Invascular plants, theroots are theorgans 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.[1] They are most often below the surface of thesoil, but roots can also beaerial or aerating, that is, growing up above the ground or especially above water.[2] Roots can be very fine like a thread or massive like those of theSitka Spruce which, in an individual named "The Octopus Tree" at Trees of Mystery in northernCalifornia, has exposed roots over four feet (1.2 meters) thick.[3]
Root morphology is divided into four zones: theroot cap, theapical meristem, the elongation zone, and the hair.[5] Theroot 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.[6] The first root in seed producing plants is theradicle, which expands from the plant embryo after seed germination.
Perhaps the most striking characteristic of roots that distinguishes them from other plant organs such as stem-branches and leaves is that roots have anendogenous[7] origin,i.e., they originate and develop from an inner layer of the mother axis, such aspericycle.[8] In contrast, stem-branches and leaves areexogenous,i.e., they start to develop from the cortex, an outer layer.
In response to the concentration of nutrients, roots also synthesisecytokinin, 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 certainfungi to formmycorrhizae, and a large range of other organisms includingbacteria also closely associate with roots.[9]
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.[10] 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.[11] 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.[11]
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
Diameter of 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 bysignal transduction pathways.[13]
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:
Together with ethylene, they promote crown primordia growth and elongation. Together with auxin, they promote root elongation. Gibberellins also inhibit lateral root primordia initiation.
Early root growth is one of the functions of theapical 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.[14]
Growth from apical meristems is known asprimary growth, which encompasses all elongation.Secondary growth encompasses all growth in diameter, a major component ofwoody plant tissues and many nonwoody plants. For example, storage roots ofsweet potato have secondary growth but are not woody. Secondary growth occurs at thelateral meristems, namely thevascular cambium andcork cambium. The former formssecondary xylem andsecondary phloem, while the latter forms theperiderm.
In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms acylinder of tissue along thestem and root.[citation needed] 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]
At this point, the cork cambium begins to form the periderm, consisting of protectivecork cells. The walls of cork cells containssuberin thickenings, which is an extra cellular complex biopolymer.[15] 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.[16] 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 theaerenchyma during waterlogging.[17] In roots, the cork cambium originates in thepericycle, a component of the vascular cylinder.[17]
The vascular cambium produces new layers of secondary xylem annually.[citation needed] 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 at Port Jackson
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 asSapindaceae (themaple family), show no correlation between root location and where the root supplies nutrients on the plant.[18]
There is a correlation of roots using the process ofplant perception to sense their physical environment to grow,[19] including the sensing of light,[20] and physical barriers. Plants also sense gravity and respond through auxin pathways,[21] resulting ingravitropism. 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.[22]
The correct environment ofair, mineralnutrients andwater directs plant roots to grow in any direction to meet the plant's needs. Roots will shy or shrink away from dry[23] or other poor soil conditions.
Gravitropism directs roots to grow downward atgermination, the growth mechanism of plants that also causes the shoot to grow upward.[24]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).[25]
Research indicates that plant roots growing in search of productive nutrition can sense and avoid soil compaction through diffusion of the gasethylene.[26]
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 ofArabidopsis thaliana found that plants sense the Red to Far Red light ratio that enters the plant through photoreceptors known asphytochromes.[27] 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.[27] 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.[27] 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.[28] In this form, PhyA stabilize thetranscription 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 thephloem, 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 transportingproteins.[28] Thus, through manipulation of ARF19, the level and activity ofauxin transporters PIN3 and LAX3 is inhibited.[28] 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 rootpericycle. 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.[27][28]
Research of Arabidopsis has led to the discovery of how this auxin mediated root response works. In an attempt to discover the role thatphytochrome plays in lateral root development, Salisbury et al. (2007) worked withArabidopsis 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.[27] In the experiments of van Gelderen et al. (2018), they wanted to see if and how it is that the shoot ofA. thaliana alters and affects root development and root architecture. To do this, they tookArabidopsis plants, grew them inagar 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 variousArabidopsis 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.[28]
A true root system consists of aprimary root andsecondary roots (orlateral roots).
the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common inmonocots. The main function of the fibrous root is to anchor the plant.
Stilt roots of maize plantCross section of an adventitous crown root of pearl millet (Pennisetum glaucum)Roots forming above ground on a cutting of anOdontonema ("Firespike")Aerating roots of amangroveThe growing tip of a fine rootAerial rootThe stilt roots ofSocratea exorrhizaVisible roots
The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions[clarification needed], 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 inmonocots and pteridophytes, but also in manydicots, such asclover (Trifolium),ivy (Hedera),strawberry (Fragaria) andwillow (Salix). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system. Adventitious root formation is enhanced in many plant species during (partial) submergence, to increase gas exchange and storage of gases like oxygen.[29] Distinct types of adventitious roots can be classified and are dependent on morphology, growth dynamics and function.[30][31]
Aerating roots (orknee root orknee orpneumatophores): roots rising above the ground, especially above water such as in somemangrove genera (Avicennia,Sonneratia). In some plants likeAvicennia 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 inepiphyticorchids. Many aerial roots are used to receive water and nutrient intake directly from the air – from fogs, dew or humidity in the air.[32] Some rely on leaf systems to gather rain or humidity and even store it in scales or pockets. Other aerial roots, such asmangrove aerial roots, are used for aeration and not for water absorption. Other aerial roots are used mainly for structure, functioning as prop roots, as inmaize or anchor roots or as the trunk instrangler 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.[32]
Canopy roots/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.[33]
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.
Contractile roots: roots that pull bulbs or corms ofmonocots, such ashyacinth andlily, and sometaproots, such asdandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.[34]
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-fixingcyanobacteria. They are only found incycads.
Fine roots: 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 inmistletoe (Viscum album) anddodder.
Photosynthetic roots: roots that are green and photosynthesize, providing sugar to the plant. They are similar tophylloclades. Several orchids have these, such asDendrophylax andTaeniophyllum.
Root nodules: roots that harbor nitrogen-fixing soil bacteria. These are often very short and rounded. Root nodules are found in virtually alllegumes.
Stilt roots: adventitious support roots, common amongmangroves. They grow down from lateral branches, branching in the soil.
Storage roots: roots modified for storage of food or water, such ascarrots andbeets. They include sometaproots 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. A type of storage root distinct from taproot.
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 (200 ft) below the ground surface, was observed during the excavation of an open-pit mine in Arizona, US. 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.
The fossil record of roots—or rather, infilled voids where roots rotted after death—spans back to the lateSilurian, about 430 million years ago.[38] 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.[39] The evolutionary development of roots likely happened from the modification of shallowrhizomes (modified horizontal stems) which anchored primitive vascular plants combined with the development of filamentous outgrowths (calledrhizoids) which anchored the plants and conducted water to the plant from the soil.[40]
Light has been shown to have some impact on roots, but it's 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 ofIndole-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 positivegravitropic responses in some situations. The effects of light on root elongation has been studied formonocotyledonous anddicotyledonous plants, with the majority of studies finding that light inhibited root elongation, whether pulsed or continuous. Studies ofArabidopsis in the 1990s showed negativephototropism and inhibition of the elongation of root hairs in light sensed byphyB.[41]
Certain plants, namelyFabaceae, formroot nodules in order to associate and form a symbiotic relationship with nitrogen-fixing bacteria calledrhizobia. 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.[42]
Soil temperature is a factor that effectsroot initiation 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 likeoats,rapeseed,rye,wheat fare better in lower temperatures than summerannuals likemaize andcotton. Researchers have found that plants like cotton develop wider and shortertaproots 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.[43]
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.[44]Since nearby plants showed no changes instomatal aperture researchers believe the drought signal spread through the roots and soil, not through the air as a volatile chemical signal.[45]
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 withAzotobacter showed higher populations in soils favorable toAzotobacter growth. Some studies have been unsuccessful in increasing the levels of certain microbes (such asP. fluorescens) in natural soil without prior sterilization.[46]
Grass root systems are beneficial at reducingsoil erosion 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 beneficialfungi,protozoa, bacteria, insects and worms release nutrients.[6]
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.[6]
Researchers studyingmaize seedlings found that calcium absorption was greatest in theapical 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.[47]
Researchers found that partial deficiencies of K or P did not change thefatty acid composition ofphosphatidyl choline inBrassica napus L. plants. Calcium deficiency did, on the other hand, lead to a marked decline ofpolyunsaturated compounds that would be expected to have negative impacts for integrity of the plantmembrane, that could effect some properties like its permeability, and is needed for theion uptake activity of the root membranes.[48]
Roots can also protect the environment by holding the soil to reduce soil erosion.Roots and tubers are some of the most widely harvested crops in the world.
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