by several phytohormones acting in concert: some substances act as stimulators, while others act as inhibitors. For instance, when acting jointly with indole-3-acetic acid, Gibberellins and Abscisic acid can function either as antagonists or as synergists depending on the context.

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The main groups of phytohormones include Auxins, gibberellins, Cytokinins, abscisic acid, and Ethylene. Recently, brassinosteroids, jasmonic acid, and phaseic acid have also been added to this list.

❖ Auxins (from the Greek word meaning to grow or increase) are plant growth stimulants. They were first described by the Danish scientist F. Went in 1926. In 1937, the Anglo-American biologist K. Thimann determined The Structure of the first representative of auxins: indoleacetic acid (heteroauxin). This compound stimulates The Biosynthesis of all forms of Cell/22.html">RNA and Protein, promotes Cell Division, enhances the elongation of cell walls, and stimulates the GROWTH AND DEVELOPMENT of The Root System. The biological action of heteroauxin is strictly concentration-dependent: low concentrations (10^-4 - 10^-12 M) exert a stimulating effect, whereas high concentrations (10^-2 - 10^-3 M) inhibit growth.

Along with plants, auxins are also produced by numerous Bacteria and Fungi. In plants, the concentration of heteroauxin ranges from 1 to 1000 mg per 1 kg of fresh weight. Auxins exist in a free state or are released upon the Hydrolysis of inactive derivatives conjugated with Amino Acids (such as β-aspartic acid) and sugars (such as D-glucose).

Biosynthetically, indoleacetic acid is derived from Tryptophan. Its precursors include indolepyruvic acid, indoleacetonitrile, indoleacetamide, and other related compounds. These precursors have also been isolated from plants and naturally exhibit auxin properties as well.

Alongside indoleacetic acid, several plants utilize phenylacetic acid—which is somewhat less active—as well as 4-chloroindoleacetic acid and indolebutyric acid to perform auxin Functions.

In agriculture, indoleacetic, indolebutyric, phenylacetic, and 1-naphthaleneacetic acids are widely used as plant growth stimulants.

Analogs of these compounds include aryloxyalkylcarboxylic acids, which are employed in agriculture as herbicides for weed control. Treating fields with such compounds induces accelerated weed growth followed by their subsequent death due to a lack of moisture and nutrients. The best-known preparations of this type are 2,4-dichlorophenoxyacetic acid (technically designated as 2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), as well as the corresponding derivatives of butyric acid (2,4-DB and MCPB).

During the 1950s and 1970s, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) was widely used as a defoliant—a substance that causes leaves to drop off. The synthesis method used for this preparation at that time led to The formation of dioxin as a byproduct, an extremely toxic compound that is highly resistant to biodegradation.

The mass application of defoliants during the Vietnam War resulted in severe harm to the local population and long-term contamination of large areas.

❖ Gibberellins - a group of phytohormones synthesized in roots, buds, and developing seeds. They were originally discovered in the fungus Gibberella fujikuroi, which infects rice and causes its excessive, spindly growth (known as "bakanae" or "foolish seedling disease"). Gibberellins stimulate overall plant growth, increase fruit size, and promote seed germination in many plant species. Plant organs and Tissues typically contain several different gibberellins. Their concentrations range from 0.01 to 1.4 mg per 1 kg of fresh weight, with the highest levels found in immature seeds.

Gibberellins were discovered by Eiichi Kurosawa in 1928. They were first obtained as a mixture in 1930 by Teijiro Yabuta. The first individual gibberellin, GA₁, was isolated by Jack Macmillan in 1958.

Gibberellins are used to stimulate The production of seedless grapes intended for raisin manufacturing. The direct application of these compounds to leaves or buds is the most effective method.

Unlike auxins, high concentrations of gibberellins do not inhibit growth. It is believed that this class of phytohormones blocks the Enzymes responsible for auxin degradation, enhances the biosynthesis of indoleacetic acid, and facilitates its release from bound forms. Furthermore, they activate the biosynthesis of Nucleic Acids and Proteins. BIOLOGICAL EFFECTS OF exogenous gibberellin are typically observed at doses around 10^-10 g.

To date, more than a hundred different gibberellins have been isolated, designated by the letters GA followed by a numerical index. The structures of some of these compounds are presented below.

From a chemical structure standpoint, they are carboxylic acids based on tetracyclic diterpenoids. There are two main groups of gibberellins: C₂₀-gibberellins (for example, GA₁₂ and GA₂₈) and nor-C20- or

C₁₉-gibberellins (e.g., GA₁, GA₃, GA₇, GA₁₀). Hormonal activity is inherent to the compounds of the second group.

✵ Gibberellin biosynthesis inhibitors. A number of chemical compounds that block the production of gibberellins are used as herbicides and retardants—substances that promote short stems and thereby prevent lodging in cereals. They are also used to prevent new leaf growth in tobacco and sprouting in potatoes. Such inhibitors include, in particular, maleic hydrazide cyclic hydrazide and chlormequat chloride.

❖ Cytokinins are compounds that stimulate cell division (cytokinesis). The action of cytokinins was discovered in 1940 by the American plant physiologist of Swedish origin, Folke Skoog.

These bioregulators accelerate seed germination, stimulate the growth of leaves and fruits, and are involved in root and stem formation as well as the differentiation of new organs. Cytokinins help prolong the lifespan of cut leaves—yellowed leaves turn green again when treated with these substances. Effective concentrations range from 10^-5 to 10^-9 M. The maximum concentration of cytokinins is found in root tips (especially abundant in nitrogen-fixing plants), germinating seeds, and ripening fruits, while lower amounts are present in roots, stems, and leaves. For instance, 1 g of zeatin can be obtained from 70 kg of corn seeds (at the milk-ripening stage). With the onset of the growing season, The transport of cytokinins from the roots to other plant organs increases dramatically.

Examples of plant cytokinins include structurally related purine derivatives such as zeatin and isopentenyladenine. They are formed in the roots via tRNA degradation, as well as through biosynthesis from adenosine phosphates and isopentenyl pyrophosphate, followed by enzymatic oxidation and hydrolysis.

Nucleoside derivatives of cytokinins, such as zeatin riboside, act as transport forms. Cytokinins can also exist as inactive storage forms—conjugates with glucose, Alanine, and certain proteins.

Benzylaminopurine, a synthetic analogue of zeatin, exhibits high stimulating activity.

❖ Abscisic acid and its analogues, such as the biological precursor xanthoxin, act as endogenous inhibitors of biochemical processes in plants. They induce leaf and fruit abscission and help plants transition into dormancy for the winter period. Abscisic acid also promotes the closure of Stomata on plant leaves, helping them better withstand drought conditions. Its concentration in plants is extremely low, approximately 10^-5 – 10^-9 mg/L.

The synthesis of abscisic acid occurs primarily in leaves, fruits, and root tips.

Abscisic acid was discovered in 1963 independently by two groups of plant physiologists led by F. Addicott (USA) and F. Wareing (UK).

❖ Ethylene and its producers. As early as 1901, Professor Dmitry Nelubov demonstrated the Physiological Effect of ethylene on pea growth. In 1934, British researcher Richard Gane showed that ethylene is produced by plants and accelerates their ripening. Plant responses are observed at a gas concentration of 10^-4–10^-6%. Thus, ethylene exhibits The properties of a phytohormone. It promotes fruit ripening, inhibits stem and root growth, and accelerates leaf abscission. Keeping fruits in an ethylene atmosphere ensures their timely ripening.

In plants, ethylene is synthesized mainly through the sequential elimination of S-adenosylmethionine and the oxidative Cleavage of 1-aminocyclopropanecarboxylic acid. The growth-regulating properties of ethylene have led to The Use of ethylene-producing compounds as herbicides: 1-aminocyclopropanecarboxylic acid and 2-chloroethylphosphonic acid (ethephon).

Several types of ethylene receptors have been discovered in plants: ETR1, ETR2, ERS, and EIN4. These transmembrane receptors are dimers linked by disulfide bridges. Their MECHANISM OF ACTION is largely analogous to that of Insulin receptors.

In the absence of ethylene, the Ser/Thr kinase CTR-1 inhibits the MAPK signaling cascade of biochemical reactions. The interaction of ethylene with the receptor leads to the inhibition of the CTR-1 kinase and the activation of the MAPK cascade. As a result, METABOLISM/31.html">Transcription-initiating factors are sequentially synthesized in The Nucleus, leading to the production of specific proteins.

❖ Brassinosteroids. In 1979, the first steroid exogenous plant hormone, brassinolide, was isolated from rapeseed pollen. Later, this compound and a number of its analogues were found in the pollen and seeds of other plants.

To date, more than 70 brassinosteroids have been isolated from various plant sources. Alongside substances containing a lactone ring, ketone and deoxy derivatives are also found, such as castasterone and 6-deoxycastasterone.

The content of brassinosteroids in plants is extremely low; for instance, 4 mg of pure brassinosteroid was isolated from 40 kg of pollen.

At low concentrations (~10^-10 M), these compounds stimulate cell division, growth, and consequently, plant elongation. A plant requires only ~10^-7 g of brassinosteroids to double its growth rate.

These phytohormones enhance plant productivity, shorten the vegetative growth period, and provide protection against environmental stresses such as drought, soil salinity, extreme temperatures, and nutrient deficiency.

❖ Jasmonic acid is found in nearly all organs across more than 200 plant genera. At concentrations of 10^-4 - 10^-6 M, it inhibits Plant Growth and seed germination, and triggers leaf and fruit senescence. Conversely, it stimulates root growth. It exerts a protective effect against phytopathogens and ultraviolet radiation in plants, partly by stimulating the synthesis of various protective enzymes. In turn, ethylene stimulates the production of jasmonic acid.

Similar biological effects are exhibited by methyl jasmonate. In its esterified form, the acid is present in numerous plants, including jasmine, in the essential oil of which it was first discovered.

❖ Phaseic acid - regulates stomatal closure, for example, in grapes.

Control Tasks

Test 3 (Option)

1. Choose the structure corresponding to thyrotropin-releasing hormone (TRH).

A. Tyr-Gly-Gly-Phe-Met; B. pyro-Glu-His-Pro-NH; C. Tyr-Pro-Trp-Phe-NH₂; D. Tyr-Arg; E. Tyr-Gly-Gly-Phe-Leu; F. Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg; G. Asp-Arg-Val-Tyr-Ile-His-Pro-Phe; H. Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu.

2. The interaction of endorphins with opioid receptors causes:

A. increased Blood pressure; B. decreased blood pressure; C. pain effect; D. analgesia; E. bronchial dilation; F. bronchospasm.

3. Match the corresponding pairs:

A. thyroliberin; B. thyroxine; C. thyroglobulin; D. triiodothyronine; E. thyrotropin.

I. a protein with a high Tyrosine content; II. a thyroid hormone; III. the active form of a thyroid hormone; IV. a pituitary peptide hormone; V. a releasing factor produced by the Hypothalamus.

4. The biosynthesis of lipoxins is mediated by the enzyme:

A. renin; B. 15-lipoxygenase; C. 5-lipoxygenase; D. cyclooxygenase; E. ACC oxidase; F. angiotensin-converting enzyme.

5. For the medicinal product aspirin, determine its mechanism of biological action and medical effect.

A. angiotensin-converting enzyme inhibitor; B. cyclooxygenase inhibitor; C. phospholipase A2 inhibitor; D. 5-lipoxygenase inhibitor.

I. hypotensive effect; II. anti-inflammatory effect; III. anti-asthmatic effect; IV. antithrombotic effect.

6. Identify the names of the presented compounds

1. leukotriene LTA₄; 2. thromboxane TXB₂; 3. prostaglandin PGH₂; 4. zeatin; 5. gibberellin GA₁; 6. thyroxine; 7. abscisic acid; 8. jasmonic acid; 9. fluorocort; 10. arachidonic acid; 11. prednisolone; 12. lipoxin LXA₄