PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

3. REGULATORY AND INTEGRATIVE SYSTEMS IN PLANTS

The complex Structure of a plant Organism requires sophisticated control systems. The integrity of any living organism is maintained by regulatory, control, and integrative systems. Regulation is generally understood as maintaining system parameters within specific limits. Control, on the other hand, is The process of transitioning a system from one state to another by acting upon its variables. In a broader sense, the term "regulation" encompasses control processes as well, which is how it is typically used in biology.

Regulation ensures organismal Homeostasis—that is, the maintenance of internal environmental parameters within defined limits—while also creating the conditions necessary for development (epigenesis). At all Levels of Organization, homeostasis is maintained by negative feedback loops, while epigenesis is driven primarily by positive feedback loops.

Regulatory and integrative systems in plants include: intracellular regulatory systems, Intercellular regulatory systems, and the Integration of regulatory mechanisms at the level of the whole organism.

Intracellular Regulatory Systems

In the course of evolution, intracellular regulatory systems were the first to emerge. These include Regulation at the enzymatic (metabolic), genetic, and membrane levels. All these systems are closely interconnected. Moreover, all forms of intracellular regulation are based on a single primary receptor-conformation principle (a molecule of a specific structure + a specific factor → conformational change → functional change).

Introduction/15.html">Regulation of enzyme Activity

ENZYME ACTIVITY REGULATION is categorized into isosteric and allosteric types. Isosteric regulation occurs at the level of catalytic sites:

· enzymatic activity depends on the presence and concentration of the substrate—the law of mass action;

· enzymatic activity depends on the presence and concentration of the enzyme itself, as well as inhibitors, activators of the catalytic site, and reaction products.

Some Enzymes, In addition to catalytic (isosteric) sites, also possess allosteric sites for binding allosteric effectors (regulators such as inhibitors or activators, metabolites, Hormones, or substrates).

The intensity of enzyme function is also determined by the presence of Coenzymes (for two-component enzymes) and Cofactors (specifically acting cations).

The activity of certain enzymes may be linked by competition for shared substrates and coenzymes, which serves as one of the ways different metabolic cycles interact.

An important method of regulating enzymatic activity is the Transformation of a latent enzyme form (zymogen) into an active one. This is achieved by cleaving specific covalent bonds using proteases, reducing disulfide groups, phosphorylation by protein Kinases using ATP, or the association of inactive subunits.

There are other ways to modify enzyme activity, including: differential compartmentation of enzymes, substrates, and regulatory substances; changes in environmental pH, ionic composition, or other physical-chemical conditions; and the degradation of enzymes by proteases.

Genetic regulatory system

The Genetic control system for intracellular processes is ensured by regulation at the levels of Replication, METABOLISM/31.html">Transcription, Processing, and Translation. The MOLECULAR MECHANISMS OF influence here are similar to those in enzyme activity regulation (component ratios, pH, ionic composition, molecular modification, inhibitors, etc.), but the saturation and complexity of the regulatory networks are significantly higher.

The Role of genes is the storage and transmission of information. Information is encoded and stored in The structure of chromosomal DNA AS A triplet nucleotide code. Information transfer occurs via: transcription → translation → processing (modification of Biopolymers). ⇒ The result is The formation of an active protein that performs diverse Functions: catalytic (enzymes); motor (contractile Proteins); transport (pumps, carriers); receptor (chemo-, photo-, and mechanoreceptors); regulatory (activator, inhibitor, and repressor proteins); protective (Lectins), and others.

All these Protein Functions are essential mechanisms for controlling the biochemical and physiological processes occurring within The Cell and the organism as a whole.

Regulation at the level of transcription and post-transcriptional processing

To retrieve specific information from chromosomal structures at a given moment, the cell employs a complex regulatory system, not all mechanisms of which are currently known.

Class="center">Organization scheme of the nucleolar DNA region encoding the synthesis of 18S, 5.8S, and 25S (28S) rRNA

Protein-coding genes consist of a regulatory Gene (promoter + acceptor zone) and structural genes (cistrons).

RNA polymerase I, which performs gene transcription, binds to the promoter (the Transcription initiation site) only after all Regions of the acceptor zone have been activated by appropriate effectors—phytohormones, metabolites, and other substances. The entire process is regulated by modulating the state of RNA polymerase (enzymatic regulation), influencing the regulatory gene (via effectors), and ensuring the availability and quantity of all other transcription components.

Structural genes (cistrons) are separated by spacers. In addition to coding regions (exons), genes may contain non-coding regions (introns). Often, blocks of structural genes in nucleolar DNA are tandemly repeated. This tandem organization of structural genes in eukaryotes is characteristic of DNA regions encoding rRNA, tRNA, and histone proteins.

As a result of gene "transcription," pre-mRNA is synthesized. Mature mRNA is formed during the processing of pre-mRNA through fragmentation, the removal of spacer regions, and splicing (the excision of introns and the ligation of the two mRNA segments).

Intronic structure is not characteristic of ribosomal genes, although it is present in The genes of Chloroplasts, Mitochondria, lower Fungi, and certain other eukaryotes. All the processes described above are enzymatic and involve a complex regulatory system.

Furthermore, there are several hypotheses regarding the mechanisms of post-Transcriptional Regulation of mRNA Gene Expression.

According to one hypothesis, nuclear DNA contains integrative regulatory genes with numerous repeats. If transcripts from specific regulatory genes appear in the nucleoplasm in large quantities, they form complementary associates with the corresponding pre-mRNA. As a result, pre-mRNA Processing is accelerated, and the activated mRNA begins to function. Simultaneously, The rate of their transcription from the corresponding structural genes increases sharply. Effectors (Eff, signals) arriving at The Nucleus from the Cytoplasm induce the selective expression of integrative regulatory genes. Eff₁, for example, a phytohormone, interacts with its receptor protein (RP₁) to induce the transcription of a specific integrative regulatory gene with numerous a...a repeats. The pre-RNA from this regulatory gene undergoes processing to form regulatory transcripts. These, in turn, form complementary associates with pre-mRNA₁, inducing faster synthesis and activation of this mRNA₁.

Unlike mRNA₁, the transcription and activation of mRNA₂, for instance, depend on the simultaneous presence of two effectors—Eff₁ and Eff₂. These may be hormones of different classes or metabolites. Thus, the synthesis of mRNA₂ is an example of genetic induction under dual control.

For example, the synthesis of nitrate reductase in plants can be induced by nitrate (substrate induction) and cytokinin; furthermore, the final product of nitrate reduction—ammonium—blocks The Development of nitrate reductase activity.

Regulation at the level of translation

The Mechanism of translation is far more complex than that of transcription. While transcription is facilitated by dozens of proteins, translation involves hundreds. Eukaryotic Ribosomes alone contain 70-100, with rRNA acting as a scaffold for the self-assembly of these proteins. Regulatory networks here are more dense, and the mechanisms more intricate. The intensity and direction of translation depend on:

1) the concentration of Messenger RNA (mRNA), the level of which is determined by its synthesis, transport, storage, activation, and degradation;

2) the availability of all Components of the translation apparatus—ribosomes, tRNA, Amino Acids, ATP, GTP, synthetases, and regulatory proteins.

3) physicochemical conditions (pH, ions, Temperature, etc.).

Membrane regulation

Membrane regulation is achieved through Changes in membrane transport, the binding or release of enzymes and regulatory proteins, and by altering the activity of functional proteins.

All Membrane Functions (transport, osmotic, energetic, etc.) are simultaneously different facets of the regulatory mechanism of intracellular metabolism. Of particular importance in these mechanisms is The system of membrane chemo-, photo-, and mechanoreceptors, which allow the cell to modify membrane properties and, consequently, its metabolism in response to environmental changes.

At the core of membrane regulation lies the state of dynamic non-equilibrium, maintained in every cell at a steady-state level by the activity of ion pumps localized within the membranes. A special role is played by proton pumps, which function based on membrane H⁺-ATPases or redox chains, creating an Electrochemical Potential of H⁺ ions (ΔμН⁺). The Membrane Potential across the Plasmalemma reaches 100-200 mV, with the inner surface of the membrane bearing a negative charge. The energy of this potential is utilized for the cellular uptake of cations, anions, sugars, and Other Compounds.

Maintaining the membrane potential at a steady-state level is a crucial component of homeostasis. When the membrane potential across the plasmalemma decreases, its value is restored through the opening of potassium channels and the efflux of K⁺ ions or by the activation of the proton pump.

Membrane regulation has a decisive influence on changes in intracellular calcium concentrations. The latter enters the cell along an electrochemical gradient through calcium channels and is pumped out via Ca²⁺-ATPase or through antiport with hydrogen ions. Calcium can be accumulated in vacuoles and The Endoplasmic reticulum, and released from them. Its concentration in the cytoplasm is maintained at a quite low level (10^-7 M). A slight increase in its concentration (up to 10^-6 M) affects the activity of Ca²⁺-dependent protein kinases, protein phosphorylation, cytoplasmic streaming, the state of the Cytoskeleton, secretory and mitotic activity, and more. Calcium regulates all these processes through its association with calmodulin and other calcium-binding proteins.

Furthermore, in plant Cells, as in animals, the phosphoinositide cycle functions as a regulatory system, which, by releasing Ca²⁺ and activating protein kinase C in the plasmalemma, phosphorylates ion channel proteins, thereby regulating their function.

Quite recently (Kaufman, 1994), it was discovered that GTP-binding signaling proteins play a significant role in the membrane regulation of higher plants. These are the so-called large GTP-binding regulatory proteins and small G-proteins. Large G-proteins consist of three (α, β, γ) subunits but interact with only one receptor and one effector system. It is believed that they transmit signals from seven-transmembrane receptors to specific cellular effector systems, such as the adenylyl cyclase system. It is known that the adenylyl cyclase system is a universal mechanism for the Transduction of external signals and their Implementation at THE MOLECULAR LEVEL in eukaryotes.

Small G-proteins are monomers with a molecular mass between 20 and 30 kDa. Three subclasses of small G-proteins have been identified with diverse regulatory functions—ranging from protein and vesicle transport to the Regulation of the Cell Cycle, cell growth, differentiation, and the organization of cytoskeletal microfilament structures.

Membranes are involved in:

- contact REGULATION OF ENZYMATIC Activity (concerning enzymes fixed to the membranes of the endoplasmic reticulum, Golgi apparatus, etc.);

- distal regulation of enzymatic activity (delivery of substrates, coenzymes, and reaction products via vesicles, as well as shifts in ion concentrations, pH, and other membrane-mediated processes). For instance, membranes regulate calcium ion levels in various cellular compartments, and the Ca²⁺ + calmodulin regulatory protein complex triggers the activation of protein kinases, which phosphorylate various proteins, thereby altering their functional activity.

Membrane-mediated Regulation of Gene activity at the levels of replication, transcription, processing, and translation occurs through:

- contact-based mechanisms ⇒ resulting from the presence of a functionally active double nuclear envelope, the association of Chromosomes with membranes (Spatial Organization of the chromosomal apparatus within the nucleoplasm), and the attachment of ribosomes to membranes (rough ER);

- distal mechanisms ⇒ transport of precursors for DNA, RNA, enzyme, and regulatory Protein Synthesis via membranes; changes in ionic potentials and pH within intermembrane spaces (an increase in Ionic strength to 0.4 mol/L, the presence of Mn, and a pH of 7.5 activate RNA polymerase II, which synthesizes mRNA; whereas at low ionic strength, in the presence of Mg and a pH of 8.5, RNA polymerase I is activated, which is responsible for rRNA synthesis); (low K⁺ ion concentration and pH 7.5 are favorable for the initiation of translation, while an increased K⁺ concentration and pH 8.4 are required for the subsequent polypeptide elongation process in the ribosomal complex).

It is hypothesized that shifts in cellular ionic homeostasis in response to external stimuli serve as the primary intracellular inducer of mitosis and differential gene activity.

Hierarchy of regulatory systems in multicellular plants

Fig. 2. Main sites of phytohormone synthesis and their transport pathways in plants.