PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

11. PLANT GROWTH AND DEVELOPMENT

Mechanisms of Morphogenesis

Elucidating the principles underlying The formation of an Organism's specific shape remains one of the most challenging and under-researched problems in biology. Solving it requires understanding how Cells and Tissues influence one another throughout ontogeny.

In a plant organism, intercellular and intracellular regulatory mechanisms are in constant interaction.

For Embryogenesis and regeneration, the sequential induction of specific genetic programs and morphophysiological orientation in space are critical. Genetic programs are triggered by specific chemical and physicochemical factors, while spatial orientation is established through polarization, which is rooted in membrane processes. Cells continuously receive signals from their external and internal environments, effectively testing their Location and surroundings. In response, they adjust their functional activity, Gene Expression, and polarization vectors. This adjustment is known as the "neighborhood effect" or "positional effect."

Triggering Genetic Programs

Plants offer excellent models for studying the GENETIC ASPECTS OF morphogenesis, notably the unicellular marine alga Acetabularia (order Dasycladales). Its cells consist of a rhizoid base containing The Nucleus, a stalk, and a spore-forming cap. Experiments on this alga have shown that the Cell Nucleus produces long-lived, species-specific morphogenetic informational RNAs. These RNAs migrate into the stalk and "direct" the morphogenesis of the cap by regulating the Synthesis of specific Proteins. Thus, the stalk and the base can independently determine the shape of the cap, which is a species-specific trait.

In many plants, intracellular gene regulation mechanisms are analogous. The development (differentiation) of a cell proceeds in a specific direction depending on which set of genes is currently active (expressed). The direction of this differentiation subsequently dictates The Cell's functional activity. Consequently, the combination of active genes determines the differentiation pathway and the Functional Characteristics of each individual cell.

However, the activation or suppression (opening or closing) of genetic programs in each cell also depends on signals received from other cells, tissues, and Organs, as well as environmental conditions. The mechanisms underlying these processes are complex and diverse.

When studying the PHYSIOLOGICAL AND BIOCHEMICAL foundations of morphogenesis in higher plants, the tissue culture method is employed. This method allows researchers to investigate METABOLISM/18.html">The Influence of trophic and hormonal factors on morphogenetic processes, which is made possible by the totipotency of plant cells.

Let us examine The Effect of phytohormones on the differentiation of callus derived from tobacco stem pith, grown on a complete nutrient medium supplemented with varying concentrations of IAA and kinetin (see table).

Class="center">Table

Studies have shown that a predominance of auxin (3) is required to trigger the genetic program for root formation, whereas a high cytokinin content (4) in the presence of IAA induces The activity of genes responsible for SHOOT formation. Other specific combinations of phytohormones, trophic factors, and physicochemical conditions trigger different genetic programs. This occurs only when the cells are competent (sensitive) to the action of these signals (effectors).

Induction of Polarization in Plants

An essential condition for morphogenesis during organismal development is the polarization of biological structures.

Polarization refers to the specific orientation of processes and structures in space, which leads to The Emergence of morphophysiological gradients.

In this process, physiological, biochemical, and anatomical-morphological differences change in a specific direction, causing opposite PARTS OF THE plant organism to differ. Polarity determines the placement of axes that dictate the shape of cells, organs, and the entire organism. As a result, the characteristic bipolar Structure of plants is formed (establishing the main axis: shoot-root).

Polarity is the physiological non-equivalence of opposite poles (apex/base) of a given cell, organ, or entire plant.

Physiologically, polarity is manifested, in particular, in regeneration processes. For instance, in stem and root cuttings, regardless of their spatial orientation, shoots develop from the morphologically apical end, while roots develop from the basal end. This is explained by the fact that IAA, moving polarly, accumulates at the morphologically lower end of the cutting, triggering the genetic program for root formation.

Individual cells also possess the property of polarity. In isolated Cells of the green filamentous alga Cladophora, rhizoids form from the morphologically lower end of the cell, while the thallus forms from the upper end.

Environmental factors often act as Inducers of polarization in biological objects. For example, in spores of horsetails and ferns, polarity arises only under conditions of unilateral illumination. Upon division, the illuminated side forms the prothallus, while the shaded side forms the rhizoid.

The mechanism of polarization has been studied in detail in the egg cell of the brown seaweed Fucus. Before Fertilization, the Fucus egg lacks a Cell wall, and the nucleus is centrally located. After fertilization, the cell settles, develops a cell wall, and a rhizoidal protrusion begins to form on its lower surface. The First Division occurs perpendicular to the established axis. The upper cell gives rise to the majority of the thallus, while the lower cell forms a small part of the thallus and the rhizoid. In darkness, rhizoids grow in random directions, whereas in light, they grow from the shaded side. Exposure to a weak electric field also induces polarization in egg cells, with the rhizoid protrusion appearing on the anode side. Local application of auxin or a unilateral influx of Ca²⁺ induces rhizoid formation at the site of stimulation. These external influences trigger the generation of local electric currents, effectively causing polarization.

Calcium induces secretory activity in the egg cell and the localized release of cell wall Polysaccharides, which facilitates the emergence of a rhizoid protrusion. Even within the nucleus, finger-like membrane projections form toward the future rhizoid, and Mitochondria and Ribosomes concentrate in this same region of the Cytoplasm.

Auxin is a class of Plant HORMONES that regulate growth, development, and various physiological processes, including cell elongation, apical dominance, and the initiation of root formation.

When an electric field is applied to an egg cell, the electrophoretic migration of positively or negatively charged lipoprotein components occurs within its Plasmalemma. These components (Ion Channels, pumps, and Enzymes) are anchored at the cell poles by cytoskeletal microfilaments and microtubules, which irreversibly fix the resulting primary polarization and determine the main axis of the plant body.

During subsequent division of the egg cell (where the plane of division is perpendicular to the axis of polarization), the daughter cell nuclei enter vastly different conditions created within the polarized cytoplasm. As a result, non-identical Genetic information is activated in the nuclei, thereby leading to Cell Differentiation.

Thus, plant polarity is determined by the polarity of the fertilized egg cell from which the new organism develops. During cell division, daughter cells inherit the polarity of the mother cell. This phenomenon ensures the specific orientation of processes and structures in space, which drives the specific cell differentiation that manifests in the formation of a polar embryo. In the embryo of higher plants, two embryonic organs are formed: the shoot with the embryonic bud and the root. Phytohormones play a leading role in establishing these polarity processes.

The direction of cell polarization changes continuously throughout embryogenesis, during the formation of primordia in the apex, in The process of leaf tissue specification, and during the initiation of root primordia in the tricyle, among other instances.

In this context, each polarized meristematic cell can divide into two unequal (differential division) or two equal (reproductive division) daughter cells. In the latter case, the tissue grows through an increase in the number of homogeneous cells.

The formation of physiological cell polarity is influenced by polar calcium ion fluxes, the gradient of bioelectric potentials, the basipetal flow of auxin, and the functional CHARACTERISTICS OF THE Cytoskeleton. Primary information regarding the polarization vector is generated by Calcium Ions. The induction of morphogenesis depends on the magnitude of Ca²⁺ ion gradients, which establish the electrical axis of polarity and influence the distribution of phytohormones within tissues. Consequently, gradients of Ca-binding and Ca-dependent proteins form within the cell, which, with the participation of corresponding cytoskeletal elements, complete the structural polarization of the cell.

Cell polarization in a multicellular organism is driven by various factors: physicochemical gradients (osmotic pressure, pH, oxygen concentration, CO₂), hormonal, electrical, and trophic gradients, as well as contacts with neighboring cells (contact polarization), and mechanical pressure and tension. Of particular importance for the integrity of the plant are the gradients created by the dominant centers of the shoot and root—their apices. The oscillatory nature of these gradients is an essential condition for the temporal integrity of the plant organism.

Position effect

The emergence of polarity is significantly influenced by cell interaction and spatial arrangement. The induction of polarity under the influence of a specific group of surrounding cells is identified as the field effect, location effect, or position effect. The cellular environment can influence the formation of polarity through uneven chemical, mechanical, or electrical stimuli.

For example, a meristematic cell is polar simply due to its location. Every meristematic cell has an apical and a basal end. If its subsequent division occurs perpendicular to the axis of polarity, the daughter cells will differ in their physiological and structural features, in the hereditary factors present in the cytoplasm, and in the presence of phytohormones that regulate genome activity. This occurs despite the fact that an equal distribution of the nuclear hereditary material (DNA) between the daughter cells has taken place.

Thus, every cell in a multicellular organism is influenced by physical, chemical, and physiological gradients, as well as by neighboring cells. As a result, cells realize those specific potentials (capabilities) that correspond to their surrounding conditions. This theory is known as the "position effect" or "field effect."

J. Bonner (1965) proposed THE PRINCIPLE OF morphogenetic testing to explain the mechanisms controlling differentiation in a multicellular organism. In order to adequately respond to changing conditions and signals received from the surrounding environment (the property of irritability), each cell constantly tests its location. The result of this testing is the activation of specific genetic programs. For instance, a certain meristematic cell divides transversely into two daughter cells. Each of them "determines" whether it is apical. For the apical cell, the result will be continued division, while the second, subapical cell, tests the size of the group of cells surrounding it. When the group is small, a division subprogram is triggered, which Functions until the number of cells in that area of the apex reaches a program-defined threshold.

Upon the Formation of the required number of cells, each cell tests its location relative to The surface of the cell population. When the analysis indicates that certain cells are On the surface of the groups, the program for their differentiation into epidermis cells is triggered. Other cells located beneath the surface perform a location test within the group; as a result, cells located in the deepest layers are induced to follow a differentiation subprogram into pith parenchyma cells, those in the vicinity into xylem cells, and those not as deep into a phloem-formation subprogram.

Cells that occupy an intermediate position become cambial and divide in a closed cycle, forming xylem and phloem elements.

This hypothesis has received experimental confirmation.