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Crop Production Department/ Instructed By: Dr. Sirwa A. Qadir Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir 1 Root Growth and Development 1.1 Root tip regions The following four regions are distinguished in a root from apex upwards. 1.1.1 Root Cap It is a cap like structure that covers the apex of the root. The main function of the root cap is to protect the root apex. The root cap is a protective cap of live parenchyma cells. It is produced by the apical meristem behind it. It protects the root meristem as it grows through the soil outermost cells are sloughed off as the root tip "pushes" through the soil. The root cap cells last 4-9 days, depending on plant species and growth rate before they become root cap cells, root cap meristem cells differentiate into: 1.1.1.1 Columella The cells of the columella a central column of cells that contain starchy amyloplasts. In response to gravity, the amyloplasts fall to the bottom of the cells, attracting hormones that promote growth in the direction of the amyloplasts. Collumella cells are also light-sensitive, and respond to pressure from soil particles, further signalling to the plant which way is down. By the time columella cells are pushed to the periphery of the root cap by other developing cells behind them, they have differentiated into peripheral root cap cells. Peripheral root cap cells secrete mucigel that is manufactured in their dictyosomes (modified Golgi apparatus). Root cap cells produce a slimy lubricant (mucilage or mucigel) it's the plant equivalent/analog of mucus hydrophilic polysaccharide (a type of pectin). Also contains sugars, organic acids, vitamins, enzymes, and amino acids. Produced in the Golgi and packaged in vesicles that attach to the cell wall and empty their contents. The goo passes through the cell wall and oozes out onto the root cap surface. Mucigel allows greater ease of transporting ions from interstitial soil water to the root surface, where it can be absorbed. Mucigel is friendly to nitrogien-bacteria, and may help them colonize roots of nitrogen-fixing plant species (primarily in the Pea Family, Fabaceae). May provide protection against desiccation. In some plants, mucigel contains inhibitors that prevent the growth of roots from competing plants. (allelopathy) aids in water and nutrient absorption by increasing soil/root contact. Mucigel can chelate soil ions, making them available to the root. Mucigel components can aid in the establishment of mycorrhizae and symbiotic bacteria. Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir 1.1.1.2 Quiescent center The discovery of the “quiescent center” in the root apex has clarified many features, however. The quiescent center is a group of cells, up to 1,000 in number, in the form of a hemisphere, with the flat face toward the root tip; it lies at the center of the meristem, in much the same position, in fact, as the tetrahedral apical cell in certain lower plants. The cells of the quiescent center are unusual in that their division rate is lower than that in the surrounding meristem. The cells of the center have other distinctive features as well, notably a lower rate of protein synthesis than that of neighboring cells. The quiescent center, a constant feature of the root tip, is apparently generally present in angiosperm and probably also in gymnosperms. The quiescent center probably plays a role comparable with that of the apical cell in some lower plant roots, maintaining the geometry of the system. It has also been suggested that it may be concerned with the synthesis of growth hormones, although no direct evidence exists. When roots are damaged mechanically or by radiation, the cells of the center can resume a rapid division rate, and they then participate in regeneration. 1.1.2 Root Meristems (Zone of cell division) The apical meristem (from the Greek merismos, which means "division") is found at the very tip of the root, just behind the root cap. As in all meristems, the apical meristem contains some cells that will always remain meristematic: one daughter cell remains in the meristem (the initial) and continues to divide, whereas its sister cell (the derivative) stays behind as the meristem grows out. It is the zone of cell division extends some distance along the length Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir of the root above the tip region. Although the girth may increase by longitudinal divisions and the widening of the daughter cells, most divisions occur in the transverse plane resulting in the formation of longitudinal files of cells. The derivative differentiates into some type of cell, depending on its gene expression. This is known as primary growth. The very end of the root tip contains the initials and the immediate derivatives, and is known as the promeristems. 1.1.3 Zone of elongation: It is a region that lies just above the meristematic zone. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. Auxin is a well-characterized hormone that influences many plant developmental processes and acts as a positive regulator of root hair development (Paque and Weijers, 2016). Most auxin responses occur as a result of transcriptional and translational changes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Auxins, especially 1-naphthaleneacetic acid (NAA) and indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indole-3-acetic acid (IAA). 1.1.4 Zone of cell maturation: This is a zone that lies above the zone of elongation. In this zone the cells differentiate into different types. In the zone of maturation, cells differentiate and serve such functions as protection, storage, and conductance. Seen in cross section, the zone of maturation of many roots has an outer layer (the epidermis), a deeper level (the cortex), and a central region that includes the conducting vascular tissue. In this region a number of root hairs are also present. The root hairs are responsible for absorbing water and minerals from the soil. Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir 1.2 Root Primary Structure Recall that any plant organ has three main layers: 1. Epidermis (and derivatives) 2. Vascular tissue 3. Ground tissue 1.3 Root Anatomy: Cross Sectional View From outermost layer to innermost: Epidermis Cortex Endodermis - selectively permeable layer; unique to roots Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir Pericycle - a secondary/lateral meristem that gives rise only to side branch roots unique to roots Vascular cylinder (a.k.a. stele) 1.3.1 Epidermis It is the surface that meets the environment, and it is the first selectively permeable membrane the plant uses to filter uptake. The epidermis is composed of thin-walled cells and is usually only one cell layer thick. In plants with secondary growth, the epidermis of roots and stems is usually replaced by a periderm through the action of a cork cambium. The absorption of water and dissolved minerals occurs through the epidermis, a process greatly enhanced in most land plants by the presence of root hairs—slender, tubular extensions of the epidermal cell wall that are found only in the region of maturation. Root Physiology course/M.Sc. students/ Crop Production Department/ instructed by: Dr. Sirwa A. Qadir The absorption of water is chiefly via osmosis, which occurs because: 1. Water is present in higher concentrations in the soil than within the epidermal cells (where it contains salts, sugars, and other dissolved organic products) 2. The membrane of the epidermal cells is permeable to water but not many of the substances dissolved in the internal fluid. These conditions create an osmotic gradient, whereby water flows into the epidermal cells. This flow exerts a force, called root pressure, that helps drive the water through the roots. Root pressure is partially responsible for the rise of water in plants, but it cannot alone account for the transport of water to the top of tall trees. 1.3.2 Cortex A cortex is an outer layer of a stem or root in a plant, lying below the epidermis but outside of the vascular bundles. The cortex is composed mostly of large thin-walled parenchyma cells of the ground tissue system and shows little to no structural differentiation. The cortex typically develops into a porous tissue called Aerenchyma, which contains air spaces produced by separation and tearing of the cortex cell walls. Aerenchyma is a spongy tissue that forms spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and the root. Aerenchyma is also widespread in aquatic and wetland plants which must grow in hypoxic soils. 1.3.3 Endodermis The endodermis is developmentally the innermost portion of the cortex. It may consist of a single layer of barrel-shaped cells without any intercellular spaces or sometimes several cell layers. The cells of the endodermis typically have their primary cell walls thickened on four sides radial and transverse with suberin, a water-impermeable waxy substance which in young endodermal cells is deposited in distinctive bands called Casparian strips.
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