Plant Physiology - Musienko, M. M. 2001

Physiology of Plant Growth and Development
Cell Differentiation

The process of functional Cell differentiation, or the accumulation of physiological differences between Cells, occurs across all growth phases. It is The formation of qualitative distinctions between individual cells associated with their specialization. Some cells differentiate very early, still within the apical meristem (for example, epidermal cells). As a rule, the future specialization of cells is determined even within the division zone of the SHOOT or ROOT apex. This process of determining future specialization is called determination (Fig. 157).

This is confirmed, for instance, by the fact that immunochemical studies of the embryonic root meristem in corn have shown that Proteins of the future vascular cylinder (xylem and phloem cells) appear long before their actual formation takes place.

Qualitative differences between cells are based on processes of differential Gene activity. Each vegetative cell contains the complete Genetic information required for The Development of the entire Organism in its genome, and under certain conditions, it can give rise to the formation of specific Organs or even an entire plant. This phenomenon is known as totipotency. However, when The Cell is situated within the organism, it expresses only a fraction of this information. A specific ratio of phytohormones, metabolites, and physicochemical factors serves as the signal (effector) for the expression of a particular program.

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Fig. 157. Troll's model of cell differentiation. Various permanent tissue cells can form from homogeneous meristematic cells (top) (from left to right): palisade, guard, parenchyma cells, "stellate" cells, vessels and sieve tubes, sclerenchymatous fibers

The most crucial problem of differentiation is its mechanism. If the primary factor of growth is Cell Division via mitosis, it follows that all cells are genetically identical and should possess identical structures and functions. Yet, in a mature organism, this is far from the case. The question arises: what mechanisms create the differences between plant cells, Tissues, and organs?

Cell Structure and function are determined by gene activity, meaning they stem from differences in genome expression. This can be achieved in two ways: either cells lose certain genes during specialization, retaining only those required for a given function, or the second possibility—different genes are "switched on" or "switched off" in different cells. Research by Professor Steward at Cornell University (USA) made it clear that the second mechanism operates in plants. He was the first to demonstrate that if differentiated cells (such as carrot phloem cells) are placed in a suitable culture medium, a new carrot plant can form from them. This means that phloem cells, despite already being differentiated, still contain all the necessary information for the complete development of a mature plant. Using such Methods, any number of genetically identical organisms can be obtained. Identical progeny derived from a single parent individual are called a clone, and the method itself is called cloning.

It should be noted that Surface Properties of cells, particularly direct interaction—surface "adhesion"—are of great importance for the differentiation process. Adhesion is caused by the presence of specific proteins on the cell surface, such as lectin-like Glycoproteins, which appear As a result of selective Gene Expression at different stages of cell development. These proteins perform the function of cell "recognition." According to modern concepts, Cell Adhesion plays a decisive role in morphogenesis.

Differentiation processes are also influenced by phytohormones. Hormones can trigger the "switching on" or "switching off" of a specific gene system in a definite sequence, thereby determining the course of development. If the nuclei of differentiated cells fully retain their genetic potential, it implies that The Cytoplasm also participates in regulating differentiation. This influence is termed induction.

It is believed that individual tissues are capable of secreting special morphogenetic substances, with Meristems being their primary source. The mutual influence of tissues is manifested, in particular, in phenomena of homo- and heterogenetic induction. Homogenetic induction implies the ability of cells of a certain tissue to induce the formation of cells similar to themselves. This is most clearly observed in isolated tissue culture. For example, callus tissue in contact with a xylem region develops xylem, whereas in contact with phloem, it develops phloem. Similar events occur during tissue grafting.

In heterogenetic induction, conversely, certain tissues or organs are capable of blocking the formation of similar tissues or organs. For instance, Stomata arise at a certain distance from one another.

The exact manner in which inductive influence is exerted remains not fully understood. It is believed that differential repression and activation of genes in various cells also take part in this process.

Finally, as early as the end of the last century, H. Driesch formulated the theory that The Fate of each part of an embryo is a function of its position within the embryo as a whole. C. Child's idea of physiological gradients became the material foundation for Driesch's law. Child believed that the dominant region of a gradient secretes certain factors that inhibit the development of identical structures from other embryonic cells. The development of these ideas led to L. Wolpert's concept of "positional information." According to his concept, the development of each cell is determined by the concentration of specific substances—"morphogens"—distributed along the axis of the entire embryo in the form of a concentration gradient, which constitutes The Essence of "positional information." Only competent cells can utilize this information.

At present, there is abundant evidence that longitudinal and radial gradients of physical (electric fields, mechanical stress), chemical (H⁺, Ca²⁺, O₂, CO₂, trophic factors, phytohormones, etc.), and physiological (Respiration, substance transport) factors exist in the tissues and organs of plant organisms. These gradients change in space and time as a result of new polarizations arising during the organism's development.

Any cell of a multicellular plant organism is subjected to METABOLISM/18.html">The Influence of physical, chemical, and physiological gradients as well as surrounding cells. As a result, the cell realizes precisely those potential capabilities that correspond to these influences. This theory is known as the "position effect."

In general, plant morphogenesis is carried out with the participation of various regulatory systems, which collectively manifest as a precise Determination of the cell's ontogenetic features depending on its Location within the tissue. It is believed that such ontogenetic determination is achieved through the spatial distribution of regulatory signal cues, a concept known as positional information (D.M. Hrodzynskyi, 1993).

The parameters of cell ontogenesis controlled by positional information are diverse: they include the type of specialization with its inherent biochemical Organization, the rate

of specialization, the topology of stem cells in tissues and the duration of their stay in the meristemoid state, the number of quantal mitosis, the duration of the mitotic cycle, and the final cell size.

These parameters of cell ontogenesis determine its function in plant organ tissues. Positional information regarding certain ontogenetic parameters is discrete, defining only the type of its realization, while at the same time, certain characteristics are realized through a non-discrete mechanism of regulation. Positional information factors, which are currently less than fully explored, are quite diverse: phytohormones, their receptors, physical factors such as light of varying quality, mechanical stresses in tissues, Temperature regime, and the distribution of electrical signals. The Mechanism of positional information realization consists of signal reception by the cell, formation of an internal signal, induction of activation of potentially active genes, and subsequent cell restructuring:

Although the positional information system is exceptionally conservative, opportunities exist to intervene in its formation and thereby influence plant morphogenesis.

The Stage of Senescence and Death

Senescence and death are the final stages of ontogenesis in differentiated cells. The fundamental principle in the development of senescence at THE MOLECULAR LEVEL is a shift in the balance between the synthesis of cellular Biopolymers and their breakdown toward enhanced degradation. The lifespan of each biological polymer and the structure it comprises is determined by its renewal capacity. This process is characterized by the half-life value, which is the time

during which half of a given substance or cell organelle ceases to exist. For example, the half-life of substrates is 10–100 minutes, of Enzymes 5–500 hours, and of Organelles 8–100 days (Leopold, 1975). As senescence develops, a slowdown in the renewal of cellular components is observed, which in turn weakens functions and disrupts regulatory mechanisms.

This stage is characterized primarily by hydrolytic processes, enhanced peroxidation of Membrane Lipids, and an increased loss of various cellular substances through membranes. Chlorophyll and Chloroplasts are degraded, the Endoplasmic reticulum and Golgi apparatus are fragmented, The Nucleus becomes vacuolated, and nucleoli are destroyed. Senescence becomes irreversible from the moment the tonoplast is disrupted and its contents, including Hydrolases, are released into the cytoplasm.

There are two hypotheses explaining the mechanism of Aging at the CELLULAR AND MOLECULAR levels: the first attributes this process to the accumulation of damage in the genetic apparatus, membranes, and other structures, while the second considers it the result of the Activation of a genetic aging program as The final stage of ontogeny.

Support for both hypotheses lies in the fact that during ontogeny, cellular damage accumulates despite the continuous self-renewal of all structures and DNA Repair systems. The processes of cell differentiation and specialization cause alterations in Protein Synthesis, such as the shutdown of parts of Introduction/26.html">The Translational Apparatus, which accelerates aging. Various Metabolic Disorders also contribute to the execution of the genetic developmental aging program. For instance, modified DNA progressively accumulates, both through the formation of chemical bonds between DNA and RNA, Histones, and non-histone proteins, and due to impaired mechanisms for repairing such damage. Chromatin can be damaged by free radicals, the levels of which gradually increase. Free radicals affect cell membrane permeability, thereby disrupting Cellular Homeostasis. Both chromatin damage and membrane alterations can trigger the synthesis of defective proteins and toxic products that accelerate aging (Polevoy, Salamatova, 1999).

Another hallmark of aging is the shift of cytoplasmic pH toward the acidic range.

The cause of cellular aging is best understood from the perspective of the PHYSIOLOGY OF THE whole plant organism. The weakening or complete failure of the proton pump, along with the loss of semi-permeability in membranes, leads to a loss of Membrane Potential and disequilibrium, ultimately resulting in cellular lysis.