Endodermis

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Ranunculus Root Cross Section

The endodermis is the central, 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 (Casparian strip) to restrict apoplastic flow of water to the inside. [1] The endodermis is the boundary between the cortex and the stele.

Contents

In many seedless vascular plants, the endodermis is a distinctly visible layer of cells immediately outside the vascular cylinder (stele) in roots and shoots. In most seed plants, especially woody types, an endodermis is absent from the stems but is present in roots.

The endodermis helps regulate the movement of water, ions and hormones into and out of the vascular system. It may also store starch, be involved in perception of gravity and protect the plant against toxins moving into the vascular system.

Structure

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. These strips vary in width but are typically smaller than the cell wall on which they are deposited. If the endodermis is likened to a brick cylinder (e.g. a smokestack), with the bricks representing individual cells, the Casparian strips are analogous to the mortar between the bricks. In older endodermal cells, suberin may be more extensively deposited on all cell wall surfaces and the cells can become lignified, forming a complete waterproof layer.

Some plants have a large number of amyloplasts (starch containing organelles) in their endodermal cells, in which case the endodermis may be called a starch sheath.

Endodermis is often made visible with stains like phloroglucinol due to the phenolic and lipid nature of the Casparian strips or by the abundance of amyloplasts.

Function

The endodermis prevents water, and any solutes dissolved in the water, from passing through this layer via the apoplast pathway. Water can only pass through the endodermis by crossing the membrane of endodermal cells twice (once to enter and a second time to exit). Water moving into or out of the xylem, which is part of the apoplast, can thereby be regulated since it must enter the symplast in the endodermis. This allows the plant to control to some degree the movement of water and to selectively uptake or prevent the passage of ions or other molecules.

The endodermis does not allow gas bubbles to enter the xylem and helps prevent embolisms from occurring in the water column. [2]

Passage cells are endodermal cells of older roots which have retained thin walls and Casparian strips rather than becoming suberized and waterproof like the other cells around them, to continue to allow some symplastic flow to the inside. Experimental evidence suggests that passage cells function to allow transfer of solutes such as calcium and magnesium into the stele, in order to eventually reach the transpiration system. [3] For the most part, however, old roots seal themselves off at the endodermis, and only serve as a passageway for water and minerals taken up by younger roots "downstream".

Endodermal cells may contain starch granules in the form of amyloplasts. These may serve as food storage, and have been shown to be involved in gravitropism in some plants. [4]

See also

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Plant cell Type of eukaryotic cell present in green plants

Plant cells are eukaryotic 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 Water transport tissue in vascular plants

Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of 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.

Root Part of a plant

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.

Bark (botany) Outermost layers of stems and roots of woody plants

Bark is the outermost layers 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.

Root pressure

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.

Amyloplast Type of plastid, double-enveloped organelles in plant cells

Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of leucoplast, a subcategory for colorless, non-pigment-containing plastids. Amyloplasts are found in roots and storage tissues and store and synthesize starch for the plant through the polymerization of glucose. Starch synthesis relies on the transportation of carbon from the cytosol, the mechanism by which is currently under debate.

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 (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis, which is the innermost cell layer of the cortex.

Suberin Hydrophobic lipid polyester in plant cell walls

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.

Casparian strip Thickening in the root endodermis of vascular plants

The Casparian strip is a band-like thickening in the center of the root endodermis of vascular plants. The composition of the region is mainly lignin, and its width varies between species. 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. The function and function of mass transportation are similar to that of animal tissues.. 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.

Apoplast Extracellular space, outside the plasma membrane of plants

Inside a plant, the apoplast is the space outside the plasma membrane within which material can diffuse freely. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.

Symplast

The symplast of a plant is the inner side of the plasma membrane in which water and low-molecular-weight solutes can freely diffuse. Symplast cells have more than one nucleus.

Gravitropism

Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

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.

Cortex (botany)

A cortex is an outer layer of a stem or root in a plant, lying below the epidermis but outside of the vascular bundles. It is also the surface layer or "skin" of the nonfruiting part of the body of some lichens.

Ground tissue Various non-vascular tissues in plants

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.

  1. Parenchyma cells have thin primary walls and usually remain alive after they become mature. Parenchyma forms the "filler" tissue in the soft parts of plants, and is usually present in cortex, pericycle, pith, and medullary rays in primary stem and root.
  2. Collenchyma cells have thin primary walls with some areas of secondary thickening. Collenchyma provides extra mechanical and structural support, particularly in regions of new growth.
  3. Sclerenchyma cells have thick lignified secondary walls and often die when mature. Sclerenchyma provides the main structural support to a plant.
Transpiration stream

In plants, the transpiration stream is the uninterrupted stream of water and solutes which is taken up by the roots and transported via the xylem to the leaves where it evaporates into the air/apoplast-interface of the substomatal cavity. It is driven by capillary action and in some plants by root pressure. The main driving factor is the difference in water potential between the soil and the substomatal cavity caused by transpiration.

Plant stem Structural axis of a vascular plant

A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, stores nutrients, and produces new living tissue.

Exodermis Part of a plant

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.

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.

References

  1. ↑ Rost, Thomas L.; Michael Barbour; C. Ralph Stocking; Terence Murphy (2006). Plant Biology, 2nd Edition. Thompson, Brooks/Cole. ISBN   978-0-534-38061-8.
  2. ↑ Taiz and Zeiger (2015). Plant Physiology and Development. Sunderland, Massachusetts U.S.A: Sinauer Associates, Inc. p. 109. ISBN   978-1-60535-353-1.
  3. ↑ Peterson. C. A.; Enstone, D. E. (1996). "Functions of passage cells in the endodermis and exodermis of root". Physiologia Plantarum. 97 (3): 592–598. doi:10.1111/j.1399-3054.1996.tb00520.x.
  4. ↑ Involvement of the Vacuoles of the Endodermis in the Early Process of Shoot Gravitropism in Arabidopsis, Miyo Terao Morita1, Takehide Kato1, Kiyoshi Nagafusaa, Chieko Saitoc, Takashi Uedac, Akihiko Nakanoc and Masao Tasaka, 2002. The Plant Cell 14:47-56
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