Plant Physiology - Musienko, M. M. 2001

Physiology of Plant Growth and Development
Ontogeny of the Plant Cell

The Concept of Plant Ontogeny

Ontogeny refers to the individual development of an Organism, encompassing the entire sequence of its transformations from inception (Fertilization of the egg Cell, the onset of independent life of a vegetative primordium) to natural death. Throughout ontogeny, the hereditary information of the organism (genotype) is realized under specific environmental conditions, resulting in The formation of the phenotype, which is the sum of all traits and Properties of the individual organism. In sexually reproducing plants, ontogeny begins with The Development of a fertilized egg cell.

A characteristic feature of plant ontogeny is the alternation of asexual (sporophyte) and sexual (gametophyte) generations. The sporophyte develops from a zygote, while the gametophyte arises from a germinating spore. In The life cycle of flowering plants, the sporophyte—the plant itself—predominates, whereas the female and male gametophytes are highly reduced. In Vegetative Reproduction, ontogeny begins with the division of somatic Cells of the parent plant, including cells from specialized Organs such as rhizomes, tubers, bulbs, and others.

During ontogeny, the Structural and functional specialization of plant cells, Tissues, and organs takes place, interactions between parts become more complex, and irreversible age-related changes arise in the organism as an integrated living system. The ontogenetic process in plants involves growth, associated with an increase in size and the de novo Formation of Structural elements, and development, which leads to qualitative Changes in the Structure AND Functions of the plant or its parts.

Since most plants lead a sessile lifestyle and their ontogeny depends heavily on environmental conditions, they have evolved various adaptive responses (dormant periods, Photoperiodism, thermoperiodism, etc.) that align their period of active life activity with the most favorable time of the year.

The GROWTH AND DEVELOPMENT of an organism, and consequently its ontogeny, are regulated primarily at THE CELLULAR LEVEL. The growth of an organ (or organism) consists of the growth of its cells, whereas morphogenesis—the formation of specific organizational forms of an organ (or organism)—is the result of the developmental pathways undertaken by individual cells. The Determination of the developmental pathway of each cell forms The basis of developmental physiology. The ontogeny of a plant cell consists of a series of successive stages: division, expansive growth, differentiation, Aging, and death.

Cell Division. In 1879, Boveri and Fleming described the events occurring within The Nucleus that result in the formation of two identical cells, and in 1887, Weismann predicted that a different type of division occurs during the formation of sexual Gametes. These Two Types of division are referred to as Mitosis and Meiosis, respectively. The processes involved are nearly identical, yet they lead to completely different outcomes.

Mitosis is a type of Cell Nucleus division that produces two daughter nuclei with identical sets of Chromosomes.

Mitosis is a continuous process comprising four phases: prophase, metaphase, anaphase, and telophase (Fig. 152).

Class="center">

Fig. 152. Mitosis: 1 — nucleolus, 2 — nuclear envelope, 3 — nucleoplasm, 4 — chromosome, 5 — pole, 6 — spindle fiber, 7 — chromatids, 8 — centromere, 9 — cell plate, 10 — phragmoplast, 11 — daughter chromosomes

The first sign of a cell entering division is the appearance of a narrow band of microtubules located directly beneath the Plasmalemma, known as the preprophase band. It disappears upon the appearance of the mitotic spindle.

Prophase. At its onset, chromosomes resemble long threads distributed within the center of the nucleus. As they shorten and thicken, it becomes apparent that each chromosome consists not of a single thread, but of two intertwined threads—chromatids. A narrow region, the centromere, divides the chromosome into two arms of unequal length. A clear zone appears around the nuclear envelope, where microtubules concentrate; in late prophase, these align along the spindle axis. The nucleolus gradually disappears, leading to The breakdown of the nuclear envelope.

Metaphase. At the beginning of metaphase, a spindle appears at the former site of the nucleus, with fibers composed of bundles of microtubules. The chromosomes arrange themselves such that their centromeres lie in the equatorial plane of the spindle. Following this positioning, the chromosomes are ready for division.

Anaphase. During the anaphase stage, the chromatids of each chromosome separate. The spindle fibers shorten, facilitating the Separation of chromatids and the movement of daughter chromosomes in opposite directions. By the end of the phase, identical daughter chromosomes separate entirely and move toward opposite poles.

Telophase. In this phase, the formation and separation of two identical groups of chromosomes are completed, and each group acquires its own nuclear envelope. The spindle apparatus disappears, chromosomes cease to be visible, and nucleoli reappear. The Formation of the phragmoplast—The Cell plate that divides the cell in half along the equatorial plane—concludes (Fig. 153). Mitosis is completed, and the two daughter nuclei enter interphase. Thus, the cell receives a double set of chromosomes identical to that of the parent cell.

Immediately following the division of the nucleus into equal parts, the Cytoplasm divides (cytokinesis), The Cell wall is restored, and two daughter cells are formed.

Cell Cycle. The sequence of events occurring between the formation of a given cell and its division into daughter cells is called the cell cycle (Fig. 154). This cycle consists of a series of stages. Mitotic cell division leads to an increase in cell number, thereby driving growth processes. In unicellular organisms, mitosis also serves as The Mechanism of asexual reproduction, resulting in population growth.

Fig. 153. Formation of the phragmoplast in anaphase and telophase of mitosis

Fig. 154. The cell cycle can be divided into mitosis and interphase—the period between two successive mitotic divisions

The actual process of parent cell division (mitosis) together with the period of cytoplasmic growth and preparation for a new division (interphase) constitute the mitotic cell cycle.

Although the morphological picture of the primary type of Introduction/5.html">Eukaryotic Cell division—mitosis—was described over 100 years ago, it has only been established relatively recently that mitosis is triggered by processes occurring during the period between two successive cell divisions, i.e., throughout the cell cycle. The latter comprises the following periods or phases: the presynthetic G₁ phase, the DNA Synthesis S phase, the postsynthetic C₂ phase, and the actual cell division phase—mitosis. METABOLISM/2.html">THE CONCEPT OF a quiescent state within The structure of the mitotic cycle, commonly designated as the G₀ period, has also been formulated. This notion arose from evidence that, under certain conditions, cells remain outside the cycle for prolonged periods while performing specific tissue functions, yet retain The ability to re-enter proliferation upon receiving an appropriate stimulus.

The intermitotic period—interphase—lasts 15–20 hours, whereas actual cell division takes place within 2–3 hours. The Cell Structure during interphase is characterized by dense cytoplasm with a well-developed Endoplasmic reticulum whose channels are constricted. Small vacuoles are present, along with numerous free Ribosomes scattered throughout the cytoplasm and a significant number of Mitochondria with a dense matrix and underdeveloped cristae. The nucleus is relatively small, featuring a round nucleolus. The nucleoplasm is homogeneous, and Chromatin appears as thread-like structures. The primary cell wall is thin, permeated by numerous plasmodesmata. It is during this period that DNA Replication occurs in the cell nucleus. Replication of mitochondrial and plastid DNA is observed throughout the entire interphase.

The presynthetic phase lasts from 3 to 8 hours, the synthetic phase from 10 to 11 hours, and the postsynthetic phase takes 4–5 hours. The most widely accepted hypothesis suggests that meristematic cells maintain a specific ratio between nuclear and cytoplasmic volumes, known as the nucleoplasmic ratio. When this ratio reaches a critical threshold where the nucleus can no longer effectively regulate the expanded cytoplasmic mass, the cell divides.

The G₁ phase begins immediately following mitosis and is characterized by an increase in cytoplasm and cellular Organelles. Necessary Enzymes, Cofactors, and NUCLEOTIDES required for nucleic acid synthesis are synthesized during this period. This phase is particularly sensitive to environmental changes.

During the S phase (synthesis), genetic material is duplicated, and specific Nuclear Proteins—Histones—are synthesized.

In the G₂ phase, structures directly involved in mitosis are formed, notably the spindle fibers. The synthesis of RNA and proteins, especially tubulin, continues. Concurrently with the division processes, certain Structural components of cell organelles are formed.

Most specialized cells lose their capacity to divide following differentiation. Some cells (such as callus tissue) retain division potential only under specific conditions, such as wounding. All Stages of the cycle are controlled by a corresponding phytohormonal balance; for instance, the G₁ and G₂ phases are regulated by Auxins, while cell division is controlled by Cytokinins.

Thus, during the initial phase of growth, the increase in cell volume is driven by division and the accumulation of cytoplasmic mass, accompanied by the formation of structural cell components. This indicates that growth is inherently tied to morphogenetic processes from its earliest stages. A cell formed through division increases in volume and subsequently undergoes division once again. After dividing 3 to 5 times, the cell transitions to the next phase of growth. The only exceptions are initial (meristematic) cells, which continue to divide throughout the entire growth period of the plant organism.

Investigating the mechanisms of endogenous Regulation of the cell cycle—which encompasses all processes occurring between two consecutive cell divisions—is a fundamental biological challenge. This is due to the practical importance of issues such as tissue regeneration and neoplasm growth, which fundamentally rely on The regulation of tissue proliferation. It has been established that specific proteins, namely cyclins and protein Kinases, play a universal role in inducing DNA SYNTHESIS AND cell division.

In particular, studying the regulation of proliferation is of great significance for plant production. Because cell division in plant meristematic tissues occurs throughout ontogeny, it serves as the basis for morphogenesis and tissue recovery following stress exposure; consequently, plant growth, development, and productivity heavily depend on its rate. Modifications in cell cycle parameters within Meristems are linked to the specific growth responses of plants to environmental factors, such as Temperature, light, and Water regimes, as well as physiologically active substances.

The scientific and Practical significance of plant cell regulation becomes especially critical with the advancement of biotechnological research. Studies show that introducing cells into culture and their transition to a differentiated state are closely linked to specific stages of the cell cycle. A cell's susceptibility to the introduction of foreign Genetic information depends on its localization within the cycle. Furthermore, a correlation has been established between the cell cycle stage of the tissue from which protoplasts are isolated and their subsequent ability to divide in culture. Significant shifts in cell distribution across cycle phases occur during the transition from vegetative to generative development. Unfortunately, the biochemical and molecular-biological processes of The plant cell cycle, along with metabolic specifics during its distinct phases, remain poorly understood and require further research.

Elongation phase. The transition to the elongation growth phase is accompanied by profound structural and physiological changes. The cytoplasm becomes less viscous and more hydrated. The channels of The endoplasmic reticulum widen, forming cisternae in various regions. Its membranes become rough due to the attachment of ribosomes. Contacts between the Endoplasmic reticulum and mitochondria increase, facilitating the supply of energy to the attached ribosomes. The nucleus assumes an irregular shape, which increases its surface area of contact with the cytoplasm. The dimensions of the nucleoli decrease.

This type of growth is characterized by an increase in cell volume driven by the fusion of small vacuoles and the formation of a large central vacuole. The enlargement of the vacuole is further facilitated by the incorporation of vesicles derived from the Golgi apparatus. Hydrolytic enzymes accumulate within the vacuoles, starch breakdown is observed, and As a result, sugar content increases. Simultaneously, the concentration of Amino Acids rises, and osmotically active substances rapidly accumulate. The relative cytoplasmic content per unit of cell mass decreases, although its absolute content per cell increases. Protein Synthesis rates rise, and all Cellular metabolic processes intensify. The protein-synthesizing machinery undergoes reorganization—cytoplasmic ribosomes attach to the endoplasmic reticulum, RNA Synthesis increases, starch accumulates in amyloplasts, and pectic substances are intensively synthesized. It is believed that during this phase, rather than newly repressed genes being derepressed, The activity of already functioning genes is dramatically upregulated.

In parallel with this process, cell walls undergo softening and stretching.

It is hypothesized that the transition to the elongation phase is triggered by changes in the levels and ratios of phytohormones and their receptors as the daughter cell moves away from the mother cell; in other words, preconditions for the elongation phase are established within the meristematic cells themselves. Elongation growth is primarily activated by auxin (IAA), which induces The Active Transport of H⁺ ions from the cytoplasm into the cell wall. This activates acidic Hydrolases and Other Enzymes that modify cell wall components.

Fig. 155. Diagram of the putative mechanism of auxin action on cellular functional activity. GA — Golgi apparatus; ER — endoplasmic reticulum; a, b, c — auxin receptors

The cell wall acquires the capacity to stretch under The Influence of turgor pressure generated within the vacuole (Fig. 155). Osmotically active substances are generated both through the breakdown of cytoplasmic components and cell wall constituents. The primary cell wall is structured in such a way that, on the one hand, it can withstand hydrostatic pressure, while on the other hand, it is capable of both elastic (reversible) and plastic (irreversible) stretching. This phenomenon is explained by the fact that the cell wall consists of microfibrils embedded in a plastic matrix. During the elongation phase, microfibrils are oriented primarily perpendicular to the longitudinal axis of the cell or arranged randomly, which permits cell elongation under the influence of hydrostatic pressure. It is important to note that elongation growth begins when turgor pressure reaches a threshold value, and further water uptake becomes possible only through the plastic (irreversible) stretching of the cell wall.

Fig. 156. Diagram of cell wall component secretion

Concurrently, active synthesis of RNA and proteins, as well as the secretion of Polysaccharides, is observed. Golgi vesicles transport these components to the cell walls, Cellulose synthesis is intensified, and as a result, microfibrils grow in length or thickness within the cell walls (Fig. 156). Cell wall growth is coupled with the de novo synthesis of its structural constituents. Cellulose microfibrils are synthesized on the inner surface of the cell wall adjacent to the plasmalemma, utilizing cytoplasmic precursors and enzymes supplied by the Golgi apparatus. During this process,

a reorientation of older molecules takes place: they assume a more vertical alignment. The overall thickness of the wall remains unchanged, maintaining approximately 0.3–0.5 µm. During the final stages of elongation growth, lignification processes intensify, inhibitors (phenols, ABA) accumulate, auxin levels decline, the secondary cell wall is formed, and elongation growth is eventually arrested. Throughout the entire elongation phase, cell volume increases by a factor of 20–50, or even up to 100.

Thus, elongation growth comprises the following stages:

·loosening of the bonds between cell wall components and an increase in its plasticity;

·influx of water, which exerts pressure against the walls, causing stretching and an increase in cell volume;

·consolidation of the expanded volume through the incorporation of new components into the cell wall structure.