Roots of angiosperms

The root• The prolongation of the radicle (first root axis arises from

cells laid down in the seed)of the embryo is ROOT.• Principally, the underground organ of the plant body which

absorbs water and minerals from the soil transporting them to other parts of the plant body.

• Due to this, root tends to grow downwards, away from light and towards water.

• As a general rule, they bear neither leaves nor buds.• The primary roles are anchorage, absorption and transport. • However, it has adapted to fulfil a variety of other functions

including storage, support and aeration.• Compared with stem, root is relatively simple and uniform in

structure.

Root Structure• The main root, primary root or Tap root is

formed from the radicle. The lateral branches of main root are called secondary roots which are further branched to form tertiary roots. Roots are absent in some angiosperms, e.g. genus utricularia ( free-floating, aquatic, canivorous plants that trap and digest very thin animal).

Four zones or regions are commonly recognized in developing roots namely:

1. the root cap,2. the meristematic zone (zone of cell division), 3. the elongation zone, and 4. the maturation zone.

Root cap: The tip of the root is covered by a cap (root cap) that is shaped like a thimble. It is composed of two types of cells, the inner columella (they look like columns) cells and the outer, lateral root cap cells that are continuously replenished by the root apical meristem. As these outer cells disintegrate they form a strong protective cover which protects the root tip from injury and damage as the root pushes its way through the soil. The root provides a continuous supply of expendale parenchyma cells that lie over the meristematic tissues.

Meristematic zone: This zone is a growing point occurring immediately behind the root cap. Meristematic region consists of meristematic tissue and is protected by the root cap. It consists of the apical and lateral meristems. Most of the activity in this zone of cell division takes place toward the edges of the dome, where the cells divide every 12 to 36 hours, often rhythmically, reaching a peak of division once or twice a day. The cells differentiate to form more specialized root tissues and additional cells are provided for the zone of elongation.

Zone of elongation: News cells produced by mitosis by the primary meristems elongate rapidly and become several times longer than wide. Thus, their width also increases slightly. The small vacuoles present merge and grow until they occupy 90% or more of the volume of each cell. No further increase in cell size occurs above the zone of elongation, and the mature parts of the root, except for an increase in girth, remain stationary for the life of the plant. This causes the root to elongate and penetrate deeper into the soil in search for water and mineral salts. This region is where the epidermal cells produce many tubular, unicellular outgrowth called hair roots. Root hairs, which can number over 35,000 per square centimeter of root surface and many billions per plant, greatly increase the surface area and therefore the absorptive capacity of the root. Thus, Water absorption mostly takes place through this area. The root hairs usually are alive and functional for only a few days before they are sloughed off at the older part of the zone of maturation, while new ones are being produced toward the zone of elongation. Above the root hair region, the root becomes thicker and secondary or lateral roots are developed. The secondary roots in turn rebranch to form tertiary roots. Each lateral branch has its own cap, root hairs, meristematic, elongation and mature regions. The roots in this region are covered by a protective cork layer.

Zone of maturation: Elongated cells differentiate into specific cell types, (e.g. xylem and phloem) in the zone of maturation. The cells of the root surface cylinder mature into epidermal cells, which have a very thin cuticle. Just above the elongation zone, there is a part of the maturation zone with is called piliferous region.

Types of roots Tap root • Generally found in dicotyledons• Main or primary root develop

from the radicle.• Grows vertically down into the soil • Later lateral or secondary

roots grow from this at an acute angle outwards and downwards, and from these other branches may arise for absorption of water and nutrients.

• When tap root is associated with many branched roots, it forms the tap root system. e.g. pastinaca sativa (parsnip) and taraxacum officinale (Dandelion)

Lateral roots

Adventitious roots• Commonly found in monocots• Growth of the radicle is usually

arrested at an early stage and is replaced by numerous roots that develop from the stem.

• These adventitious roots are slender and equal in size.

• Adventitious roots associated with branched or lateral roots and form the adventitious root system.

• Also known as fibrous roots.

Modifications in roots

• Root modification occurs when there is a permanent change in the structure of tap or adventitious roots. This is to perform additional specific functions to those of anchorage and absorption for adaptation to their surrounding environment.

Roots modified for the storage of food:• Conical roots: modified tap root of

conical shape throughout which they store food. It is broad at the base and gradually tapers towards its apex - e.g. Carrot (Daucas carota).

• Napiform roots: modified tap root, fleshy with the upper portion inflated or swollen and abrupt narrowing of the base into a tail-like portion. - e.g. Turnip (Brassica rapa) and Beetroot (Beta vulgaris) .

• Fusiform roots modified tap root with thickened middle portion containing food and tapered towards both ends – e.g. radish (Raphanus sativus) Turnip

• Tuberous/storage root is modified fibrous root with irregular shape. Many single roots are modified to form several similar swollen roots for the storage of food. They occur in a bunch - e.g. Mirabilis jalapa

• Nodulated roots are fibrous/adventitious roots of the plants of the Leguminosae family. Nodules like structures are present on branches of root in which nitrogen fixing bacteria can be found.The apex of these roots become swollen because of the accumulation of food e.g. Curcuma amada, Ginger.

Roots modified for mechanical support:• Prop roots – the name is related to the

pillar like appearance. They are aerial adventitious roots that develop from branches and give support to the branches. They grow vertically downward, penetrate the soil and become thick to provide additional support to the plant. they brace the plant against wind - e.g. Ficus bengalensis, banyan tree (Ficus macrophylla) and maize.

• Stilt roots - roots develop from nodes of the lowermost portion of the stem and provide mechanical support to the plant by fixing it in soil firmly. e.g. sugarcane, Pandanus Tectorius (screw pine).

• Climbing roots - in some weak stemmed plants roots develop from nodes which are useful to climb on the hard object.

Screw pine roots

Maize roots

Decumaria barbara

• Contractile roots widely distributed among monocotyledons and herbaceous perennial dicotyledons. The roots from the bulbs of lilies and of several other plants such as dandelions and colocassia contract by spiraling to pull the plant a little deeper into the soil each year until they reach an area of relatively stable temperatures. The roots may contract to a third of their original length as they spiral like a corkscrew due to cellular thickening and constricting.

• Floating roots - in some aquatic plants, the roots will float on the surface of the water. These roots store air, become inflated and spongy, project above the level of water and make the plant light. They also help in exchange of gases. E.g. Jussiaea

Bulbs and roots of

lilies

Water primrose

(Jussiaea )

Roots modified for vital function:

• Pneumatophores or Respiratory roots - The roots of some aquatic plants and plants which grow on marshy areas, such as mangroves, develop outgrowths (pneumatophores). This type of roots arises from underground branches of tap root, grows in upwards direction and usually extends several centimeters above water, facilitating the oxygen supply to the roots beneath. In aquatic plants, floating roots are acting as respiratory roots. In marshy area, the roots are not getting sufficient oxygen and hence they grow upwards from the ground.

• Epiphytic roots are aerial roots of plants such as orchids. A distinguishing feature is that these roots absorb moisture from atmosphere with the help of velamen tissue (outer layer of dead cells). Most of the cells are water absorbing while the others are filled with air and thus facilitate the exchange of gases with the inner cortex. Epiphytic roots have only physical contact and cause no harm to the host plant.

• Photosynthetic or Assimilatory roots: The roots are green, flattened, and ribbon like. The root tip cells contain chloroplasts and thus, perform photosynthesis. In some cases such as in the case of leafless orchids of the genera Taeniophyllum and Chiloschista, they are the only photosynthetic tissues. E.g. Tinospora (-aerial roots) , Trapa (-Hydrophyte is with submerged green roots)

Orchids roots

Trapa

Taeniophyllum

• Parasitic roots: The stems of certain plants that lack chlorophyll, such as dodder (Cuscuta), produce peg-like roots called haustoria that penetrate the host plants around which they are twined. The haustoria establish contact with the conducting tissues of the host and effectively parasitize their host. Thus, they are food sucking roots.

• Root thorns: roots of some plants arise from the stem and change into thorns performing the protective function e.g. Pothos (money plant)

Cuscuta on host plant

Electromacrograph to show the haustoria.

Thorns of Pothos

Other modifications of roots:• Fasciculated roots: from the bas or

lower nodes of the stem, these tuberous roots arise in groups. E.g Dahlia

• Moniliform roots: these are also called beaded roots because of their bead-like appearance e.g. Momordica (bitter gourd)

• Annulated roots: these thickened roots look as if formed by a number of discs placed on above another. Ipecac (Cephalis)

• Water storage roots. Some members of the pumpkin family (Cucurbitaceae), especially those that grow in arid regions, may produce water-storage roots weighing 50 or more kilograms.

• Buttress roots. Certain species of fig and other tropical trees produce huge buttress roots toward the base of the trunk, which provide considerable stability.

Root growth • Most individual root growth can be divided into two main

phases; the indeterminate growth phase and the termination growth phase. The indeterminate growth phase is one where growth is maintained for an undefined period of time. Growth is usually accomplished by division of RAM while the terminate phase is a phase where growth stops usually after a certain period of time, size or length of roots or when appropriated conditions are unavailable. According to Wilcox 1962, the RAM can become dormant for example during droughts but can restart its division and growth after some time.

Primary growth

• Primary growth is a longitudinal growth occurring in all vascular plants and involves all elongation of the roots. Primary growth takes place only in the apical meristem.

Secondary growth • Secondary growth occurs mainly in dicots but

rarely in monocots. It is the result of division in the lateral meristems, the cork cambium and the vascular cambium. Secondary growth takes place in woody as well as in non woody plants. It involves all growth in diameter of the roots.

• 1. Vascular cambium

2. The vascular cambium cells divides longitudinally.

3. One of the new cells remains vascular cambium and the other becomes xylem.

Vascular cambium

4 & 5. The cells can be seen enlarging in this diagram

Vascular cambiumVascular

cambium

6. Occasionally, the inner cells remain vascular cambium and the outer ones become phloem.

Vascular cambium

phloem

Root meristem

• Meristems are areas in plants where mitosis occurs, and due to this cell division, it is also where growth occurs. Apical meristems are responsible for vertical growth and they can be found at the root tips.

• The planes of cell division in the root meristem are strictly ordered, and are primarily transverse divisions that provide growth of the root in length.

• A primary root meristem generates two tissues simultaneously, the main root axis extending proximally towards the shoot, and the root cap pushing relentlessly forward into the soil. Primary roots arise through controlled cell divisions in the apical meristem and subsequent expansion and differentiation of these cells.

Apical meristem and development• Divisions can be in any of three planes,

either anticlinal (normal to the root axis), periclinal (tangential to the root axis) or radial to the axis. These divisions will give rise, respectively, to increased root length, increased root thickness (more layers of cells through the root), or increased root circumference.

• The apical meristem supplies all the cells for the primary root axis and the consequences of the planes of cell division are evident long after meristematic activity ceases.

• Separate cell divisions at the leading edge of the root meristem generate a root cap which extends forward as a protective structure.

• The root apical meristem is covered by the root cap which provides a protective and lubricative function as growth takes place through soil particles. Root caps advance at a dramatic speed: a root might elongate by 5 cm per day and new root cap cells can be pushed in advance of the apex of the primary axis at about the same rate.

• A remarkable feature of root apices is the quiescent centre, a paradox at the heart of the meristem. The quiescent centre is a zone of relatively inactive, slowly dividing cells, numbering about 500–600 in a mature maize root.

Diagram showing longitudinal section of a maise (Zea mays)

root tip. The quiescent centre is shaded dark green.

• The Picture shows the Root Cap (Thimble-like covering which protects the delicate apical meristem), the Apical Meristem (Region of rapid cell division of undifferentiated cells), the Quiescent Center (Populations of cells in apical meristem which reproduce much more slowly than other meristematic cells), the Zone of Cell Division - Primary Meristems (Three areas just above the apical meristem that continue to divide for some time), the Zone of Elongation (Cells elongate up to ten times their original length )and the Zone of Maturation (Region of the root where completely functional cells are found) of a root.

Lateral meristem and development• Lateral root formation is a major determinant of root systems

architecture. The degree of root branching impacts the efficiency of water uptake, acquisition of nutrients and anchorage by plants.

• Lateral roots extend horizontally from the primary root and serve to anchor the plant securely into the soil. This branching of roots also contributes to water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant.

• Many different factors are involved in the formation of lateral roots. Regulation of root formation is tightly controlled by plant hormones such as auxin, and by the precise control of aspects of the cell cycle. Such control can be particularly useful: increased auxin levels, which help to promote lateral root development, occur when young leaf primordia form and are able to synthesise the hormone. This allows coordination of root development with leaf development, enabling a balance between carbon and nitrogen metabolism to be established.

• Lateral root primordia originate from the mature pericycle of the parent root. One of the first events is a periclinal division that generates a double layer of pericycle-derived cells. Cells of the primordia begin to differentiate almost immediately after initiation, as evidenced by differential gene expression in the inner and outer layers.

• Lateral root primordia develop through a characteristic program of cell divisions and expansions to create a fully patterned structure that resembles the primary root tip.

• After the lateral root primordium is formed, it becomes a mature lateral root by a two stage process. First, the primodium emerges through the overlaying tissues by cell expansion. The increase in cell size is particularly apparent in cells near the base of the primordium, while cell number remains relatively unchanged. Second, the new lateral root begins to elongate, and cell numbers increase at the root tip. This is characteristic of mature root elongation via division of cells in the root apical meristem.

• It was found that lateral root formation can be divided into four stages:

1) Differentiation of pericycle cells 2) First morphological event is series of asymmetric

transverse divisions of pericycle cells in three cell files positioned opposite the xylem pole. Although all pericycle cells are morphologically identical, these three files are already fated differently from their neighbors.

Followed by ordered cell divisions and differentiation that generates a lateral root primordium

3) Emergence via cell expansion 4) 'Activation' of lateral root primordia to form a functional

root.

Root-stem transition • The root transition zone concept states that root cells leaving the apical

meristem need to accomplish a transitional stage of cyto-architectural rearrangement, especially of the actin cytoskeleton, in order to perform rapid cell elongation. Cells of this zone also have unique functional and sensorial properties.

• The root and the stem make a continuous structure called the axis of the plant. The vascular bundles are continuous from the root to the stem, but the arrangement of vascular bundles is quite different in the two organs; the stems possess collateral bundles with endarch xylem, whereas the roots possess radial bundles with exarch xylem. The change of position involving inversion and twisting of xylem strands from exarch to endarch type is referred to as vascular transition, and the part of the axis where these changes occur is called transition region.

There exist four types of root-stem transition.

1. In Fumaria, Mirabilis and Dipsacus, and others, each xylem strand of the root divides by radial division forming branches, they swing in their lateral direction; one towards right and the other goes to the left. These branches join the phloem strands on the inside. The phloem strands, do not change their position and also remain unchanged in their orientation. They remain in the form of straight strands continuously from the root into the stem. In this type as many primary bundles are formed in the stem as many phloem strands are formed in the root. Dipsacus

2. In Cucurbita, Phaseolus, Acer and Trapaeolum and others, the xylem and phloem strands fork, the branches of the strands of both swing in lateral direction and join in pairs. After joining in the pairs they remain in the alternate position of the strands in the root. The xylem strands become inverted in their position and the phloem strands do not change their orientation. This way, in the stem, the number of bundles becomes double of the phloem strands found in the root. This type of transition is more commonly found.

Phaseolus vulgaris

3. In Lathyrus, Medicago and Phoenix, the xylem strands do not fork and continue their direct course into the stem. These strands twist through 180 degrees. The phloem strands divide soon and the resulting halves swing in the lateral direction to the xylem positions. The phloem strands join the xylem strands on the outside. In this type as many bundles are formed as there are phloem strands in the root (as in type 1). Lathyrus

tuberosus

4. This type is rarely found and is known in only a few monocotyledons (e.g., Anemarrhena). In this type half of the xylem strands fork and the branches swing in their lateral direction to join the other undivided strands of xylem which become inverted. The phloem strands do not divide but they become united in pairs. These united phloem strands unite with the triple strands of the xylem. Thus, a single bundle of the stem consists of five united strands. In this type half as many bundles are formed in the stem as there are phloem strands in the root.

Zhi-Mu (Anemarrhena asphodeloides)

• The transition zone of root apices is a unique part of the whole plant body. Apart from tip-growing cells, such as root hairs and pollen tubes, the cells of the transition zone have the highest rate of vesicle recycling activity, and their auxin transport shows the highest degree of activity. In this root apex zone synchronized electrical activity has been reported. The activity of auxin-secreting domains of the transition zone is sensitive not only to internal developmental cues but, importantly, also to environmental inputs such as light and gravity.

Dicot roots: • The parenchyma cells of the cortex

store starch and other substances. They have air spaces between the cells which are essential for aeration of the root tissue as they are non photosynthetic.

• The vascular tissue, i.e. the xylem and the phloem forms a central cylinder through the root and is surrounded by the pericycle which is a ring of cells from which lateral roots arise.

• The primary xylem of Dicot roots forms a star shape in the center of the vascular cylinder with usually 3 or 4 points, unlike monocots, there is no central pith of parenchyma cells.

• The Epidermis of the dicot root contains dermal tissue and acts mostly to protect the root

• The Cortex contains ground tissue which stores photosynthetic products. It is active in the uptake of water and minerals.

• The Endodermis is a cylinder once cell thick that forms a boundary between the cortex and the stele. It contains the casparian strip.

• The Pericycle is found just inside of the endodermis. It may become meristematic. It is responsible for the formation of lateral roots.

• Vascular Tissue contains the Xylem and Phloem and forms an X-shaped pattern in very center of root

Figure showing the cross section of a dicot root

Monocot roots • In the Fibrous root system of Monocots, the

primary root is almost non-existent. The secondary roots are important in absorption, but are not as deep as the primary root of most Dicot.

• Because many monocots have shallow root systems (the fibrous system has many secondary roots that spread more on top than they grow deep into the ground), secondary or adventitious roots will be produced.

• Most monocotyledon plants such as grass and onions have fibrous foot systems. The actual root structure differs from the dicots as Monocots tend to have parallel vein systems in their stems.

• In monocots, the first root to emerge from the seed dies off, and so no strong, central tap root forms. Instead, monocots sprout roots from shoot tissue near the base, called adventitious roots. The familiar fibrous root system of grasses is an example of this rooting pattern.

• Many monocots form bulbs, such as onion, gladiolus, and tulips. These are not root structures, but rather modified stems, made of compact leaves.

• The Epidermis contains dermal tissue and it protects the root.

• The Cortex contains ground tissue which stores photosynthetic products. It is also active in the uptake of water and minerals

• The Endodermis is a cylinder once cell thick that forms a boundary between the cortex and the stele. It is even more distinct than Dicot counterpart. It also contains the casparian strip.

• The Vascular Tissue contains the Xylem and the Phloem. It forms a ring near center of plant

• The Pith is the center of most region of root.

Figure showing the cross section of a

monocot

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