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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.


Function

The major functions of roots are absorption of water, plant nutrition and anchoring of the plant body to the ground.


Anatomy

Root morphology is divided into four zones: the root cap, the apical meristem, the elongation zone, and the hair. The root cap of new roots helps the root penetrate the soil. These root caps are sloughed off as the root goes deeper creating a slimy surface that provides lubrication. The apical meristem behind the root cap produces new root cells that elongate. Then, root hairs form that absorb water and mineral nutrients from the soil. The first root in seed producing plants is the radicle, which expands from the plant embryo after seed germination. When dissected, the arrangement of the cells in a root is root hair,
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
,
epiblem In botany, epiblem is a tissue that replaces the epidermis in most roots and in stems of submerged aquatic plants. It is usually located between the epidermis and cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermos ...
, cortex, endodermis, pericycle and, lastly, the vascular tissue in the centre of a root to transport the water absorbed by the root to other places of the plant. Perhaps the most striking characteristic of roots that distinguishes them from other plant organs such as stem-branches and leaves is that roots have an ''endogenous'' origin, ''i.e.'', they originate and develop from an inner layer of the mother axis, such as pericycle. In contrast, stem-branches and leaves are ''exogenous'', ''i.e.'', they start to develop from the cortex, an outer layer. In response to the concentration of nutrients, roots also synthesise cytokinin, which acts as a signal as to how fast the shoots can grow. Roots often function in storage of food and nutrients. The roots of most vascular plant species enter into symbiosis with certain fungi to form
mycorrhiza   A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
e, and a large range of other organisms including bacteria also closely associate with roots.


Root system architecture (RSA)


Definition

In its simplest form, the term root system architecture (RSA) refers to the spatial configuration of a plant's root system. This system can be extremely complex and is dependent upon multiple factors such as the species of the plant itself, the composition of the soil and the availability of nutrients. Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support. The configuration of root systems serves to structurally support the plant, compete with other plants and for uptake of nutrients from the soil. Roots grow to specific conditions, which, if changed, can impede a plant's growth. For example, a root system that has developed in dry soil may not be as efficient in flooded soil, yet plants are able to adapt to other changes in the environment, such as seasonal changes.


Terms and components

The main terms used to classify the architecture of a root system are: All components of the root architecture are regulated through a complex interaction between genetic responses and responses due to environmental stimuli. These developmental stimuli are categorised as intrinsic, the genetic and nutritional influences, or extrinsic, the environmental influences and are interpreted by
signal transduction pathways Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...
. Extrinsic factors affecting root architecture include gravity, light exposure, water and oxygen, as well as the availability or lack of nitrogen, phosphorus, sulphur, aluminium and sodium chloride. The main hormones (intrinsic stimuli) and respective pathways responsible for root architecture development include:


Growth

Early root growth is one of the functions of the apical meristem located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these are sacrificed to protect the meristem), and undifferentiated root cells. The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues. Growth from apical meristems is known as primary growth, which encompasses all elongation. Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of
sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant that belongs to the Convolvulus, bindweed or morning glory family (biology), family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a r ...
have secondary growth but are not woody. Secondary growth occurs at the
lateral meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s, namely the vascular cambium and cork cambium. The former forms
secondary xylem 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 ...
and
secondary phloem 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 c ...
, while the latter forms the periderm. In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the
stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...
and root. The vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem including the epidermis and cortex, in many cases tend to be pushed outward and are eventually "sloughed off" (shed). At this point, the cork cambium begins to form the periderm, consisting of protective
cork Cork or CORK may refer to: Materials * Cork (material), an impermeable buoyant plant product ** Cork (plug), a cylindrical or conical object used to seal a container ***Wine cork Places Ireland * Cork (city) ** Metropolitan Cork, also known as G ...
cells. The walls of cork cells contains suberin thickenings, which is an extra cellular complex biopolymer. The suberin thickenings functions by providing a physical barrier, protection against pathogens and by preventing water loss from the surrounding tissues. In addition, it also aids the process of wound healing in plants. It is also postulated that suberin could be a component of the apoplastic barrier (present at the outer cell layers of roots) which prevents toxic compounds from entering the root and reduces radial oxygen loss (ROL) from the aerenchyma during waterlogging. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder. The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity (in some) but are responsible for most water transport through the vascular tissue in stems and roots. Tree roots usually grow to three times the diameter of the branch spread, only half of which lie underneath the trunk and canopy. The roots from one side of a tree usually supply nutrients to the foliage on the same side. Some families however, such as
Sapindaceae The Sapindaceae are a family of flowering plants in the order Sapindales known as the soapberry family. It contains 138 genera and 1858 accepted species. Examples include horse chestnut, maples, ackee and lychee. The Sapindaceae occur in tempera ...
(the maple family), show no correlation between root location and where the root supplies nutrients on the plant.


Regulation

There is a correlation of roots using the process of plant perception to sense their physical environment to grow, including the sensing of light, and physical barriers. Plants also sense gravity and respond through auxin pathways, resulting in gravitropism. Over time, roots can crack foundations, snap water lines, and lift sidewalks. Research has shown that roots have ability to recognize 'self' and 'non-self' roots in same soil environment. The correct environment of air, mineral nutrients and water directs plant roots to grow in any direction to meet the plant's needs. Roots will shy or shrink away from dry or other poor soil conditions. Gravitropism directs roots to grow downward at
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
, the growth mechanism of plants that also causes the shoot to grow upward. Different types of roots such as primary, seminal, lateral and crown are maintained at different gravitropic setpoint angles i.e. the direction in which they grow. Recent research show that root angle in cereal crops such as barley and wheat is regulated by a novel gene called Enhanced Gravitropism 1 (EGT1). Research indicates that plant roots growing in search of productive nutrition can sense and avoid soil compaction through diffusion of the gas
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
.


Shade avoidance response

In order to avoid shade, plants utilize a shade avoidance response. When a plant is under dense vegetation, the presence of other vegetation nearby will cause the plant to avoid lateral growth and experience an increase in upward shoot, as well as downward root growth. In order to escape shade, plants adjust their root architecture, most notably by decreasing the length and amount of lateral roots emerging from the primary root. Experimentation of mutant variants of
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
found that plants sense the Red to Far Red light ratio that enters the plant through photoreceptors known as phytochromes. Nearby plant leaves will absorb red light and reflect far- red light which will cause the ratio red to far red light to lower. The phytochrome PhyA that senses this Red to Far Red light ratio is localized in both the root system as well as the shoot system of plants, but through knockout mutant experimentation, it was found that root localized PhyA does not sense the light ratio, whether directly or axially, that leads to changes in the lateral root architecture. Research instead found that shoot localized PhyA is the phytochrome responsible for causing these architectural changes of the lateral root. Research has also found that phytochrome completes these architectural changes through the manipulation of auxin distribution in the root of the plant. When a low enough Red to Far Red ratio is sensed by PhyA, the phyA in the shoot will be mostly in its active form. In this form, PhyA stabilize the transcription factor HY5 causing it to no longer be degraded as it is when phyA is in its inactive form. This stabilized transcription factor is then able to be transported to the roots of the plant through the phloem, where it proceeds to induce its own transcription as a way to amplify its signal. In the roots of the plant HY5 functions to inhibit an auxin response factor known as ARF19, a response factor responsible for the translation of PIN3 and LAX3, two well known auxin transporting proteins. Thus, through manipulation of ARF19, the level and activity of
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...
transporters PIN3 and LAX3 is inhibited. Once inhibited, auxin levels will be low in areas where lateral root emergence normally occurs, resulting in a failure for the plant to have the emergence of the lateral root primordium through the root pericycle. With this complex manipulation of Auxin transport in the roots, lateral root emergence will be inhibited in the roots and the root will instead elongate downwards, promoting vertical plant growth in an attempt to avoid shade. Research of Arabidopsis has led to the discovery of how this auxin mediated root response works. In an attempt to discover the role that phytochrome plays in lateral root development, Salisbury et al. (2007) worked with ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
'' grown on agar plates. Salisbury et al. used wild type plants along with varying protein knockout and gene knockout Arabidopsis mutants to observe the results these mutations had on the root architecture, protein presence, and gene expression. To do this, Salisbury et al. used GFP fluorescence along with other forms of both macro and microscopic imagery to observe any changes various mutations caused. From these research, Salisbury et al. were able to theorize that shoot located phytochromes alter auxin levels in roots, controlling lateral root development and overall root architecture. In the experiments of van Gelderen et al. (2018), they wanted to see if and how it is that the shoot of ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
'' alters and affects root development and root architecture. To do this, they took Arabidopsis plants, grew them in
agar gel Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
, and exposed the roots and shoots to separate sources of light. From here, they altered the different wavelengths of light the shoot and root of the plants were receiving and recorded the lateral root density, amount of lateral roots, and the general architecture of the lateral roots. To identify the function of specific photoreceptors, proteins, genes, and hormones, they utilized various Arabidopsis knockout mutants and observed the resulting changes in lateral roots architecture. Through their observations and various experiments, van Gelderen et al. were able to develop a mechanism for how root detection of Red to Far-red light ratios alter lateral root development.


Types

A true root system consists of a primary root and secondary roots (or lateral roots). * the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common in monocots. The main function of the fibrous root is to anchor the plant.


Specialized

The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions, described in the introduction. * Adventitious roots arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in monocots and pteridophytes, but also in many
dicot The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...
s, such as clover (''Trifolium''), ivy (''Hedera''),
strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown hybrid species of the genus '' Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The fruit is widely ap ...
(''Fragaria'') and willow (''Salix''). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system. * Aerating roots (or knee root or knee or pneumatophores): roots rising above the ground, especially above water such as in some mangrove genera ('' Avicennia, Sonneratia''). In some plants like ''Avicennia'' the erect roots have a large number of breathing pores for exchange of gases. * Aerial roots: roots entirely above the ground, such as in ivy (''Hedera'') or in
epiphytic An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...
orchids. Many aerial roots are used to receive water and nutrient intake directly from the air – from fogs, dew or humidity in the air. Some rely on leaf systems to gather rain or humidity and even store it in scales or pockets. Other aerial roots, such as mangrove aerial roots, are used for aeration and not for water absorption. Other aerial roots are used mainly for structure, functioning as prop roots, as in maize or anchor roots or as the trunk in
strangler fig Strangler fig is the common name for a number of tropical and subtropical plant species in the genus ''Ficus'', including those that are commonly known as banyans. Some of the more well-known species are: * ''Ficus altissima'' * ''Ficus aurea'', ...
. In some Epiphytes – plants living above the surface on other plants, aerial roots serve for reaching to water sources or reaching the surface, and then functioning as regular surface roots. *
Canopy root A canopy root, also known as an arboreal root, is a type of root that grows out of a tree branch underneath an epiphytic mat. These adventitious roots form in response to moist, dark, nutrient-rich conditions that are found in “canopy soils”. C ...
s/arboreal roots: roots that form when tree branches support mats of epiphytes and detritus, which hold water and nutrients in the canopy. They grow out into these mats, likely to utilize the available nutrients and moisture. * Contractile roots: roots that pull bulbs or corms of monocots, such as
hyacinth Hyacinth or Hyacinthus may refer to: Nature Plants * Hyacinth (plant), genus ''Hyacinthus'' ** ''Hyacinthus orientalis'', common hyacinth * Grape hyacinth, ''Muscari'', a genus of perennial bulbous plants native to Eurasia * Hyacinth bean, ''Labl ...
and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface. * Coarse roots: roots that have undergone secondary thickening and have a woody structure. These roots have some ability to absorb water and nutrients, but their main function is transport and to provide a structure to connect the smaller diameter, fine roots to the rest of the plant. *
Dimorphic root system A dimorphic root system is a plant root system with two distinct root forms, which are adapted to perform different functions. One of the most common manifestations is in plants with both a taproot, which grows straight down to the water table, from ...
s: roots with two distinctive forms for two separate functions *
Fine root A fine root is most commonly defined as a plant root that is two millimeters or less in diameter. Fine roots may function in acquisition of soil resources (eg. nutrients, water) and/or resource transport, making them functionally most analogous to t ...
s: typically primary roots <2 mm diameter that have the function of water and nutrient uptake. They are often heavily branched and support mycorrhizas. These roots may be short lived, but are replaced by the plant in an ongoing process of root 'turnover'. * Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in
mistletoe Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. ...
(''Viscum album'') and dodder. * Propagative roots: roots that form adventitious buds that develop into aboveground shoots, termed suckers, which form new plants, as in Canada thistle,
cherry A cherry is the fruit of many plants of the genus ''Prunus'', and is a fleshy drupe (stone fruit). Commercial cherries are obtained from cultivars of several species, such as the sweet ''Prunus avium'' and the sour ''Prunus cerasus''. The nam ...
and many others. * Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in
Proteaceae The Proteaceae form a family of flowering plants predominantly distributed in the Southern Hemisphere. The family comprises 83 genera with about 1,660 known species. Together with the Platanaceae and Nelumbonaceae, they make up the order Pro ...
and some plants from the following families Betulaceae, Casuarinaceae, Elaeagnaceae, Moraceae,
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...
and Myricaceae. * Stilt roots: adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil. * Storage roots: roots modified for storage of food or water, such as
carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', nat ...
s and beets. They include some taproots and tuberous roots. * Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees. * Surface roots: roots that proliferate close below the soil surface, exploiting water and easily available nutrients. Where conditions are close to optimum in the surface layers of soil, the growth of surface roots is encouraged and they commonly become the dominant roots. * Tuberous roots: fleshy and enlarged lateral roots for food or water storage, e.g.
sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant that belongs to the Convolvulus, bindweed or morning glory family (biology), family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a r ...
. A type of storage root distinct from taproot. *Photosynthetic roots: roots that are green and photosynthesize, providing sugar to the plant. They are similar to phylloclades. Several orchids have these, such as ''
Dendrophylax ''Dendrophylax'' is a genus of leafless neotropical orchids (family Orchidaceae) native to Mexico, Central America, the West Indies, and Florida. The name is from Greek δένδρον ("tree") and φύλαξ ("guard; keeper"). One species, ''De ...
'' and '' Taeniophyllum''. * Root nodules: roots that harbor nitrogen-fixing soil bacteria. These are often very short and rounded. Root nodules are found in virtually all
legumes A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock fo ...
. *Coralloid roots: similar to root nodules, these provide nitrogen to the plant. They are often larger than nodules, branched, and located at or near the soil surface, and harbor nitrogen-fixing
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
. They are only found in cycads.


Depths

The distribution of vascular plant roots within soil depends on plant form, the spatial and temporal availability of water and nutrients, and the physical properties of the soil. The deepest roots are generally found in deserts and temperate coniferous forests; the shallowest in tundra, boreal forest and temperate grasslands. The deepest observed living root, at least 60 metres below the ground surface, was observed during the excavation of an open-pit mine in Arizona, USA. Some roots can grow as deep as the tree is high. The majority of roots on most plants are however found relatively close to the surface where nutrient availability and aeration are more favourable for growth. Rooting depth may be physically restricted by rock or compacted soil close below the surface, or by anaerobic soil conditions.


Records


Evolutionary history

The fossil record of roots—or rather, infilled voids where roots rotted after death—spans back to the late
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, about 430 million years ago. Their identification is difficult, because casts and molds of roots are so similar in appearance to animal burrows. They can be discriminated using a range of features. The evolutionary development of roots likely happened from the modification of shallow
rhizomes In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
(modified horizontal stems) which anchored primitive vascular plants combined with the development of filamentous outgrowths (called rhizoids) which anchored the plants and conducted water to the plant from the soil.


Environmental interactions

Light has been shown to have some impact on roots, but its not been studied as much as the effect of light on other plant systems. Early research in the 1930s found that light decreased the effectiveness of Indole-3-acetic acid on adventitious root initiation. Studies of the pea in the 1950s shows that lateral root formation was inhibited by light, and in the early 1960s researchers found that light could induce positive
gravitropic Gravitropism (also known as geotropism) 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 featu ...
responses in some situations. The effects of light on root elongation has been studied for monocotyledonous and dicotyledonous plants, with the majority of studies finding that light inhibited root elongation, whether pulsed or continuous. Studies of ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model organi ...
'' in the 1990s showed negative
phototropism Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...
and inhibition of the elongation of root hairs in light sensed by
phyB Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-red ...
. Certain plants, namely
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...
, form
root nodules Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known a ...
in order to associate and form a symbiotic relationship with nitrogen-fixing bacteria called
rhizobia Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In gene ...
. Owing to the high energy required to fix nitrogen from the atmosphere, the bacteria take carbon compounds from the plant to fuel the process. In return, the plant takes nitrogen compounds produced from ammonia by the bacteria. Soil temperature is a factor that effects
root initiation 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 sur ...
and length. Root length is usually impacted more dramatically by temperature than overall mass, where cooler temperatures tend to cause more lateral growth because downward extension is limited by cooler temperatures at subsoil levels. Needs vary by plant species, but in temperate regions cool temperatures may limit root systems. Cool temperature species like oats, rapeseed,
rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is a member of the wheat tribe (Triticeae) and is closely related to both wheat (''Triticum'') and barley (genus ''Hordeum''). Rye grain is u ...
, wheat fare better in lower temperatures than summer annuals like maize and cotton. Researchers have found that plants like cotton develop wider and shorter taproots in cooler temperatures. The first root originating from the seed usually has a wider diameter than root branches, so smaller root diameters are expected if temperatures increase root initiation. Root diameter also decreases when the root elongates.Encyclopedia of Soil Science
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Plant interactions

Plants can interact with one another in their environment through their root systems. Studies have demonstrated that plant-plant interaction occurs among root systems via the soil as a medium. Researchers have tested whether plants growing in ambient conditions would change their behavior if a nearby plant was exposed to drought conditions. Since nearby plants showed no changes in
stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...
tal aperture researchers believe the drought signal spread through the roots and soil, not through the air as a volatile chemical signal.


Soil interactions

Soil microbiota can suppress both disease and beneficial root symbionts (mycorrhizal fungi are easier to establish in sterile soil). Inoculation with soil bacteria can increase internode extension, yield and quicken flowering. The migration of bacteria along the root varies with natural soil conditions. For example, research has found that the root systems of wheat seeds inoculated with '' Azotobacter'' showed higher populations in soils favorable to Azotobacter growth. Some studies have been unsuccessful in increasing the levels of certain microbes (such as ''
P. fluorescens ''Pseudomonas fluorescens'' is a common Gram-negative, rod-shaped bacterium. It belongs to the ''Pseudomonas'' genus; 16S ribosomal RNA, 16S rRNA analysis as well as phylogenomic analysis has placed ''P. fluorescens'' in the ''P. fluorescens'' gr ...
'') in natural soil without prior sterilization. Grass root systems are beneficial at reducing
soil erosion Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, and ...
by holding the soil together. Perennial grasses that grow wild in rangelands contribute organic matter to the soil when their old roots decay after attacks by beneficial fungi,
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, bacteria, insects and worms release nutrients. Scientists have observed significant diversity of the microbial cover of roots at around 10 percent of three week old root segments covered. On younger roots there was even low coverage, but even on 3-month-old roots the coverage was only around 37%. Before the 1970s, scientists believed that the majority of the root surface was covered by microorganisms.


Nutrient absorption

Researchers studying maize seedlings found that calcium absorption was greatest in the apical root segment, and potassium at the base of the root. Along other root segments absorption was similar. Absorbed potassium is transported to the root tip, and to a lesser extent other parts of the root, then also to the shoot and grain. Calcium transport from the apical segment is slower, mostly transported upward and accumulated in stem and shoot. Researchers found that partial deficiencies of K or P did not change the fatty acid composition of
phosphatidyl choline Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or soyb ...
in ''
Brassica napus L. Rapeseed (''Brassica napus ''subsp.'' napus''), also known as rape, or oilseed rape, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturally contains a ...
'' plants. Calcium deficiency did, on the other hand, lead to a marked decline of polyunsaturated compounds that would be expected to have negative impacts for integrity of the plant membrane, that could effect some properties like its permeability, and is needed for the ion uptake activity of the root membranes.


Economic importance

The term root crops refers to any edible underground plant structure, but many root crops are actually stems, such as potato tubers. Edible roots include cassava,
sweet potato The sweet potato or sweetpotato (''Ipomoea batatas'') is a dicotyledonous plant that belongs to the Convolvulus, bindweed or morning glory family (biology), family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a r ...
, beet,
carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', nat ...
, rutabaga, turnip, parsnip, radish,
yam Yam or YAM may refer to: Plants and foods *Yam (vegetable), common name for members of ''Dioscorea'' * Taro, known in Malaysia and Singapore as yam * Sweet potato, specifically its orange-fleshed cultivars, often referred to as yams in North Amer ...
and horseradish. Spices obtained from roots include sassafras, angelica, sarsaparilla and licorice.
Sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet (''Beta vulgaris''). Together wi ...
is an important source of sugar.
Yam Yam or YAM may refer to: Plants and foods *Yam (vegetable), common name for members of ''Dioscorea'' * Taro, known in Malaysia and Singapore as yam * Sweet potato, specifically its orange-fleshed cultivars, often referred to as yams in North Amer ...
roots are a source of estrogen compounds used in birth control pills. The fish
poison Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...
and
insecticide Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to b ...
rotenone is obtained from roots of '' Lonchocarpus'' spp. Important medicines from roots are ginseng,
aconite Aconite may refer to: *'' Aconitum'', a plant genus containing the monkshoods *Aconitine Aconitine is an alkaloid toxin produced by various plant species belonging to the genus ''Aconitum'' (family Ranunculaceae), known also commonly by the na ...
, ipecac, gentian and
reserpine Reserpine is a drug that is used for the treatment of high blood pressure, usually in combination with a thiazide diuretic or vasodilator. Large clinical trials have shown that combined treatment with reserpine plus a thiazide diuretic reduces m ...
. Several legumes that have nitrogen-fixing root nodules are used as green manure crops, which provide nitrogen fertilizer for other crops when plowed under. Specialized bald cypress roots, termed knees, are sold as souvenirs, lamp bases and carved into folk art. Native Americans used the flexible roots of white spruce for basketry. Tree roots can heave and destroy concrete sidewalks and crush or clog buried pipes.Zahniser, David (February 21, 2008
"City to pass the bucks on sidewalks?"
'' Los Angeles Times''
The aerial roots of
strangler fig Strangler fig is the common name for a number of tropical and subtropical plant species in the genus ''Ficus'', including those that are commonly known as banyans. Some of the more well-known species are: * ''Ficus altissima'' * ''Ficus aurea'', ...
have damaged ancient Mayan temples in Central America and the temple of
Angkor Wat Angkor Wat (; km, អង្គរវត្ត, "City/Capital of Temples") is a temple complex in Cambodia and is the largest religious monument in the world, on a site measuring . Originally constructed as a Hinduism, Hindu temple dedicated ...
in Cambodia. Trees stabilize soil on a slope prone to landslides. The root hairs work as an anchor on the soil. Vegetative propagation of plants via cuttings depends on adventitious root formation. Hundreds of millions of plants are propagated via cuttings annually including
chrysanthemum Chrysanthemums (), sometimes called mums or chrysanths, are flowering plants of the genus ''Chrysanthemum'' in the family Asteraceae. They are native to East Asia and northeastern Europe. Most species originate from East Asia and the center ...
,
poinsettia The poinsettia ( or ) (''Euphorbia pulcherrima'') is a commercially important flowering plant species of the diverse spurge family Euphorbiaceae. Indigenous to Mexico and Central America, the poinsettia was first described by Europeans in 1834 ...
, carnation, ornamental
shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...
s and many houseplants. Roots can also protect the environment by holding the soil to reduce soil erosion. This is especially important in areas such as
sand dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...
.


See also

* Absorption of water *
Cypress knee A cypress knee is a distinctive structure forming above the roots of a cypress tree of any of various species of the subfamily Taxodioideae, such as the bald cypress. Their function is unknown, but they are generally seen on trees growing in swa ...
* Drought rhizogenesis * Fibrous root system *
Mycorrhiza   A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
– root symbiosis in which individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant. * Mycorrhizal network *
Plant physiology Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (bi ...
* Rhizosphere – region of soil around the root influenced by root secretions and microorganisms present *
Root cutting A plant cutting is a piece of a plant that is used in horticulture for vegetative reproduction, vegetative (asexual) plant propagation, propagation. A piece of the Plant stem, stem or root of the source plant is placed in a suitable medium such as ...
* Rooting powder *
Stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...
*
Tanada effect The Tanada effect refers to the adhesion of root tips to glass surfaces. It is believed to involve electric potentials. It is named for the scientist who first described the effect, Takuma Tanada. The phenomenon was observed while Dr. Tanada was r ...
* Taproot


References


Further reading

* * * * * * * * *


External links


Botany – University of Arkansas at Little Rock
* {{Authority control