The Casparian strip is a band-like thickening in the center of the root endodermis (radial and tangential walls of endodermal cells) of vascular plants (Pteridophytes [1] and Spermatophytes). The composition of the region is mainly suberin, lignin and some structural proteins, which are capable of reducing the diffusive apoplastic flow of water and solutes into the stele and its width varies between species. [2] [3] The Casparian strip is impervious to water so can control the transportation of water and inorganic salts between the cortex and the vascular bundle, preventing water and inorganic salts from being transported to the stele through the apoplast, so that it must enter the cell membrane and move to the stele through the symplastic pathway, blocking the internal and external objects of the cell.[ clarification needed ] The function of mass transportation are similar to that of animal tissues.[ clarification needed ]. [4] [5] The development of the Casparian strip is regulated by transcription factors such as SHORT-ROOT (SHR), SCARECROW (SCR) and MYB36, as well as polypeptide hormone synthesised by midcolumn cells. [6] [7]
The chemistry of the Casparian strip has been described as composed of suberin. According to some studies, [8] the Casparian strip begins as a localized deposition of phenolic and unsaturated fatty substances in the middle lamella between the radial walls, as partly oxidized films. The primary wall becomes encrusted with and later thickened by deposits of similar substances on the inside of that wall. The encrustation of the cell wall by the material constituting the Casparian strip presumably plugs the pores that would have otherwise allowed the movement of water and nutrients via capillary action along that path. [9] The cytoplasm of the endodermal cell is firmly attached to the Casparian strip so that it does not readily separate from the strip when the cells are subjected to contraction of the protoplasts. At the root, the Casparian strip is embedded within the cell wall of endodermal cells in the non-growing region of the root behind the root tip. [10] Here, the Casparian strip serves as a boundary layer separating the apoplast of the cortex from the apoplast of the vascular tissue thereby blocking diffusion of material between the two. [11] This separation forces water and solutes to pass through the plasma membrane via a symplastic route in order to cross the endodermis layer. [10]
Casparian strips differentiate after an outward growth of the cortex is completed. At this level of the root development, the primary xylem of its vascular cylinder is only partly advanced. In gymnosperms and angiosperms displaying secondary growth, the roots commonly develop only endodermis with Casparian strips. In many of those, the endodermis is later discarded, together with the cortex, when the periderm develops from the pericycle. If the pericycle is superficial and the cortex is retained, either the endodermis is stretched or crushed or it keeps pace with the expansion of the vascular cylinder by radial anticlinal divisions, and the new walls develop Casparian strips in continuity with the old ones. [12]
In the absence of secondary growth (most monocotyledons and a few eudicots), the endodermis commonly undergoes wall modifications. There are two developmental stages beyond the development of the Casparian strip. In the second stage suberin (or endoderm [9] ) coats the entire wall on the inside of the cell. As a result, the Casparian strip is separated from the cytoplasm and the connection between the two ceases to be evident. In the third stage, a thick cellulose layer is deposited over the suberin, sometimes mainly on the inner tangential walls. The thickened wall, as well as the original wall in which the Casparian strip is located, may become lignified, creating a secondary cell wall. The Casparian strip may be identifiable after the thickening of the endodermal wall has occurred. The thickened endodermal wall may have pits. The successive development of endodermal walls is clearly expressed in monocotyledons. [13] [14]
The discovery of the Casparian strip dates back to the mid-19th century, and advances in the understanding of the endodermis of plant roots. [15] In 1865, the German botanist Robert Caspary first described the endodermis of the root of plants, found that its cell wall was thickened, and named it Schuchtzscheide. Later scholars called the thickened part of it the Carls Belt, which was named after Casbury[ clarification needed ]. [5] [16] The term "Caspary'schen fleck" (German : Caspary'schen fleck) appeared in the 1870s literature, [17] [18] and after the 20th century, it was often called the Casparian strip. In 1922, researchers first left the Casparian strip from the root of plants to study its composition.[ clarification needed ] [19] [20]
The chemical composition of the Casparian strip has been controversial for a long time. Casbury pointed out that this structure may be composed of lignin or suberin. Later scholars mostly thought it was suberin. [21] It was not until the 1990s that after analyzing the Casparian strip of several plants, it was found that lignin was the main component, but many textbooks have not been updated. [4] Although the cell wall of the endothelium is rich in woodbolic, this is the result of the sublevel differentiation of the endothelium. [note 1] In the past, some scholars believe that the formation of the endodermis of Casparian strip is the beginning of sublevel differentiation, but there is no direct relationship between the two. The casparian strip has formed after the primary differentiation, and the secondary differentiation begins with the slash cut of the root, not where the Casparian strip is. [1]
The casparian strip is fully filled with the gap between endothelial cells, including the middle lamella, making the cell walls of the two cells almost fused. [1] In the transportation of water and inorganic nutrients at the root of plants, the Casparian strip mainly affects the transportation of primary in vitro, that is, the transportation of water and inorganic salts through the interstitial cells of the epidermis and cortex cells. When water and inorganic salt come to the endothelial cells, they need to enter the cell through the cell membrane because the casparian strip is not water-permeable, and then transported by the protoplasmic inner path to reach the lignan cells of the stele, and then to other organs such as the stems and leaves. [16] When the growth environment is unfavourable, the casparian strip can act as a barrier between plant cells and the outside world, avoiding the entry of ions or outflow of their own ions in the environment. [7] In addition, the thickening of the carcass belt and the cortex also prevents toxic substances or pathogen invasion, as well as the function of preventing water dispersion. [22] Some studies have shown that plants may form thicker Casparian strip in high-salt environments, and in areas closer to the tip of the roots, which may be an adaptation to the environment, [23] [24] but compared with the endothelial sublevel differentiated wooden bolt walls, which are significantly thickened in high-salt adversity, the Casparian strip changes is smaller. [25]
The Casparian strip is mainly located in the endodermis of the root, [26] but some plants also have the Casparian strip in the outer cortex on the outer side of the root cortex, stem or leaf. [27] For example, the conifers of Pinus bungeana and the stems of Pelargonium have the Casparian strip, which may be related to preventing water dispersion or pathogenic invasion. [28] [29]
The development of the Casparian strip is started after the endogenic cells are fully delayed, [21] [30] and there is currently two news signal transduction that promote endothelial cell formation of Casparian strip. The first is transcription factor Short-root (SHR) Activated additional two transcription factors Myb36 and Scarecrow (SCR), the former can stimulate Casparian Strip Proteins (Casp1-5), Peroxidase (PER64) and ESB1 (Enhanced) Suberin 1), etc., the latter affects the position of the Casparian strip in the inner skin cell, which causes the position of the Casparian strip to be too close to the Stele; [6] the second is medium Casparian Strip Integrity Factor (CIF1-2) and the GSO1 (SGN3) and GSO2 receptor bonded to the endothelial cell radial wall and the GSO2 receptor in the lateral wall. CASP in the cells is concentrated to a cell membrane region corresponding to the position of the Casparian strip, forming a Casparian Strip Membrane Domain (CSD), and the CSD is incorporated in the region. The GS01 receptor is surrounded by the edge of each CSD region, promoting CSD fused into a continuous strip region, that is, the region where the Casparian strip is to be formed. [7] [31]
Casparian strip protein is a membrane protein that interacts with each other and can bind to proteins needed to synthesize lignin such as PER64, ESB1 and respiratory oxidase homologer F (RBOHF) to activate the downstream reaction of Casparian strip development. [1] [5] In mutant plants lacking GSO1 receptors or at the same time lacking CIF1 and CIF2 polypeptides, CASP1 is abnormally distributed on the endothelial cell membrane, and the CSD cannot normally fuse into a continuous and complete band structure, thus eventually forming a broken and discontinuous Casparian strip. [7] [31]
Environmental factors such as light, soil salinity and water deficit can affect the development of the Casparian strip. [28]
A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier. Another vital role of the cell wall is to help the cell withstand osmotic pressure and mechanical stress. While absent in many eukaryotes, including animals, cell walls are prevalent in other organisms such as fungi, algae and plants, and are commonly found in most prokaryotes, with the exception of mollicute bacteria.
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of the xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word xylem is derived from the Ancient Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout a plant. The term was introduced by Carl Nägeli in 1858.
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.
Phloem is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to the rest of the plant. This transport process is called translocation. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Ancient Greek word φλοιός (phloiós), meaning "bark". The term was introduced by Carl Nägeli in 1858. Different types of phloem can be distinguished. The early phloem formed in the growth apices is called protophloem. Protophloem eventually becomes obliterated once it connects to the durable phloem in mature organs, the metaphloem. Further, secondary phloem is formed during the thickening of stem structures.
Vascular plants, also called tracheophytes or collectively tracheophyta, form a large group of land plants that have lignified tissues for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms, and angiosperms. Scientific names for the group include Tracheophyta, Tracheobionta and Equisetopsida sensu lato. Some early land plants had less developed vascular tissue; the term eutracheophyte has been used for all other vascular plants, including all living ones.
Bark is the outermost layer of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost layer of the periderm. The outer bark on older stems includes the dead tissue on the surface of the stems, along with parts of the outermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the living periderm is also called the rhytidome.
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.
Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.
The endodermis is the innermost layer of cortex in land plants. It is a cylinder of compact living cells, the radial walls of which are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside. The endodermis is the boundary between the cortex and the stele.
Suberin, cutin and lignins are complex, higher plant epidermis and periderm cell-wall macromolecules, forming a protective barrier. Suberin, a complex polyester biopolymer, is lipophilic, and composed of long chain fatty acids called suberin acids, and glycerol. Suberins and lignins are considered covalently linked to lipids and carbohydrates, respectively, and lignin is covalently linked to suberin, and to a lesser extent, to cutin. Suberin is a major constituent of cork, and is named after the cork oak, Quercus suber. Its main function is as a barrier to movement of water and solutes.
The apoplast is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extracellular space are called apoplastic flow or apoplastic transport. The apoplastic pathway is one route by which water and solutes are transported and distributed to different places through tissues and organs, contrasting with the symplastic pathway.
The symplast of a plant is the region enclosed by the cell membranes, within which water and solutes can diffuse freely. By contrast the apoplast is any fluid-filled space within the cell wall and extracellular space. Neighbouring cells are interconnected by microscopic channels known as plasmodesmata that traverse the cell walls. These channels, allow the flow of small molecules such as sugars, amino acids, and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures. The symplast allows direct cytoplasm-to-cytoplasm flow of water and other nutrients along concentration gradients. In particular, symplastic flow is used in the root systems to bring in nutrients from soil. Nutrient solutes move in this way through three skin layers of the roots: from cells of the epidermis, the outermost layer, through the cortex into the endodermis.
The pericycle is a cylinder of parenchyma or sclerenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.
In botany, a cortex is an outer layer of a stem or root in a vascular 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 outer cortical cells often acquire irregularly thickened cell walls, and are called collenchyma cells.
The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. This tissue system is present between the dermal tissue and forms the main bulk of the plant body.
The exodermis is a physiological barrier that has a role in root function and protection. The exodermis is a membrane of variable permeability responsible for the radial flow of water, ions, and nutrients. It is the outer layer of a plant's cortex. The exodermis serves a double function as it can protect the root from invasion by foreign pathogens and ensures that the plant does not lose too much water through diffusion through the root system and can properly replenish its stores at an appropriate rate.
Johann Xaver Robert Caspary was a German botanist.
Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.
Niko Geldner is a German-Swiss biologist specialised in the study of Plant Cell and Developmental Biology. He is a full professor and the director of the plant cell biology laboratory at the University of Lausanne.
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