Casparian strip
Encyclopedia
In plant anatomy, the Casparian strip is a band of cell wall
material deposited on the radial and transverse walls of the endodermis
, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele
of a plant. The band was first recognized as a wall structure by Robert Caspary (1818–1887).
The strip forms during the early ontogeny
of the cell
and is a part of the primary wall. It varies in width and is often much narrower than the wall in which it occurs. It is typically located closer to the inner tangential wall than the outer.
The chemistry of the Casparian strip has been variously described as composed of lignin
or suberin
or both. According to some studies, the Casparian strip is initiated as a localized deposition of phenolic
and unsaturated fat
ty 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 the wall. The encrustation of the cell wall by the material constituting the Casparian strip presumably blocks the submicroscopic
capillaries
in the wall. 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 the effects of plasmolytic or other agents normally causing a contraction of protoplast
s. Thus, the Casparian strip appears to form a barrier at which the apoplastic flow is forced to pass through the selectively permeable plasma membrane into the cytoplasm (the symplast).
Casparian strips differentiate after the centripetal growth of the cortex
is completed. At this level of the root, primary xylem
development in the vascular cylinder may be more or less advanced. In gymnosperm
s and angiosperms having secondary growth, the roots commonly develop no other kind of endodermis than that with casparian strips. In many of these plants 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, the new walls developing Casparian strips in continuity with the old ones.
In the absence of secondary growth (most monocotyledon
s and a few eudicots), the endodermis commonly undergoes certain wall modifications. Workers distinguish two developmental stages in addition to the first stage when only the Casparian strip is present. In the second stage a suberin (or endodermin) lamella covers the entire wall on the inside of the cell, so 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 lamella, sometimes mainly on the inner tangential walls. The thick wall, as well as the original wall in which the Casparian strip is located, may become lignified. The Casparian strip may or may not be identifiable after the thickening of the endodermal wall has occurred. The thick endodermal wall, a secondary cell wall, may have pits. The successive development of endodermal walls is clearly expressed in monocotyledons. In dicotyledons, the distinction between the second and third stages of endodermal development may not be sharp (Guttenberg, 1943), and in the seedless vascular plant
s the differentiation is terminated with the deposition of the suberin lamella. An endodermis with Casparian strips and later wall modifications occurs in aerial roots).
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
material deposited on the radial and transverse walls of the endodermis
Endodermis
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...
, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele
Stele (biology)
In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue and a pericycle, which, if present, defines the outermost boundary of the stele...
of a plant. The band was first recognized as a wall structure by Robert Caspary (1818–1887).
The strip forms during the early ontogeny
Ontogeny
Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...
of the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
and is a part of the primary wall. It varies in width and is often much narrower than the wall in which it occurs. It is typically located closer to the inner tangential wall than the outer.
The chemistry of the Casparian strip has been variously described as composed of lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
or suberin
Suberin
Suberin is a waxy substance found in higher plants. Suberin is a main constituent of cork, and is named after the Cork Oak, Quercus suber.-Anatomy and physiology:...
or both. According to some studies, the Casparian strip is initiated as a localized deposition of phenolic
Natural phenol
Natural phenols, bioavailable phenols, plant phenolics, low molecular weight phenols or phenoloids are a class of natural products. They are small molecules containing one or more phenolic group. These molecules are smaller in size than polyphenols, containing less than 12 phenolic groups...
and unsaturated fat
Unsaturated fat
An unsaturated fat is a fat or fatty acid in which there is at least one double bond within the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond. Where double bonds are formed, hydrogen atoms are...
ty 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 the wall. The encrustation of the cell wall by the material constituting the Casparian strip presumably blocks the submicroscopic
Submicroscopic
Submicroscopic is an English adjective used to describe particles of matter that cannot be seen under the most powerful optical microscope available. Atoms are examples of such submicroscopic particles....
capillaries
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
in the wall. The cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
of the endodermal
Endodermis
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances to restrict apoplastic flow of water to the inside...
cell is firmly attached to the Casparian strip so that it does not readily separate from the strip when the cells are subjected to the effects of plasmolytic or other agents normally causing a contraction of protoplast
Protoplast
Protoplast, from the ancient Greek πρῶτον + verb πλάθω or πλάττω , initially referred to the first organized body of a species.Protoplast has several biological definitions:...
s. Thus, the Casparian strip appears to form a barrier at which the apoplastic flow is forced to pass through the selectively permeable plasma membrane into the cytoplasm (the symplast).
Casparian strips differentiate after the centripetal growth of the cortex
Cortex (botany)
In botany, the cortex is the outer layer of the stem or root of a plant, bounded on the outside by the epidermis and on the inside by the endodermis. It is composed mostly of undifferentiated cells, usually large thin-walled parenchyma cells of the ground tissue system. The outer cortical cells...
is completed. At this level of the root, primary xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
development in the vascular cylinder may be more or less advanced. In gymnosperm
Gymnosperm
The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds...
s and angiosperms having secondary growth, the roots commonly develop no other kind of endodermis than that with casparian strips. In many of these plants the endodermis is later discarded, together with the cortex, when the periderm develops from the pericycle
Pericycle
The pericycle is a cylinder of parenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.Although it is composed of non-vascular parenchyma cells, it is still considered part of the vascular cylinder because it arises from the procambium as do the...
. 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, the new walls developing Casparian strips in continuity with the old ones.
In the absence of secondary growth (most monocotyledon
Monocotyledon
Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon , in contrast to the two cotyledons typical of dicots...
s and a few eudicots), the endodermis commonly undergoes certain wall modifications. Workers distinguish two developmental stages in addition to the first stage when only the Casparian strip is present. In the second stage a suberin (or endodermin) lamella covers the entire wall on the inside of the cell, so 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
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
layer is deposited over the suberin lamella, sometimes mainly on the inner tangential walls. The thick wall, as well as the original wall in which the Casparian strip is located, may become lignified. The Casparian strip may or may not be identifiable after the thickening of the endodermal wall has occurred. The thick endodermal wall, a secondary cell wall, may have pits. The successive development of endodermal walls is clearly expressed in monocotyledons. In dicotyledons, the distinction between the second and third stages of endodermal development may not be sharp (Guttenberg, 1943), and in the seedless vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s the differentiation is terminated with the deposition of the suberin lamella. An endodermis with Casparian strips and later wall modifications occurs in aerial roots).