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root
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{{About|the part of a plant}}{{Missing information| details on function, root-environment/soil interactions|date=March 2016}}File:Primary and secondary cotton roots.jpg|thumb|Primary and secondary roots in a cottoncottonIn vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. Roots can also be aerial or aerating, that is,growing up above the ground or especially above water. Furthermore, a stem normally occurring below ground is not exceptional either (see rhizome). Therefore, the root is best defined as the non-leaf, non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist.

Evolutionary history

{{further|Evolution of plants#Evolution of roots}}The fossil record of roots—or rather, infilled voids where roots rotted after death—spans back to the late Silurian, about 430 million years ago.BOOK, The fossil record of soils, Retallack GJ, Paleosols: their Recognition and Interpretation, 1–57, Wright VP, Blackwell, Oxford, 1986,weblink no,weblink" title="web.archive.org/web/20170107005159weblink">weblink 2017-01-07, 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.JOURNAL, Sedimentological evidence for rooting structures in the Early Devonian Anglo–Welsh Basin (UK), with speculation on their producers, 2008, 10.1016/j.palaeo.2008.01.038, Hillier R, Edwards D, Morrissey LB, Palaeogeography, Palaeoclimatology, Palaeoecology, 270, 366–380, 3–4, 2008PPP...270..366H,

Definitions

The first root that comes from a plant is called the radicle. A root's four major functions are:
  1. absorption of water and inorganic nutrients;
  2. anchoring of the plant body to the ground, and supporting it;
  3. storage of food and nutrients;
  4. vegetative reproduction and competition with other plants.
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 mycorrhizae, and a large range of other organisms including bacteria also closely associate with roots.{{citation needed|date=March 2016}}(File:Kiental entre Herrsching y Andechs, Alemania 2012-05-01, DD 12.JPG|thumb|Large, mature tree roots above the soil)

Anatomy

File:CSIRO ScienceImage 11626 Barley root.jpg|thumb|The cross-section of a barley root]]When dissected, the arrangement of the cells in a root is root hair, epidermis, epiblem, 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.{{clarify|reason=need diagram |date=March 2016}}(File:Ranunculus Root Cross Section.png|thumb|Ranunculus Root Cross Section)Perhaps the most striking characteristic of roots (that makes it distinguishable from other plant organs such as stem-branches and leaves) is that, roots have an endogenousBOOK, College Botany, 1, Gangulee HC, Das KS, Datta CT, Sen S, New Central Book Agency, Kolkata, origin, i.e. it originates and develops from an inner layer of the mother axis (Such as PericycleBOOK, BOTANY For Degree Students, 6th, Dutta AC, Dutta TC, Oxford University Press, ). Whereas Stem-branching and leaves (those develop as buds) are exogenous, i.e. start to develop from the cortex, an outer layer.

Architecture

File:Tree Roots at Riverside.jpg|thumb|Tree roots at Cliffs of the Neuse State ParkCliffs of the Neuse State ParkIn its simplest form, the term root architecture 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.JOURNAL, Malamy JE, Intrinsic and environmental response pathways that regulate root system architecture, Plant, Cell & Environment, 2005, 28, 67–77, 10.1111/j.1365-3040.2005.01306.x, The configuration of root systems serves to structurally support the plant, compete with other plants and for uptake of nutrients from the soil.JOURNAL, Caldwell MM, Dawson TE, Richards JH, Hydraulic lift: consequences of water efflux from the roots of plants, Oecologia, 113, 2, 151–161, January 1998, 28308192, 10.1007/s004420050363, 1998Oecol.113..151C, 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.Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support. The main terms used to classify the architecture of a root system are:BOOK, Fitter AH, The ecological significance of root system architecture: an economic approach, Plant Root Growth: An Ecological Perspective, Atkinson D, 1991, 229–243, Blackwell,
  • Branch magnitude: the number of links (exterior or interior).
  • Topology: the pattern of branching, including:
  • Herringbone: alternate lateral branching off a parent root
  • Dichotomous: opposite, forked branches
  • Radial: whorl(s) of branches around a root
  • Link length: the distance between branches.
  • Root angle: the radial angle of a lateral root’s base around the parent root’s circumference, the angle of a lateral root from its parent root, and the angle an entire system spreads.
  • Link radius: the diameter of a root.
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.JOURNAL, Malamy JE, Ryan KS, Environmental regulation of lateral root initiation in Arabidopsis, Plant Physiology, 127, 3, 899–909, November 2001, 11706172, 129261, 10.1104/pp.010406, The extrinsic factors that affect 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:
  • Auxin – Auxin promotes root initiation, root emergence and primary root elongation.
  • Cytokinins – Cytokinins regulate root apical meristem size and promote lateral root elongation.
  • Gibberellins – Together with ethylene they promote crown primordia growth and elongation. Together with auxin they promote root elongation. Gibberellins also inhibit lateral root primordia initiation.
  • Ethylene – Ethylene promotes crown root formation.

Growth

(File:Root of a Tree.JPG|thumb|Roots of trees)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.BOOK, Russell PJ, Hertz PE, McMillan B, Biology: The Dynamic Science, Cengage Learning, 2013, 978-1-285-41534-5,weblink 2017-04-24, 750, no,weblink 2018-01-21, 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 have secondary growth but are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm.{{citation needed|date=March 2016}}In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root.{{citation needed|date=March 2016}} 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).{{citation needed|date=March 2016}}At this point, the cork cambium begins to form the periderm, consisting of protective cork cells containing suberin.{{citation needed|date=March 2016}} In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.{{citation needed|date=March 2016}}The vascular cambium produces new layers of secondary xylem annually.{{citation needed|date=March 2016}} The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems and roots.{{citation needed|date=March 2016}}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 maple family), show no correlation between root location and where the root supplies nutrients on the plant.{{citation needed|date=March 2016}}

Regulation

There is a correlation of roots using the process of plant perception to sense their physical environment to grow,JOURNAL, Nakagawa Y, Katagiri T, Shinozaki K, Qi Z, Tatsumi H, Furuichi T, Kishigami A, Sokabe M, Kojima I, Sato S, Kato T, Tabata S, Iida K, Terashima A, Nakano M, Ikeda M, Yamanaka T, Iida H, 6, Arabidopsis plasma membrane protein crucial for Ca2+ influx and touch sensing in roots, Proceedings of the National Academy of Sciences of the United States of America, 104, 9, 3639–44, February 2007, 17360695, 1802001, 10.1073/pnas.0607703104, 2007PNAS..104.3639N, including the sensing of light,{{citation|title=UV-B light sensing mechanism discovered in plant roots|url=http://phys.org/news147979343.html|publisher=San Francisco State University|date=December 8, 2008}} and physical barriers. Plants also sense gravity and respond through auxin pathwaysJOURNAL, Marchant A, Kargul J, May ST, Muller P, Delbarre A, Perrot-Rechenmann C, Bennett MJ, AUX1 regulates root gravitropism in Arabidopsis by facilitating auxin uptake within root apical tissues, The EMBO Journal, 18, 8, 2066–73, April 1999, 10205161, 10.1093/emboj/18.8.2066, , resulting in gravitropism. Over time, roots can crack foundations, snap water lines, and lift sidewalks.{{citation needed|date=March 2016}} Research has shown that roots have ability to recognize 'self' and 'non-self' roots in same soil environment.JOURNAL, Hodge A, Root decisions, Plant, Cell & Environment, 32, 6, 628–40, June 2009, 18811732, 10.1111/j.1365-3040.2008.01891.x, 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 dryJOURNAL, Carminati, Andrea, Vetterlein, Doris, Weller, Ulrich, Vogel, Hans-Jörg, Oswald, Sascha E., vanc, When roots lose contact, Vadose Zone Journal, 2009, 8, 3, 805–809, 10.2136/vzj2008.0147, or other poor soil conditions.Gravitropism directs roots to grow downward at germination, the growth mechanism of plants that also causes the shoot to grow upward.JOURNAL, Chen R, Rosen E, Masson PH, Gravitropism in higher plants, Plant Physiology, 120, 2, 343–50, June 1999, 11541950, 1539215, 10.1104/pp.120.2.343, (File:ArabidopsisLatRoot.jpg|thumb|Fluorescent imaging of an emerging lateral root.)

Shade Avoidance Root 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 Arabidospis thaliana found that plants sense the Red to Far Red light ratio that enters the plant through photoreceptors known as phytochromes.JOURNAL, Salisbury FJ, Hall A, Grierson CS, Halliday KJ, Phytochrome coordinates Arabidopsis shoot and root development, The Plant Journal, 50, 3, 429–38, May 2007, 17419844, 10.1111/j.1365-313x.2007.03059.x, 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.JOURNAL, van Gelderen K, Kang C, Paalman R, Keuskamp D, Hayes S, Pierik R, Far-Red Light Detection in the Shoot Regulates Lateral Root Development through the HY5 Transcription Factor, The Plant Cell, 30, 1, 101–116, January 2018, 29321188, 5810572, 10.1105/tpc.17.00771, 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 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 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 alters and affects root development and root architecture. To do this, they took Arabidopsis plants, grew them in agar gel, 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

{{Unreferenced section|date=March 2010}}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

(File:Prop roots of Maize plant.jpg|thumb|Stilt roots of Maize plant)(File:Adventitious roots on Odontonema aka Firespike.jpg|thumb|Roots forming above ground on a cutting of an Odontonema ("Firespike"))File:Mangroves.jpg|thumb|Aerating roots of a mangrovemangrove(File:Root tip.JPG|thumb|The growing tip of a fine root)(File:Aerial root.jpg|thumb|Aerial root)File:Socratea exorriza2002 03 12.JPG|thumb|The stilt roots of Socratea exorrhizaSocratea exorrhiza(File:Visible roots.jpg|thumb|Visible roots)The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions{{clarify|reason=there's no intro section|date=March 2016}}, 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 dicots, such as clover (Trifolium), ivy (Hedera), strawberry (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 orchids. Many aerial roots are used to receive water and nutrient intake directly from the air - from fogs, dew or humidity in the air.JOURNAL, Nowak, Edward J., Martin, Craig E., vanc, Physiological and anatomical responses to water deficits in the CAM epiphyte Tillandsia ionantha (Bromeliaceae), International Journal of Plant Sciences, 1997, 158, 6, 818–826,weblink 2475361, 10.1086/297495, 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. 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 Roots/ Arboreal Roots: forms when tree branches support mats of epiphytes and detritus, which hold water and nutrients in the canopy. Tree branches send out canopy roots into these mats, likely to utilize the available nutrients and moisture.JOURNAL, Nadkarni NM, Canopy roots: convergent evolution in rainforest nutrient cycles, Science, 214, 4524, 1023–4, November 1981, 17808667, 10.1126/science.214.4524.1023, 1981Sci...214.1023N,
  • Contractile roots: these pull bulbs or corms of monocots, such as hyacinth and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.BOOK, Pütz, Norbert, Waisel Y., Eshel A., Kafkafi U., vanc, Plant roots: The hidden half, 2002, Marcel Dekker, New York, 975–987, 3rd, Contractile roots,
  • 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 systems: roots with two distinctive forms for two separate functions
  • Fine roots: typically primary roots


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