PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 2. A BRIEF HISTORY OF CELL RESEARCH. STRUCTURAL FEATURES OF HIGHER PLANT CELLS

2.3. Formation of the Cell Wall

The formation of The Cell wall is closely linked to Cell Division (cytokinesis). At the end of mitosis telophase, a phragmoplast forms in the equatorial zone of the dividing cell between the threads of the achromatic spindle (mitotic spindle) (Fig. 4). This Structure was first discovered and described in 1888 by the German botanist L. Herrera, who noted its barrel-like shape and fibrillar composition. It is now well established that the phragmoplast represents a microtubule system. Microtubules are membrane-free Organelles that appear as cylindrical structures approximately 24 nm in diameter and up to several micrometers in length. A microtubule has a hollow center, and its wall is formed by 13 rows of globular protein dimer complexes known as tubulins. Microtubules are located predominantly in the peripheral layers of the Cytoplasm. During cell division, the mitotic spindle and the phragmoplast remain closely associated due to shared microtubules. The phragmoplast has been found in all Higher Plants and in A number of green Algae (e.g., Coleochaetae).

The division of the mother cell into two daughter Cells begins with the Formation of the cell plate (Fig. 4), to which the dictyosomes of the Golgi complex (Golgi apparatus) contribute. Dictyosomes are single-membrane organelles composed of flat, rounded cisternae. A plant dictyosome typically consists of two to seven (rarely more) stacked cisternae arranged one above the other. Small gaps filled with fibrillar and tubular elements of unknown function are found between individual cisternae in the stack. Dictyosomes perform a secretory function, actively producing vesicles (Golgi vesicles, Golgi bodies) that contain highly viscous secreted substances, such as Polysaccharides or polysaccharide-Protein Complexes. The high viscosity of Golgi vesicle contents is due to a large amount of pectic substances, which is why they are frequently referred to as pectin vesicles.

Class="center">Fig. 4. Phragmoplast (1) and cell plate formation (2) during mother cell division

Pectin vesicles migrate toward the phragmoplast region, align horizontally in its central part, fuse, and begin to form The Cell plate. In the early Selection/3.html">Stages of development, the cell plate is laid down as a disk in the central part of the phragmoplast. The pectin vesicles do not fit tightly against one another; as they fuse, small gaps remain in the cell plate—plasmodesmatal channels lined with the Plasmalemma. These channels contain plasmodesmata—minute cytoplasmic strands that connect the protoplasts of daughter cells into a unified structure known as the symplast. In the center of each plasmodesma lies a desmotubule, which is not a tubular structure proper, but rather a compact strand of structural Proteins that contacts the cisternae of The Endoplasmic reticulum on both sides in the adjacent cells. Gradually, as microtubules disappear in the central part of the phragmoplast, new ones appear at its peripheral ends, causing the phragmoplast to expand toward the walls of the dividing mother cell. Along with it, the cell plate expands as its formation proceeds from the center to the periphery. Once the cell plate reaches the wall of the dividing cell, the division process is essentially complete. The resulting daughter cells must then reach a certain size (grow) and form their own cell walls.

During the cell growth period, the primary cell wall is formed. Both the Golgi complex and The cell membrane (plasmalemma) take part in this process. In the Initial Stages of development, the wall of the daughter cells consists of remnants of the mother cell wall and newly formed primary cell wall regions. The primary cell wall begins to form between the cell plate and the plasmalemma. It is believed that hemicelluloses and pectic substances are delivered to the site of wall synthesis by Golgi vesicles, whereas Cellulose synthesis is associated with The activity of the plasmalemma. The primary cell wall is quite elastic, consisting of about 15% cellulose, with the remainder made up of matrix components. The incorporation of cellulose molecules into the cell wall can occur in two ways: by intussusception and by apposition. In intussusception, new cellulose molecules are inserted between molecules already integrated into the wall. In apposition, new cellulose molecules are laid down from the inside onto previously deposited cell wall elements. Like the cell plate, the primary cell wall contains regions through which plasmodesmata pass. The thinner Regions of the primary cell wall penetrated by plasmodesmata are called primary pit fields (Fig. 5).

Fig. 5. Diagram of primary cell wall (cell wall) structure: 1—primary wall; 2—middle lamella; 3—primary pit field with plasmodesmata

Because the primary cell wall is dominated by hemicelluloses and pectic substances, contains a high amount of Water, and has an unorganized arrangement of cellulose molecules (Fig. 6), the wall is capable of stretching, allowing the cell to grow in both length and width.

Fig. 6. Arrangement of cellulose micelles in the primary cell wall

The pattern of growth determines the future morphological Features of the developing cell. If a cell expands more or less uniformly in length and width, it becomes isodiametric (with equal longitudinal and transverse diameters) or tabular (where one diameter is no more than twice the other). Such cells are termed parenchymatous. If length significantly exceeds width, the cells are called prosenchymatous (Fig. 7).

Fig. 7. Schematic representation of parenchymatous (A–G) and prosenchymatous (D) cell shapes: A—geometrically regular 14-hedron with eight hexagonal and six quadrangular faces; B—cell from the pith of Ailanthus (Ailanthus sp.); V–G — pith cells of boneset (Eupatorium sp.); D—cambium cells: 1—longitudinal tangential section; 2—longitudinal radial section; 3—transverse section

Primary cell walls are characteristic of meristematic cells, young growing cells, and cells of certain permanent Tissues (parenchyma, epidermis, collenchyma, phloem).

In most cells that have reached their maximum size and stopped growing, the secondary cell wall begins to form from the inside over the primary cell wall via apposition (Fig. 8). The secondary cell wall contains the same components as the primary wall, but The ratio of polysaccharides differs. The majority (up to 60%) is cellulose, and in the secondary walls of flax bast fibers, the cellulose content can reach up to 95%. Furthermore, the degree of polymerization of its molecules increases, and the arrangement of microfibrils in the matrix becomes more ordered. The matrix volume and water content decrease significantly, while the bonds between the Components of the secondary cell wall strengthen.

Fig. 8. Diagram of secondary cell wall structure: A—general layout: 1—primary cell wall; 2—middle lamella; 3—simple pit aperture; 4—pit closing membrane; 5—secondary wall; 6—pit entrance; B—orientation of cellulose micelles in different layers of the secondary cell wall

The secondary wall is typically layered. This stratification arises during wall formation and is related to the orientation of cellulose microfibrils within it. Most commonly, the secondary wall consists of three layers that differ in microfibril orientation. Within each layer, microfibrils run strictly parallel to one another, but the angle of inclination of the microfibrils changes from one layer to the next relative to the previous one.

The secondary cell wall is not continuous. Wherever primary pit fields formed in the primary wall, the secondary wall is interrupted. This interruption in the secondary cell wall located over a primary pit field is called a pit (Fig. 9). Pits in adjacent cells form opposite one another, creating a pit pair. These pits are connected by pit canals extending from the cell lumen to the primary cell wall, which Functions as the pit closing membrane. Pits can be simple or bordered. Simple pits form when the secondary cell wall abruptly terminates above the primary pit fields. The pit canals of simple pits maintain a uniform diameter throughout their length. In contrast, during the formation of a bordered pit, portions of the secondary cell wall overhang the internal pit entrance from the cell lumen side, making the inner opening much narrower than the outer opening that abuts the primary cell wall (the pit closing membrane), thus giving the pit canal a funnel-like shape. Simple pits are characteristic of parenchyma cells and bast fibers, whereas bordered pits form in water-conducting elements—tracheids. In conifers, the center of the bordered pit closing membrane features a thickened region formed by the middle lamella and bordered on both sides by the primary cell walls of two adjacent cells. This thickening is called the torus. Because the primary cell wall between the torus and the pit canal wall is extremely thin, the torus is mobile. As water moves through the tracheids, the torus can partially block certain pits, thereby regulating the speed of water transport through the conducting elements.

Fig. 9. Diagram of pit structure: A—simple; B—bordered; V—half-bordered: 1—pit entrance; 2—pit exit; 3—torus

2.4. Secondary Modifications of the Secondary Cell Wall

During the plant's life cycle, the secondary cell wall may undergo secondary modifications: lignification, suberization, cutinization, mineralization, and mucilaginous degeneration.

Lignification (from Lat. lignum — wood) is a process associated with the deposition of Lignin within the matrix of the secondary cell wall. Lignin is a complex phenolic polymer whose exact chemical structure remains unelucidated. As one of the most critical components of secondary cell walls, lignin is synthesized by the protoplast and deposited exclusively in the cell walls of higher plants (excluding bryophytes). It is embedded within the wall matrix of specialized cells (sclerenchyma, tracheids, vessel elements) between microfibrils, substantially increasing structural rigidity while reducing elasticity. Simultaneously, lignin renders the cell wall impermeable to water and dissolved mineral solutes, disrupts gas exchange, and ultimately drives the programmed cell death of the living cell contents. Lignified cell walls possess an ultrastructure analogous to reinforced concrete.

Among lipid-like substances, suberin (from Lat. suber — cork) is deposited in the secondary walls of certain tissues, such as the phellem and endodermis. This process causes cell wall suberization. Suberin is insoluble in organic and inorganic Solvents as well as acids, though it is degraded by concentrated alkalis. It may be deposited on the inner surface of the cell wall either as discrete lamellae or as a continuous layer. In the latter case, the protoplast becomes completely isolated from both the external and internal environments and subsequently dies.

Chemically, suberin is structurally related to the hydrophobic polymers cutin and wax. Cutin is deposited on the outer surface of the cell wall, forming a thin film known as the cuticle. The cuticle performs a protective function, shielding the aerial Organs of plants from excessive transpirational water loss.

Analogous functions are fulfilled by the waxy bloom coating various plant organs. Unlike cutin and suberin, wax is soluble in organic solvents and melts readily. Wax is deposited as an amorphous or crystalline layer with a protective role. Furthermore, wax may be incorporated into the cuticle, thereby enhancing its barrier properties.

Cell wall mineralization results from the deposition of silica or calcium salts (oxalates, carbonates). Amorphous silica is deposited in the epidermal cell walls of certain taxa, such as grasses, horsetails, and stinging hairs of nettles. Calcium oxalate and carbonate may be embedded directly within cell walls or form cystoliths and diverse crystals intracellularly.

Mucilaginous degeneration of cell walls is promoted by high concentrations of hemicelluloses and, notably, pectic substances. This process entails extensive Hydration, leading to the formation of surface mucilages comprising a mixture of polysaccharides and their derivatives. Mucilaginous modification serves an adaptive function: in some instances, it assists plants in surviving arid periods and enhances desert plant viability; in others, it AIDS reproduction, as seeds with mucilaginous coats adhere securely to the substrate and germinate rapidly.

Thus, the cell wall of higher plants constitutes a complex, multifunctional structure that varies according to age, cellular functional specialization, and plant species Specificity.

2.5. Plastids as Specialized Organelles of the Plant Cell

Plastids (from Greek plastos — molded, formed) are mandatory organelles of The plant cell. The plastids of higher plants are double-membraned organelles whose Morphology varies depending on their type. Higher plants possess three primary plastid types: METABOLISM/14.html">Chloroplasts (from Greek chloros — green, and plastos), chromoplasts (from Greek chroma — color, and plastos), and leucoplasts (from Greek leukos — white, and plastos) (Fig. 10). These color-based designations were introduced by the German botanist A. Schimper (1856–1901): chloroplasts are green, chromoplasts are orange-red, and leucoplasts are colorless. Plastid coloration depends on their resident pigments. In higher plants, chloroplasts contain the green pigments chlorophyll a and b, alongside carotenoids—specifically carotene and xanthophyll (orange and yellow). Chromoplasts accumulate exclusively carotenoids, whereas leucoplasts lack pigments entirely.

Fig. 10. Submicroscopic structure of plastids: A—chloroplast; B—leucoplast; C—amyloplast; D—chromoplast: 1—outer membrane; 2—inner membrane; 3—matrix (stroma); 4—stroma lamellae; 5—grana; 6—thylakoid; 7—starch grain; 8—lipid droplet with pigments

The submicroscopic architecture of all plastids is fundamentally similar. A double-membraned, agranular envelope encloses the stroma—a colorless proteinaceous matrix that houses an ordered system of pigment-bearing membranes (thylakoids). Additionally, the stroma contains Ribosomes, DNA, Enzymes responsible for the synthesis and Hydrolysis of reserve nutrients, and other components. Plastids are the sites of both Primary and secondary synthesis of Organic compounds, as well as their storage.

Plastid number per cell (ranging from 15 to 50) and dimensions (3–10 µm) vary considerably. Plastid shape is similarly diverse, determined primarily by submicroscopic Organization, which is governed by the quantity and spatial arrangement of thylakoids.

Chloroplasts exhibit the most intricate submicroscopic structure (Fig. 10, A). Chloroplasts in higher plants share a uniform spherical-lenticular or elliptical morphology, maintained by a rigorously ordered system of internal membranes—thylakoids—suspended in the stroma. Thylakoids originate from the inner membrane of the chloroplast envelope and form flattened, sac-like or discoid structures. Within the chloroplast stroma, discoid thylakoids stack vertically like piles of coins to form grana. Higher plant chloroplasts typically develop between 40 and 60 or more grana. Thylakoids within grana are interconnected via frets, while individual grana are linked by single thylakoids known as stroma lamellae. The principal function of chloroplasts is Photosynthesis.

Unlike chloroplasts, the stroma of chromoplasts either lacks thylakoids entirely or contains only isolated, sparse thylakoids. Owing to the absence of complex submicroscopic organization, chromoplasts can alter their shape. Chromoplast morphology is largely dictated by the physical state of their resident pigments. Carotenoids may dissolve in Lipids and accumulate within plastoglobules—spherical lipid droplets containing dissolved carotenoids; in such cases, chromoplasts maintain a relatively defined shape. If carotenoids accumulate within protein fibrils or crystallize out, chromoplasts assume varied morphologies as their envelope tightly Molds around the formed crystalline structures (Fig. 10, D; 11). The primary function of chromoplasts is the synthesis and accumulation of carotenoids.

Leucoplasts are the smallest plastids, typically developing in underground plant organs and more rarely in epidermal cells (Fig. 10, B). The inner membrane of the leucoplast envelope generates a limited number of isolated, dispersed thylakoids. Leucoplasts are most frequently spherical, though alternative shapes occur. They contain DNA, ribosomes, and enzymes that drive the synthesis and hydrolysis of reserve substances, which dictates their functional profile. Leucoplasts mediate the storage or secondary synthesis of CARBOHYDRATES, proteins, and lipids. Starch-storing leucoplasts are termed amyloplasts (Fig. 10, C; 12) and are abundant in rhizomes, tubers, seed storage tissues, etc. Leucoplasts that accumulate or secondarily synthesize proteins are designated proteinoplasts. Proteinoplasts are found in the leaf epidermis of members of the Commelinaceae family (Tradescantia, Zebrina, Setcreasea, etc.) and in the pollen grain walls of Asclepiadaceae plants (Cynanchum, Asclepias). Oleoplasts are lipid-storing leucoplasts characteristic of many monocots; they may occur in leaf epidermis (Orchidaceae), perianth segments (Ornithogalum in Liliaceae), and Ovary walls (Hosta in Liliaceae).

Fig. 11. Morphological diversity of chromoplasts in the flesh cells of mature fruits: A—dog rose (Rosa canina); B—lily of the valley (Convallaria majalis); C—rowan/mountain ash (Sorbus aucuparia); D—Blood-red hawthorn (Crataegus sanguinea): 1—chromoplasts; 2—Cell Nucleus; 3—cell wall

All plastids are genetically interrelated, originating from proplastids—colorless precursors bounded by two elementary membranes but lacking an internal membrane system. Proplastids are invariably present in the Cell Cytoplasm and are transmitted to daughter cells during division. Typically, a cell contains only a single plastid type. The complete Complement of plastids within a cell is referred to as the plastidome.

Throughout the plant Organism's life cycle, plastids can modify their Structure and function. Upon The breakdown of their internal architecture, chloroplasts can transform into chromoplasts, which may likewise develop from leucoplasts. Conversely, chromoplasts are virtually incapable of giving rise to other plastid types, as they represent specialized organelles with a comparatively low level of internal structural organization relative to chloroplasts and leucoplasts. Consequently, chromoplasts are frequently regarded as the terminal stage of plastid differentiation.

2.6. Plant Cell Vacuoles

Vacuoles (from Lat. vacuus — empty) in plant cells are cytoplasmic cavities bounded by a selectively permeable (semipermeable) membrane, the tonoplast, and filled with cell sap (Fig. 12). The primary component of cell sap is water, in which various metabolic by-products and solutes are dissolved. The Biochemical Composition of cell sap varies among plant species and tissues. The fruits, taproots of sugar beets, and sugarcane are rich in sugars (sucrose, glucose, fructose). Seed cell vacuoles are protein-rich, storing proteins either as colloidal solutions or in crystalline form. The cell sap of immature fruits accumulates organic acids, including citric, oxalic, succinic, and acetic acids. The cell sap of certain plants contains Tannins—astringent non-nitrogenous cyclic compounds. Tannins are abundant in the bark of oaks and willows, tea leaves, and unripe walnuts. For specific cell types, tannin accumulation constitutes a primary metabolic function. Alkaloids—chemically diverse, bitter-tasting, nitrogen-containing heterocyclic compounds—are restricted exclusively to the cell sap of higher plants. Approximately 10,000 alkaloids are known (e.g., caffeine, atropine, nicotine), most commonly existing as salts within the cell sap. Furthermore, cell sap may contain Glycosides, which are conjugates of sugars with alcohols, aldehydes, phenols, and other molecules. The cell sap pigment anthocyanin is itself a glycoside.

Fig. 12. Lower epidermal cells of wandering jew leaf (Zebrina pendula): 1—cell wall; 2—cell nucleus; 3—vacuoles; 4—leucoplasts

In the vacuoles of certain plants, a high concentration of mineral salts leads to the formation of crystals in the form of druses, raphides, styloids, and crystal sand.

Plant cell vacuoles perform a variety of functions. They regulate Water and Salt Metabolism, maintain turgor pressure within the cell, serve as storage sites for reserve nutrients, and sequester toxic metabolic byproducts. The turgor pressure generated within vacuoles causes the cell walls to round out, resulting in the formation of intercellular spaces. This system of intercellular spaces and cell walls is termed the apoplast, which serves as the primary pathway for The transport of aqueous mineral solutions throughout the plant.

The exact mechanism of vacuole formation in plant cells remains incompletely understood. Some researchers suggest they originate from the expansion of endoplasmic reticulum cisternae, though other hypotheses exist. It is quite likely that vacuoles can form through various pathways: either by the fusion of agranular vesicles budding off from the endoplasmic reticulum, or from elements of the Golgi apparatus.

Thus, the distinct structural FEATURES OF PLANT Cells determine their properties and, ultimately, the CHARACTERISTICS OF THE plant tissues they form.