Fundamentals of Biochemistry - A. A. Anisimov 1986

Integration and Regulation of Metabolism
Phytohormones and Features of Metabolic Regulation in Plants

There are four main groups of phytohormones (Plant HORMONES): Auxins, Gibberellins, Cytokinins, and growth inhibitors.

Auxins. These are indole-derived compounds, the principal one being ß-indoleacetic acid (ß-IAA), which is typically synthesized in meristematic Tissues from shikimic acid via Tryptophan.

Class="center">

At optimal concentrations, auxins significantly accelerate Cell elongation and, in some cases, stimulate Cell Division. They regulate the influx of Water and nutrients. A crucial aspect of their MECHANISM OF ACTION involves the activation of RNA and METABOLISM/35.html">Protein Biosynthesis, as well as the enhancement of energy-yielding processes (such as Respiration intensity and the Coupling of oxidation and phosphorylation). According to V. V. Polevoy (1967), the interaction of IAA with cell membranes triggers an increased proton efflux onto their outer surface, resulting in membrane hyperpolarization and acidification of The Cell wall. This acidification activates cell wall-loosening Enzymes, thereby facilitating wall extension and cell growth. Mitochondrial membrane hyperpolarization may also account for the enhanced ATP synthesis observed during Oxidative Phosphorylation.

Increasing recognition has recently been gained by the hypothesis that plant Cells contain specific primary receptors for auxin action (certain enzymes, specific domains within various membranes). It is suggested that the interaction of auxins with such receptors triggers profound changes in biochemical processes, which in turn mediate the diverse Physiological effects of these phytohormones.

The concentration of auxins in plant tissues varies in response to light conditions, water availability, mineral Nutrition, and other environmental factors.

Gibberellins. This group comprises over 40 tetracyclic carboxylic acids. The most widespread is gibberellin A₃, commonly known as gibberellic acid (GA).

The remaining gibberellins differ primarily in The Structure of their side chains. They are synthesized from acetyl-CoA via mevalonic acid (see Section 8.8.3) and are produced predominantly in highly active growing Organs (young leaves, roots, and developing seeds). Gibberellins substantially promote stem elongation and overall growth by activating both cell division and cell extension. The Mechanism of gibberellin action is based on upregulating The biosynthesis of specific enzymes and stimulating non-cyclic Photophosphorylation.

Cytokinins. These are the primary regulators of plant cell division, from which they derive their name (whereas auxins and gibberellins primarily regulate growth via cell elongation). Cytokinins are adenine derivatives in which the 6-amino group is substituted with various chemical groups. They are synthesized in the roots and transported to aerial organs via the xylem. Acting in concert with auxins, they participate in Organogenesis, stimulate protein and chlorophyll synthesis, and delay leaf senescence. Similar to auxins, they enhance the translocation of metabolites toward tissues rich in these hormones. They increase The rate of cyclic photophosphorylation (unlike gibberellins), elevate the levels of all RNA species within tissues, and activate Chromatin.

Growth inhibitors. The most prominent among these is Abscisic acid (ABA).

Like gibberellins, ABA is synthesized from mevalonic acid. It is found in significant quantities in various plant organs during dormancy. In terms of its physiological effects, ABA frequently acts as an antagonist to IAA, gibberellins, and cytokinins. The balance between phytohormones and ABA dictates a given organ's capacity for growth and morphogenesis. ABA sharply reduces membrane permeability, which appears to be central to its mechanism of action. This compound is also regarded as a universal Gene repressor that prepares the plant for dormancy.

Seed coats and fruit pulps contain phenolic growth inhibitors—such as coumarin and coumaric acid—which suppress seed germination. These compounds are also present in other plant tissues, particularly dormant ones. Phenolic growth inhibitors reduce phytohormone levels, specifically by inhibiting IAA biosynthesis, and uncouple oxidation from phosphorylation in Mitochondria.

Ontogeny involves complex interactions among phytohormones, with their quantitative ratios playing a critical role. For instance, an increased auxin/kinetin ratio promotes ROOT differentiation, whereas a decreased ratio favors SHOOT differentiation.

A crucial role in regulating plant metabolism is played by the substance-cycling system. Serving to some extent as an analog of animal Blood Circulation, it encompasses the phloem Transport of Assimilates from leaves to stems and roots, coupled with the return flow via the xylem of sap containing products resulting from the interaction of assimilates with mineral salts in the roots. Research by A. L. Kursanov and coworkers (1960) demonstrated that this circular flow coordinates and integrates metabolic conversions across individual organs into a unified whole-plant metabolism. Because phytohormones (auxins, cytokinins) are transported alongside assimilates and their metabolic derivatives, this substance cycle holds profound regulatory significance.

Given that plants are continuously interacting with substrates containing mineral salts (soil or water) and that mineral nutrition forms a cornerstone of their metabolism, mineral ions play a remarkably prominent role in plant tissues as regulators of metabolic exchange. They exert this function primarily by modulating enzyme activity (by incorporating into enzymes, altering their conformation, and affecting their biosynthesis rates) as well as by modifying the Structure and properties of Biomembranes.

Work by A. A. Anisimov and coworkers (1984) demonstrated that phytohormones, the transport system, and mineral ions constitute an integrated regulatory network within the plant Organism, characterized by constant interaction and feedback loops among its components. The salt composition and ionic gradients of plant tissues, by influencing enzyme activity and cell membrane properties, exert a profound impact on phytohormone biosynthesis and distribution throughout the plant. Possessing strong sink strength—the capacity to attract assimilates—phytohormones regulate both the rate and direction of assimilate transport. Shifts in the quantity and composition of assimilates delivered to a given organ subsequently trigger changes in its metabolism. For example, a deficiency in nitrogen or phosphorus slows down sugar transport from wheat leaves to the roots, leading to a drop in root respiration. Artificially halting assimilate export by severing a leaf from the stem inhibits starch Hydrolysis within the leaf. The intensity of substance cycling also exerts some influence on the translocation and distribution of phytohormones across the plant, representing a feedback mechanism. In this context, the dependence of root mineral ion uptake on the influx of assimilates from the shoot should likewise be evaluated.