Roots and Root Systems Introductory article

Nancy M Kerk, Yale University, New Haven, Connecticut, USA Article Contents Ian M Sussex, Yale University, New Haven, Connecticut, USA . Introduction . Parts of the Root System Roots, together with stems and leaves, constitute the major organs of vascular plants. The . The Root Apical Meristem main functions of the root system are to anchor and support the plant, to seek out, absorb, . Anatomy and Histology and transport water and ions from the soil, and to transport and store the products of . Root Branching and the Origin of Lateral Roots photosynthesis from the shoot system. . Root Specializations . Root Physiology and Function . Root Genetics Introduction . Root Hairs . Signalling Systems Roots, together with stems and leaves, constitute the major organs of vascular plants. Nonvascular plants such as algae and bryophytes and the most primitive group of vascular plants, the Rhyniophyta, lack a root system, and The Root Apical Meristem other organs perform its functions. The major functions of Each root grows in length through the activity of a the root system are to anchor and support the plant, to seek meristem located at its tip. A meristem may be defined as a out, absorb, and transport water and ions from the soil, population of cells which by their mitotic and cell division and to transport and store the products of photosynthesis activity generate the cells which contribute to the develop- from the shoot system. These functions are facilitated by ing organ and maintain themselves as initial cells. expansion of the root system through continued growth of Meristem cells have frequently been referred to as stem the root tips which are also regions of gravity and moisture cells because of their ability to self-perpetuate. perception. Roots may therefore penetrate to remarkable In the different groups of vascular plants, the cellular depths into the soil. For example, roots of the desert shrub organization of meristems is quite diverse, but their mesquite (Prosopis juliflora) were found at a depth of 53 ultimate functions are similar. In most lower vascular metres below the soil surface. The total extent of the root plants the root meristem is dominated by a single enlarged system may exceed by many times that of the shoot system tetrahedral cell, the apical cell. In gymnosperms and of the same plant. For example, in a plant of winter rye angiosperms there is no single cell or small group of cells (Secale cereale) the surface area of the root system is 130 that differ greatly in size from other cells in the meristem. times that of the shoot system. However, because most Instead, cells are arranged in layers and files that radiate roots are very narrow, the biomass of the root system is out from a common centre. In some plants these cell layers usually less than that of the shoot system. Typical root/ are very conspicuous and converge at the tip of the shoot ratios are in the range 0.2–0.4. meristem. This type of meristem is referred to as closed and is further defined by a sharp boundary between the root tip and root cap. The meristem of Arabidopsis is an extremely Parts of the Root System reduced form of a closed meristem because of the small number of cells that comprise it. The first root of a plant is called the radicle and it originates Within the root apical meristem, the frequency of cell in the embryo. In germinating gymnosperm and dicotyle- divisions is unequal. At the tip there is a group of cells in don seeds the radicle develops as the primary root which which cell division is essentially nonexistent or cell cycle grows directly downward as the tap root and initiates times are very long, in some cases in excess of one week. For lateral roots. This is called a tap root system. Branching this reason, this part of the meristem has been termed the can be elaborated further by the formation of other orders quiescent centre. Proximal to the quiescent centre the rate of lateral roots. In these plants, the root system has its of cell division increases rapidly and this region is referred origin in the embryo independent of the shoot system and is to as the proximal meristem. Distal to the quiescent centre termed allorhyzic. In monocotyledons and the ‘non-seed’ is a layer of cells called the distal or root cap meristem that vascular plants the primary root may be short-lived and the initiates all or much of the root cap. In other plants cell main root system of the plant develops from roots that layering is less conspicuous in the meristem and there is no arise from the shoot either adventitiously or from nodal discrete boundary between the meristem and the root cap. regions. This is called a fibrous root system and is termed This is referred to as an open meristem. homorhyzic because the functional root system originates from shoots.

ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net 1 Roots and Root Systems

Anatomy and Histology typically elongated tracheids or vessel elements. In the latter the cell end walls are perforated so that the cells form Proximal to the root apical meristem, cells that are derived a continuous tubular system whereas in tracheids the end from it undergo progressive differentiation to form the walls are not perforated and they are connected in tissues and cell types of the mature root. During these staggered files with pit connections that permit water processes, cells are dividing and elongating, and this results movement from cell to cell. The secondary walls of xylem in the root tip being pushed forward through the cells are thick and lignified in patterns that may be annular substratum. Because of the restriction of certain develop- or spiral ringlike thickenings in protoxylem or more mental events to particular regions of the root, zones extensively thickened in scalariform or pitted patterns in recognized as the zone of cell division, the zone of metaxylem. The conducting elements of the phloem elongation, the zone of differentiation, and the zone of transport photosynthates from the shoot system which maturation have been identified. However, there is overlap support root growth and metabolic processes and which of the boundaries of these zones and the zones themselves may be accumulated in storage tissues in the root. The may have different extents at different times during root principal cell types in the phloem are sieve tube elements, development and in different species. which are the conducting cells, and associated companion In the mature root, tissues are arranged in concentric cells. The end walls of the sieve tube elements contain cylinders. The external cylinder is the epidermis. In most perforated sieve plates that permit movement of materials roots this consists of a single cell layer. Some of the from one element to the next. This interconnected series is epidermal cells produce tubular outgrowths which are the called a sieve tube. root hairs. Internal to the epidermis is the cortex which is The root apical meristem actually occupies a subterm- usually several cell layers in thickness, but consists of only inal position at the root tip because in most species of two cell layers in Arabidopsis. The majority of cortical cells vascular plants it is covered by the root cap. The root cap is are parenchymatous and function as storage cells for the formed by anterior derivatives of the apical meristem and it plant. The outermost layer of cortical cells may have probably serves a protective function as the meristem is lignified and suberized radial walls and are then distin- forced through the soil by elongation of the differentiating guished as the exodermis. The innermost cortical cell layer cells behind it. In ferns the root cap is initiated by a division has specialized walls and is identified as the endodermis. at right angles to the long axis of the root that cuts off a This layer is characterized by the presence of Casparian derivative cell from the anterior face of the tetrahedral strips in its radial cell walls. The Casparian strip is a region apical cell. In the higher vascular plants, root cap initial of the wall that is impregnated with suberin and sometimes cells that lie anterior to the quiescent centre divide lignin and its function is to prevent the direct apoplastic transversely to form files of columella cells. In some roots movement of water and ions between the cortex and the the entire root cap appears to be initiated in this way but in vascular cylinder. others lateral parts of the root cap share a common origin Internal to the endodermis is the vascular cylinder. The with cells of the root epidermis. At the surface of the root outermost one or few cell layers of the vascular cylinder is cap mature cells become separated from one another and the pericycle. The pericycle is the principal site of lateral slough off the root tip together with mucigel that lubricates root initiation. Internal to the pericycle are alternating the root penetration of the soil. These cells have been bands of xylem and phloem. In the dicotyledons xylem cells named ‘border cells’ and they perform special functions in typically extend to the centre of the root, but in most relation to root–microbial interactions. monocotyledons the centre of the root is occupied by a pith composed of parenchyma cells. In the monocotyledons there are numerous separate bundles of xylem and phloem that are arranged in a cylinder. In dicotyledons the number Root Branching and the Origin of of alternating xylem and phloem units is much smaller, Lateral Roots typically numbering between two and seven and this number is relatively consistent within each species. The The production of branch roots generates the complex root vascular anatomy of a root is described as diarch if there system. Root branching can occur at many sites on the are two xylem units, triarch if three, and if there are many plant, some as parts of normal development and others as units it is referred to as polyarch. Within each unit of responses to wounding and environmental factors. What xylem, the narrowest cells are closest to the pericycle. They typifies branching in root systems, and distinguishes it mature earliest and are called the protoxylem. Internal to from branching in shoot systems, is that new root these are cells of larger diameter that mature later and are meristems are formed internally whereas the shoot branch the metaxylem. meristems are formed from superficial cell layers. The The principal function of the xylem is water transport exception to this is terminal root branching in the through the root to the shoot system. Xylem cells, which Lycophyta in which the apical meristem itself divides, perform this function, are dead at maturity and are resulting in the dichotomous formation of two root tips.

2 Roots and Root Systems

These can grow equally or one may outgrow the other to Root Specializations form an unequal branching system. A third mode of branching that is intermediate between lateral and terminal The preceding account of roots has focused on root branching is pseudodichotomous branching in which the systems that are typical of the great majority of plants. main root tip persists, but the tip of the new root is formed However, within the root system not all roots necessarily very close to it. perform the same function and in some plants roots have The origin of lateral roots as part of normal development become evolutionarily specialized in response to the in root systems occurs in the pericycle of most higher plants unique demands of physical or biological environments. although in some cases both pericycle and endodermis are This has resulted in modification of the root structures that involved. In the typical case, several elongated cells of the perform the major functions of the root system: anchoring pericycle divide anticlinally several times and then each cell and support, absorption, and storage. In addition, roots begins a succession of periclinal divisions to form the have become specialized in relation to their interactions hemispherical lateral root primordium. At the tip of the with microorganisms. primordium, a group of cells establishes the root apical meristem which generates the lateral root with tissues arranged in patterns like that of the parent root. There can Anchoring and support specializations be many orders of lateral root branching resulting in the Because monocotyledons do not typically develop second- much-branched condition characteristic of fibrous root ary tissues that would support and strengthen a tall shoot systems. The radial placement of lateral roots corresponds system, some have developed specialized roots that to tissues within the vascular system of the parent root. In perform this function. Prop roots develop along the stems most dicotyledonous species, lateral roots are initiated in of some arborescent monocotyledons such as Pandanus. the pericycle near protoxylem, but in the monocotyledons These roots can be more than one inch in diameter and can lateral roots are initiated outside the phloem. Lateral roots emerge several metres up the stem. The monocotyledons are initiated successively in acropetal sequence so that the Monstera and Philodendron grow as scrambling vines and youngest lateral root is nearest the tip of the parent root produce roots from nodal regions that encircle support with a characteristic spacing in the longitudinal direction. structures such as the stem of other plants. One of the most Both the radial and longitudinal spacing are under genetic spectacular examples of prop roots is the banyan habit. In control. Roots can also arise on root systems outside the certain species of the genus Ficus, adventitious roots bounds of normal development and these are referred to as formed on branches of the stem grow downwards, adventitious roots. These arise due to wounding or other penetrate the soil and branch profusely. These roots environmental factors. These roots usually have their undergo considerable secondary thickening and become origin in the pericycle and they can arise between existing major supports for horizontally growing branches. Large laterals and sometimes outside of the normal radial numbers of these roots may be formed serving as a positional restrictions. scaffolding for the massive crown. Roots borne laterally on shoot systems may arise at the Another specialization for support is the formation of nodes. In Zea mays, for example, there are several types of buttress roots commonly found in tropical tree species such shoot-borne roots. Seminal roots are initiated in the as the Tahitian chestnut, Inocarpus. These are positioned at embryo, but usually die early in development of the plant. the base of the trunk and form flattened platelike structures Permanent nodal roots initiate from parts of the stem that that extend for some distance up the trunk. An extreme remain below ground. Prop roots are initiated from the example of a root system specialized for support and lowermost aerial nodes and serve to support the growing anchorage is that of so-called strangler plants such as shoot system. These branch extensively to form the Metrosideros robusta of New Zealand. The seeds of this functional root system of the plant. plant begin their development epiphytically on limbs of Other shoot-borne roots arise adventitiously in the stem, forest trees. Numerous roots grow downwards and graft that is in less precisely defined locations and typically along laterally with one another to form a more or less parts of an internode. In ivy, these adventitious roots continuous lattice around the supporting tree which appear to be part of the normal developmental programme ultimately dies and decays, leaving the Metrosideros shoot of the plant, but in other cases they are formed in response supported on a tall pseudotrunk composed of the to wounding, such as occurs when a stem cutting is made integrated root system. for plant propagation. In ferns, the entire functional root system is derived from shoot-borne roots. Absorptive specializations Roots require oxygen for their metabolic processes, but plants in which the root system is in an anaerobic environment, such as occurs in marshes and swamps, have

3 Roots and Root Systems developed specializations of their root system that facil- potato (Ipomoea batatas) and sugar beet (Beta vulgaris), or itate absorption and exchange of gases. In mangroves, such more usually are converted to osmotically inactive as Rhizophora mangle and Avicennia, which grow in substances, principally starch. The original cambium saltwater mud flats, specialized roots called pneumato- may continue to function throughout the development of phores develop. These roots are negatively gravitropic and the storage root, as in carrot (Daucus carota), but in other grow upwards into the aerial environment as silt and mud plants such as the sugar beet a succession of cambia is accumulate over them. The pneumatophores have lenticels formed in concentric circles. which are small openings in the corky surface layer of the root that permit gas exchange between the environment and the root, and the cortical tissues of the root contain large airspaces that permit movement of gases through the Root–microbial specializations root system. In water lilies such as the sacred lotus, Nelumbo nucifera, Because roots grow through the soil it is not unexpected the roots which grow in mud in fresh water environments that they encounter some of the large number of develop aerenchyma, a parenchymatous tissue with large microorganisms that inhabit this environment. Some of intercellular spaces. The aerenchyma in these roots is part these encounters have resulted in evolutionary specializa- of an elaborate aeration system that is continuous from the tions of root systems that provide a benefit to both partners leaves to the roots. of these mutualistic associations. Other root specializations have permitted roots to The best known of these are the symbiotic relationships accumulate and store water and exchange gases in an between soil bacteria of the genera Rhizobium and aerial environment. For example, in orchids such as the Bradyrhizobium and roots of many legume species. These vanilla orchid (Vanilla planifolia), which grows in the result in the formation of root nodules in which atmo- canopy of humid tropical rainforests, the epidermis is spheric nitrogen is fixed and becomes available to the plant numerous cell layers in thickness and called a velamen. in the form of organic nitrogenous compounds. Bacteria These cells are dead at maturity and act as a sponge to penetrate the plant by attaching to a developing root hair. absorb water from the atmosphere. In many epiphytic The root hair curves over the bacteria, forming a orchids the roots also contain chlorophyll and in some, ‘shepherd’s crook’ within the fold of which the cell wall termed ‘shoot-less orchids’, the shoot system is greatly lyses. Bacteria move into the plant through this lesion in an reduced and the roots perform the principal photosyn- infection thread that is formed by invagination of the plant thetic and gas exchange functions. plasma membrane. The infection thread penetrates Although plants are typically autotrophic, synthesizing through cells to the inner cortex where cell division has all of their organic molecules from inorganic materials of already begun. Here the bacteria are released from the the environment, some have evolved a heterotrophic habit infection thread into the plant cell cytoplasm but are still in which some or all of the organic nutrient supply as well enclosed in a peribacteroid membrane derived from the as water and ions are absorbed from a parasitized host plant plasma membrane. Within these membranes bacteria plant. Mistletoes such as Viscum album are hemiparasites differentiate as bacteroids that are incapable of further because they produce some photosynthetic leaves. The division and they synthesize the enzyme nitrogenase plant grows epiphytically and produces modified roots, required for the fixation of atmospheric nitrogen. called haustoria, that penetrate and form close associa- During this time growth of the dividing cortical cells has tions with the vascular system of the host plant through produced a root nodule that ruptures overlying layers of which nutrients move to the mistletoe. Dodder (Cuscuta)is cortical cells and emerges from the parent root. A mature a holoparasite producing no chlorophyll. It grows as a vine root nodule is an elongated or lobed structure a few that twines around a potential host plant and in areas of millimetres in size within which there is a terminal contact between the two stems adventitious root-like meristem, a zone of differentiating cells, and a large zone haustoria are produced that penetrate the host vascular of nitrogen fixation. The majority of cells in this zone tissue. contain bacteroids and are pink in colour due to the presence of leghaemoglobin, a plant gene encoded protein that maintains the low oxygen environment that protects Storage specializations nitrogenase from oxygen poisoning. Surrounding these zones are layers of cortical parenchyma and endodermal In plants with tap root systems, the primary root and cells, and a network of vascular tissue that connects with sometimes lateral roots can undergo secondary thickening that of the parent root and transports nitrogenous as the result of cambial meristem activity. In storage roots, compounds from the nodule to growing regions of the cells derived from the cambium differentiate predomi- plant. Because of the anatomical similarities between nantly as parenchyma, and photosynthates transported nodules and roots it has been suggested that nodules may from the shoot accumulate in them as sucrose in the sweet represent an abnormal form of lateral root development.

4 Roots and Root Systems

Cycads also form coralloid lateral roots that become a tomato root system (tomato does not synthesize nicotine) invaded by nitrogen-fixing cyanobacteria of the genus do not contain nicotine. Anabaena.InCycas revoluta the microbial cells gain access Roots have evolved special physiological processes that to roots through breaks in the epidermis and become are important for growth in a subterranean environment. concentrated in a cylindrical zone of the cortex where they These include the uptake and transport of water and ions, are located both within and between cells. Nitrogenous the transport, metabolism and storage of photosynthates, compounds fixed by Anabaena may be transferred to other and the role of hormones in the maintenance of root parts of the plant. growth and function. Symbiotic interactions between fungi and roots occur in the great majority of vascular plants and produce mycorrhizae, which is the name given to these fungus- Water and ion uptake and transport infected roots. Among the angiosperms only a few families, Soil water enters the root system principally through root including the Brassicaceae and Cyperaceae, fail to form hairs although some uptake occurs through other epider- mycorrhizae. There are two general types of mycorrhizae, mal cells. Water transport across the cortex occurs by both endomycorrhizae in which the fungus penetrates into root the symplastic (intracellular cytoplasmic) and the apoplas- cells, and ectomycorrhizae in which they do not. The tic (intercellular wall) pathways. At the endodermis, the former is the more common, but the latter involves greater apoplastic pathway is blocked by the presence of suberized specialization of root structure. Casparian strips in the radial cell walls, and all water In ectomycorrhizae fungal filaments surround the root passing into the vascular system does so via the symplastic as a sheath and grow between cells of the epidermis, and in route. Water movement through the xylem to the shoot some cases between cortical cells also. Infected roots system occurs as a transpiration stream in response to the remain short and are thickened by radial expansion of negative water potential generated by transpiration from epidermal cells. They are highly branched, sometimes in a stomata and other parts of the shoot/atmosphere interface. dichotomous manner, probably in response to growth This negative pressure is sufficient to raise the water substances such as auxin, released by the fungus, and they column to the tops of the tallest trees. do not produce root hairs. The effect of both ecto- and endomycorrhizae is to expand the root–soil interface greatly by the growth of fungal filaments into the soil Photosynthate transport, utilization, and structure. This facilitates transfer of ions, especially storage phosphorus, and water into the roots. Among many other structural specializations of roots With regard to photosynthates the plant can be considered are root clusters and proteoid roots, root spines, contrac- as a source–sink system in which photosynthates synthe- tile roots, and long root–short root dimorphisms. sized in the source leaves are transported to storage sinks in the root system. Photosynthates are transported as sucrose and may be stored as such, or more commonly as starch or other polysaccharides in specialized root storage organs or in the parenchyma cells of the root cortex. Conversion of Root Physiology and Function sucrose to starch occurs through the activity of the enzyme sucrose synthase (EC 2.4.1.13) in storage cells. Stored The basic aspects of root cell metabolism are similar to reserves are made available again through their enzymatic those of shoot cells, but there are aspects of root physiology breakdown and are transported to growing regions, and function that are unique to the root system. Nitrogen, reversing the source–sink relationships. Of the total which is an essential component of proteins, enters the amount of photosynthate that the plant synthesizes, plant through root tips and root hairs as nitrate ions. In the approximately 20% is utilized in growth and metabolic root this is reduced to ammonia, and transported to processes in the root system. In legumes, nodule produc- plastids in leaves and other root cells where it is tion and function involves expenditure of up to 25% of the resynthesized into amino acids that are incorporated into photosynthate production of the plant. new proteins. Legume plants that produce bacteroid- containing root nodules are able to fix atmospheric Hormones and tropisms nitrogen through the activity of the enzyme nitrogenase, convert this to reduced nitrogen compounds such as Hormones play an important role in the response of roots amides and transport these to other parts of the plant. to environmental conditions, and form communication Some secondary compounds are synthesized only in pathways between the root and shoot systems. Tropisms roots. For example, the alkaloid nicotine is synthesized in such as gravitropism, phototropism, thigmotropism and roots of the tobacco plant and transported to the shoot hydrotropism are hormone-mediated growth responses system where it accumulates. Tobacco shoots grafted onto that allow the plant to react to environmental stimuli.

5 Roots and Root Systems

The majority of roots are positively gravitropic, growing Other roles of hormones in root physiology towards the centre of gravity. Others are apogravitropic and function and grow at an angle to the gravitational field or are negatively gravitropic, such as pneumatophores. These Auxin regulates many developmental processes in root different responses permit the root system to colonize systems. The major site of auxin synthesis is in shoot tips. different parts of the soil environment. Apogravitropic From here auxin is transported in a polar fashion through roots can explore large expanses of soil in a horizontal the stem and the root system. In roots auxin accumulates in direction and capture frequent but sparse rains or tips and high concentrations are avoided by oxidative irrigation water, while positively gravitropic tap roots metabolism or by conjugation to inactive forms. These can exploit deep-seated water tables. Gravitropic re- latter may represent storage forms of auxin that are sponses are brought about by bending of the growing available for subsequent use. The processes for reducing zone of the root and are caused by unequal elongation of auxin concentration are especially important in roots cells on the upper and lower sides. The locus of gravity because root cells are sensitive to much lower auxin perception in the root is the columella cells of the root cap. concentrations than are shoot cells (approximately two to These cells contain starch grains that are thought to three orders of magnitude lower), so that their growth is function as statoliths. In a downward growing positively inhibited by relatively low auxin concentrations. A gravitropic root the statoliths are located near the lower- primary function of auxin is to stimulate the initiation most cell wall, but if the root is reoriented, statoliths fall and early growth of lateral roots from root pericycle cells towards the lower side wall of the cell. This activates and to stimulate the initiation of shoot-borne roots. membrane transport proteins that cause a lateral redis- The ratio of auxin to other hormones also affects tribution of auxin towards the lower side of the root cap. developmental processes. Because cytokinins, which are The auxin indoleacetic acid, which accumulates or is synthesized in root tips and transported to the shoot synthesized in the root cap, is transported to the growing system, are inhibitory to lateral root initiation it has been region and lateral redistribution causes unequal amounts proposed that lateral roots are initiated at sites where the of cell elongation and subsequent bending of the root tip auxin/cytokinin ratio is optimal, and that this is respon- towards gravity. sible for their longitudinal spacing pattern. Phototropism is response to the direction of incident Abscisic acid is termed a stress hormone because it is light. Light penetrates several centimetres into the soil and synthesized by cells that are subjected to abiotic stresses roots typically respond negatively to light. The photo- such as dehydration or low temperature. Under conditions tropic response, together with the gravitropic response, is of water stress, abscisic acid is synthesized by root cells and important in directing growth of roots of germinating seeds stimulates water flow and ion flux into roots by increasing downward into the soil. The negative phototropism of hydraulic flow through the apoplast and cell membranes. roots involves perception by a membrane-bound blue light Abscisic acid transported from the root to the shoot system receptor and subsequent unequal distribution of auxin in xylem sap is a major factor accounting for stomatal such as occurs in the gravitropic response. closure in water-stressed plants. Thigmotropism is the response to surface contact, the so-called touch response. Thigmotropic responses occur in roots of climbing plants, such as the tendril roots of the vanilla orchid, and cause the roots to curve around and Root Genetics clasp a supporting structure. A thigmotropic response is also thought to be responsible for the response of root tips Because roots are subterranean organs and therefore not that encounter an immovable object in the soil or which readily observable, the genetic analysis of roots has lagged grow in compacted soils that are a consequence of no-till behind that of shoots. Nevertheless, numerous root agricultural practices. Cells in the growing region expand mutants have been identified in crop species. It was only laterally and their cell walls become thickened, probably in when attention focused on Arabidopsis as a model system response to increased ethylene production in the affected for genetic analysis of development that root genetics cells. This growth response allows the root to generate became an active field of research. Arabidopsis roots are additional force to overcome the soil resistance. very thin and translucent and can be analysed micro- Hydrotropism is the response to the presence of water. scopically without the need for sectioning so the identifica- Roots in dry soils grow towards a source of water. The tion and analysis of mutants in root structure and mechanism of this directional root growth is poorly development has been facilitated. understood but the hormone abscisic acid, which is The following discussion of root genetics covers first, synthesized in plant organs under conditions of low water genes that are involved in initiation and growth of the potential or partial desiccation, stimulates the elongation primary root, then genes that are involved in tissue and cell of root cells and may be involved in the hydrotropic identity, and finally genes involved in the initiation of response. lateral and adventitious roots.

6 Roots and Root Systems

The radicle develops during embryogeny but in the was estimated to have over 14 billion root hairs. Thus root monopteros mutants of Arabidopsis the basal pole of the hairs contribute greatly to the root/soil interface. Root embryo, including the radicle, fails to form, indicating that hairs are initiated in acropetal sequence from epidermal MONOPTEROS gene function is required for proper cells that have almost completed their elongation and they development of the radicle. In mutants named root typically live for only a few days. meristemless the radicle is organized correctly but the In grasses and some other monocotyledons an epidermal primary root is very short and contains the same number of cell undergoes an asymmetric division to form a larger cells as are in the radicle. These mutants appear to be atrichoblast cell and a smaller trichoblast cell that is defective in a function required for cell division in the root located towards the root tip. The root hair develops from meristem although cell division in the embryo and the the trichoblast cell and elongates by growth at its tip. In shoot is not affected. dicotyledons all root epidermal cells are potential tricho- Other Arabidopsis root mutants affect the presence or blasts but root hair formation is usually limited to fewer identity of the concentric cell layers, and these are known cells. In Arabidopsis the root epidermis consists of long- collectively as radial pattern mutants. In the short root itudinal files of cytoplasmically rich trichoblasts separated mutants the endodermal layer is missing and the cortex by one to two files of atrichoblasts. Trichoblast cells overlie consists of a single layer of parenchyma cells between the the anticlinal walls of the underlying layer of cortical epidermis and the pericycle. In contrast, in the pinocchio parenchyma cell files. This has suggested that positional mutant it is the cortical parenchyma cell layer that is information specifies the trichoblast, and local production missing and the endodermis is formed normally. In the of ethylene in these regions has been proposed as providing scarecrow mutant there is a single layer of cortical cells but the information needed for such specification. Several the cells appear to have characteristics of both the Arabidopsis mutants in which root hairs are formed from parenchyma and endodermal layers. They have Casparian every epidermal cell and others that constitutively produce strips in their walls and they react positively to a ethylene have supported this model. Mutations are also monoclonal antibody that specifically identifies cortical known in which root hairs are not initiated or their parenchyma cells. In other mutants the number of cells in development is arrested at a specific stage. From these it different tissue layers is affected. In the wooden leg mutant has been possible to propose a genetic pathway for root the vascular system is reduced, consisting of only a few cell hair development. layers, but other tissues contain the expected number of cells. In the fass mutant all tissues contain additional cells and the cortex consists of multiple cell layers. Mutants of Arabidopsis have been identified in which lateral roots either are not initiated or undergo early Signalling Systems growth arrest although growth of the primary root is not The development of a whole plant requires the functioning affected. These mutants indicate that early developmental of signals that allow integration of events in the different events in the formation of lateral roots are different from parts. Some of these signals occur over long distances such those in the primary root. Study of root mutants in crop as auxin signalling or cytokinin signalling between the root plants has confirmed this finding and has revealed further and shoot systems, but other signals operate over short that the initiation of different classes of roots is under ranges and may move from a signalling cell to the diageotro- different systems of control. For example, the contiguous responding cell by permeating the intervening pica Lycopersicon esculentum mutant of tomato ( ) fails to plasma membranes and cell walls or they may move initiate lateral roots, but adventitious roots are initiated on through interconnecting plasmodesmata. the stem and develop normally. The maize mutant, rtcs-1, fails to initiate the shoot-borne seminal or nodal prop roots, but development of the primary root and laterals Further Reading formed on it is normal. Howell SH (1998) Molecular Genetics ofPlant Development . Cambridge: Cambridge University Press. Jackson MB (1986) New Root Formation in Plants and Cuttings. Root Hairs Dordrecht, The Netherlands: Martinus Nijhoff Publishers. Raven PH, Evert RF and Eichhorn SE (1999) Biology ofPlants , 6th edn. New York: WH Freeman/Worth Publishers. Root hairs are tubular, usually unbranched outgrowths of Steeves TA and Sussex IM (1989) Patterns in Plant Development,2nd epidermal cells and may be up to 1.5 mm in length. The edn. Cambridge: Cambridge University Press. number of root hairs on a root system is usually very large. Waisel Y, Eshel A and Kafkafi U (1991) Plant Roots: The Hidden Half. The winter rye plant (Secale cerea) mentioned previously New York: Marcel Dekker.

7