PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

10. BIOGENESIS OF CELLULAR STRUCTURES AND PLANT CELL ONTOGENY

Phases of Plant Cell Ontogeny

During The life cycle of a Cell, the specific composition of genes undergoing expression changes gradually and systematically, causing The Cell to progress through several developmental stages.

The Cell life cycle, or ontogeny, is the period of a cell's existence from its formation through the division of a mother cell to its own subsequent division or death. Plant cell ontogeny consists of a series of sequential stages: division, growth, differentiation, maturity, senescence, and death.

Cell Division

Mitosis. Mitosis is a mode of cell division in which the chromosome number is doubled, ensuring that each daughter cell receives a double set of Chromosomes identical to the chromosomal Complement of the mother cell.

The first phase of mitosis is prophase, a phase of cellular reorganization. During this stage, the following changes occur: Organelles move to the periphery; The Nucleus enlarges; Chromatin condenses into chromosomes with kinetochores; the nucleolus dissociates; the nuclear envelope breaks down; and spindle structures form (without centrioles, but with the aid of ER membrane clusters at the cell poles). The spindle is composed of interpolar and chromosomal microtubules. Associated with the microtubules (MTs) are regulatory Proteins for MT synthesis and calmodulin, which participates in their disassembly. At this stage, the kinetochores of the chromosomes are not yet attached to the spindle elements.

In prometaphase (metakinesis), chromosome movement begins. Kinetochores increase in size, and chromosomal MTs form from them. Chromosomes move first toward the poles and then toward the center of the spindle. During these movements, the two sister chromatids within a chromosome, connected only at the kinetochore, begin to uncoil. Constantly moving, the chromosomes gather along the spindle equator to form the metaphase plate (metaphase). During this process, they exhibit slight oscillatory movements along the spindle.

The transition of the cell into anaphase is accompanied by the division of the kinetochore, the physical Separation of the two sister chromatids, and the movement of the separated chromosomes toward the poles, with the kinetochore leading the way. A redistribution of MTs occurs: their number near the poles decreases and increases in the equatorial region. Here, a zone of vesicle accumulation begins to form, marking the initiation of the cell plate.

Chromosome movement is explained by both physical forces and biochemical mechanisms: the sequential removal of MT subunits from the spindle by a protein system located on the kinetochore, and the interaction between MTs and Actin microfilaments.

After chromosome separation, telophase, the final phase of mitosis, begins. Near the poles, spindle MTs disintegrate, nucleoli and nuclei form, and The Development of the cell plate—the phragmoplast—is completed. This occurs through the accumulation of Golgi apparatus (GA) vesicles and ER membranes along the equator. The vesicles contain pectic substances. Upon fusing, the vesicles form two membranes—the plasmalemmae of the daughter Cells—separated by a semi-liquid layer consisting of pectic substances.

The phragmoplast grows from the center to the periphery through The addition of new vesicles, but the cytoplasms of the daughter cells remain in contact via plasmodesmata. These form in the Regions of the cell plate where spindle fibers with MTs and ER elements are localized. From the side of the daughter cell cytoplasms, the Formation of primary cell walls begins, and the phragmoplast transforms into the middle lamella. Cellulose microfibrils in the primary cell walls have a loose texture, but their primary orientation is perpendicular to the longitudinal axis of the cell. Initially, the cellulose content does not exceed 2-3%.

Hemicelluloses, pectic substances, and the glycoprotein extensin are delivered to the forming Cell wall in Golgi vesicles. The polypeptide extensin, synthesized on the rough ER, is glycosylated in the GA dictyosomes. Components of the cellulose synthase enzyme complex are localized on the outer surface of the Plasmalemma. Enzymes required for the assembly and modification of wall polymers are also evidently delivered to the wall by ER or GA vesicles. UDP-glucose and GDP-glucose for cellulose synthesis are transported from the soluble phase of the cell across the plasmalemma. New cellulose molecules are integrated into The cell wall from within.

Mitotic Cycle. The period of cytoplasmic growth and preparation for division (interphase) and The process of division (mitosis) together constitute the mitotic cycle of the cell. Depending on the specifics of biochemical processes, four periods of the mitotic cycle are distinguished:

- presynthetic period — G₁ (gap) - the most sensitive to environmental factors; during this period, all conditions for DNA Synthesis are prepared;

- synthetic period - S (DNA and histone synthesis);

- postsynthetic (premitotic) - G₂ (continued synthesis of RNA and general cellular proteins; a sharp increase in tubulin synthesis);

- mitosis proper - M (proteins and RNA are synthesized until the end of metaphase).

In plants, the periods of the mitotic cycle are controlled by Hormones. Auxin is required for the normal progression of the G₁ and G₂ periods. Cytokinin is apparently necessary for the cell to transition into division.

In dividing cells, the establishment of organelle structures also takes place. Cells that have exited the meristematic zone cease dividing (the genetic program for division is closed) and transition to the next stage of ontogeny - growth by elongation.

Cell Elongation

The increase in the size of dividing cells occurs through the synthesis of new cytoplasmic structures and the influx of significant amounts of nitrogen compounds and other nutrients. After completing division, plant cells transition to more rapid growth—elongation—which is characteristic only of plants. It serves as the primary and essential method for increasing leaf surface area, as well as stem and ROOT system length. During elongation growth, the increase in cell volume is achieved through The formation of a large central vacuole. Simultaneously with vacuolar growth, the cell walls soften and stretch.

The transition of meristematic cells to elongation is not yet fully understood.

The transition process itself is resistant to adverse factors (such as PROTEIN AND NUCLEIC acid Biosynthesis inhibitors or X-rays). Cells undergo vacuolization, the protein-synthesizing system is reorganized, the number of granular ER membranes and RNA increases, and the synthesis of simple and complex CARBOHYDRATES and pectic substances intensifies. The mechanisms by which cells transition to elongation remain poorly understood. It is believed that Changes in the content and ratio of phytohormones (Auxins, Cytokinins) and their receptors may play a significant role. Auxin is essential for elongation growth.

The cell elongation growth phase is well-documented. During this period, the established growth mechanism is maintained. Under METABOLISM/18.html">The Influence of auxin, the plasticity of the cell wall in higher plants increases, enhancing its extensibility. The Mechanism of auxin action is as follows: the enzyme induces active H+ Transport from the Cytoplasm to the cell wall. A decrease in pH within the cell wall phase promotes the Cleavage of acid-labile bonds and activates acid Hydrolases and Other Enzymes that modify cell wall components. Wall expansion is also facilitated by increasing intracellular turgor pressure, which is maintained by the influx of Water into the growing vacuole.

The Effect of auxin on elongation growth is accompanied by the synthesis of RNA, proteins, and enzymes, as well as the secretion of Polysaccharides and proteins required for the entire process. Cell wall growth is supported by the active function of the Golgi apparatus (GA). Enzyme systems responsible for forming new wall components are activated on the outer surface of the plasmalemma, and cellulose synthesis increases. The epidermis of shoots is most sensitive to auxin. The growth of internal parenchymal Tissues is more strongly activated by increased acidity due to CO₂. The enhancement of growth under low pH values (within the range of 3–5) is known as the acid growth effect. It acts only for 1–3 hours and is not supported by Protein Synthesis.

At the end of elongation growth, cell wall lignification increases, phenolic inhibitors and Abscisic acid accumulate, The activity of peroxidase and IAA oxidase rises, auxin content decreases, and a secondary cell wall is formed. All these factors lead to the inhibition and cessation of cell elongation. Throughout the entire elongation phase, cell volume increases 20- to 50-fold, or even up to 100-fold. Thus, elongation growth consists of the following stages: cleavage of bonds between cell wall components and an increase in its plasticity; influx of water into the vacuole, which grows and exerts pressure on the walls, causing stretching and an increase in cell volume; and stabilization of the increased volume by incorporating new components into The Structure of the renewed cell wall.

At all stages of cell elongation, the self-assembly of various structures occurs (during the Organization and activation of the 'elongation' genetic apparatus, the auxin action mechanism, enzyme systems for cellulose, hemicellulose, and protein (extensin) synthesis, and the expansion of the plasmalemma).

Cell Differentiation

Cell differentiation is The Emergence of qualitative differences between cells associated with their specialization. Cell specialization is ensured by biochemical and structural features that allow the cell to perform specific physiological or other Functions necessary for the plant's vital activity.

Some cells differentiate very early, even in the apical meristem, such as epidermal cells. Most other meristematic cells appear similar to one another externally. However, their future specialization is often determined as early as the division zone (as evidenced by immunochemical analysis data).

The qualitative differences between cells are based on the processes of differential Gene activity. Every vegetative cell of a plant Organism contains the complete information for the Development of the entire organism in its genome and, under certain conditions, can form a whole plant (the property of totipotency). However, while part of the organism, this cell will express only a portion of its Genetic information. Specific combinations of phytohormones, metabolites, and physicochemical factors serve as signals (effectors) for the expression of a particular genetic program.

Upon completion of the differentiation process, the cell enters its mature stage and begins to perform the functions inherent in its organization.

Cell Aging and death

Aging and senescence are the final Stages of the ontogeny of differentiated, mature cells. These phenomena have been best studied in aging leaves.

Aging cells are characterized by a weakening of synthetic processes and an intensification of hydrolytic ones. RNA and Protein content decreases, the activity of hydrolases and peroxidases increases, membrane lipid oxidation rises, membrane semi-permeability declines, autophagic vacuoles form, chlorophyll and Chloroplasts are destroyed, the ER and GA dissociate, and Mitochondria and nuclei disintegrate. Aging becomes irreversible from the moment the tonoplast ruptures and its contents are released into the cytoplasm.

There are two groups of hypotheses to explain the mechanism of aging.

1. The first group of hypotheses links the transition of cells to aging and senescence with the accumulation of damage in the genetic apparatus, membranes, and other structures, as well as an increase in the concentration of toxic substances within the cell.

2. Proponents of the second group of hypotheses believe in the existence and Activation of a genetic program of aging as The final stage of plant cell ontogeny.

Both hypotheses have quite convincing justifications.

The cause of cell aging can be better understood from the perspective of the PHYSIOLOGY OF THE whole plant. A decrease in the supply of certain phytohormones (auxins, cytokinins, Gibberellins) and nutrients to the cell sharply accelerates aging processes. These hormones activate Protein and RNA synthesis, restore chloroplast structures, and 'rejuvenate' cells. Conversely, Ethylene and ABA accelerate aging processes.

One sign of cell aging is a shift in pH toward the acidic side. This negatively affects the physicochemical state of proteins and activates acid hydrolases. Auxin, by activating the H⁺-pump, protects the cytoplasm from acidification and organelles from degradation.

The weakening of the H⁺-pump and the loss of membrane semi-permeability lead to the dissipation of the Membrane Potential and the inability to absorb and retain substances. From this moment, the cell ceases to be alive and undergoes lysis.