Handbook of Plant Science root covered by cuticle but, there is a different muatan dari wax-nya shg msh bs permeable

Plants can grow indeterminately. The ongoing growth of roots is made possible by the capability of the plant to maintain a pool of stem cells. These stem cells keep proliferating and do not differentiate. The daughters of the stem cells differentiate and form the different structures of the root. In the plant root, stem cells are located in the root tip. The stem cells are located in an organized structure, the stem cell niche. The stem cell niche functions as a microenvironment where, by short range cell signaling, the stem cells are maintained. The Quiescent Center (QC) lies in the centre of the root stem cell niche and provides the signals for maintenance of the surrounding stem cells. The organization of the niche is set up early in embryogenesis and stays unchanged in the mature plant. Essential for root patterning in embryogenesis is the phytohormone auxin. Several transcription factors are involved in both root patterning in the embryo and stem cell maintenance in the mature plant. The processes of organization and maintenance of root stem cells are very well studied in the Arabidopsis thaliana and new insights from this model organism will be discussed here.

The root system of a plant constantly provides the stems and leaves with water and dissolved minerals.  In order to accomplish this the roots must grow into new regions of the soil.  The growth and metabolism of the plant root system is supported by the process of photosynthesis occurring in the leaves.   Photosynthate from the leaves is transported via the phloem to the root system.   Root structure aids in this process.  This section will review the different kinds of root systems an look at some specialized roots, as well as describe the anatomy of the roots in monocots and dicots.

Root Systems:

Taproot System:Characterized by having one main root (the taproot) from which smaller branch roots emerge.  When a seed germinates, the first root to emerge is the radicle, or primary root.  In conifers and most dicots, this radicle develops into the taproot.   Taproots can be modified for use in storage (usually carbohydrates) such as those found in sugar beet or carrot.  Taproots are also important adaptations for searching for water, as those long taproots found in mesquite and poison ivy.

Fibrous Root System:Characterized by having a mass of similarly sized roots.  In this case the radicle from a germinating seed is short lived and is replaced by adventitious roots. Adventitious roots are roots that form on plant organs other than roots.  Most monocots have fibrous root systems.  Some fibrous roots are used as storage; for example sweet potatoes form on

fibrous roots.  Plants with fibrous roots systems are excellent for erosion control, because the mass of roots cling to soil particles.

Root Structures and Their Functions:

Root Tip: the end 1 cm of a root contains young tissues that are divided into the root cap, quiescent center, and the subapical region.Root Cap: root tips are covered and protected by the root cap.   The root cap cells are derived from the rootcap meristem that pushes cells forward into the cap region.  Root cap cells differentiate first into columella cells.Columella cells contain amylopasts that are responsible for gravity detection.   These cells can also respond to light and pressure from soil particles.  Once columella cells are pushed to the periphery of the root cap, they differentiate into peripheral cells.  These cells secrete mucigel, a hydrated polysaccharide formed in the dictyosomes that contains sugars, organic acids, vitamins, enzymes, and amino acids.   Mucigel aids in protection of the root by preventing desiccation.  In some plants the mucigel contains inhibitors that prevent the growth of roots from competing plants.  Mucigel also lubricates the root so that it can easily penetrate the soil.   Mucigel also aids in water and nutrient absorption by increasing soil:root contact.   Mucigel can act as a chelator, freeing up ions to be absorbed by the root.   Nutrients in mucigel can aid in the establishment of mycorrhizae and symbiotic bacteria.Quiescent Center: behind the root cap is the quiescent center, a region of inactive cells.  They function to replace the meristematic cells of the rootcap meristem.  The quiescent center is also important in organizing the patterns of primary growth in the root.Subapical Region:  this region, behind the quiescent center is divided into three zones.  Zone of Cell Division - this is the location of the apical meristem (~0.5 -1.5 mm behind the root tip).  Cells derived from the apical meristem add to the primary growth of the root.  Zone of Cellular Elongation - the cells derived from the apical meristem increase in length in this region.  Elongation occurs through water uptake into the vacuoles.  This elongation process shoves the root tip into the soil.  Zone of Cellular Maturation - the cells begin differentiation.  In this region one finds root hairs which function to increase water and nutrient absorption.  In this region the xylem cells are the first of the vascular tissues to differentiate.

Mature Root: the primary tissues of the root begin to form within or just behind the Zone of Cellular Maturation in the root tip.  The root apical meristem gives rise to three primary meristems: protoderm, ground meristem, and procambium.

Epidermis: the epidermis is derived from the protoderm and surrounds the young root one cell layer thick.  Epidermal cells are not covered by cuticle so that they can absorb water and mineral nutrients.  As roots mature the epidermis is replaced by the periderm.Cortex: interior to the epidermis is the cortex which is derived from the ground meristem.  The cortex is divided into three layers: the hypodermis, storage parenchyma cells, and the endodermis.  The hypodermis is the suberinized protective layer of cells just below the epidermis.  The suberin in these cells aids in water retention.  Storage parenchyma cells  are thin-walled and often store starch.  The endodermis is the innermost layer of the cortex.  Endodermal cells are closely packed and lack intercellular spaces.  Their radial and transverse walls are impregnated with lignin an suberin to form the structure called the Casparian Strip.   The Casparian Strip forces water and dissolved nutrients to pass through the symplast (living portion of the cell), thus allowing the cell membrane to control absorption by the root. Stele: all tissues inside the endodermis compose the stele.   The stele includes the outer most layer, pericycle, and the vascular tissues.   The pericycle is a meristematic layer important in production of branch roots.   The vascular tissues are made up of the xylem and phloem.  In dicots the xylem is found as a star shape in the center of the root with the phloem located between the arms of the xylem star.  New xylem and phloem is added by the vascular cambium located between the xylem and phloem.  In monocots the xylem and phloem form in a ring with s the central portion of the root made up of a parenchymatous pith.

Why does the epidermis on a leaf have no chloroplasts in it?The epidermis has numerous functions: protection against various chemical and physical influences, against being fed upon by animals and against infestation by parasites protection of the plant against desiccation participation in gas exchange, in secretion of metabolic compounds and in absorption of water 

site of receptors for light and mechanical stimuli that help to transform signals from the surrounding to the plant The epidermis has accordingly a number of differentiated cell types to serve the various functions.Variations typical for certain species and different organizations of the epidermis in the miscellaneous plant organs add to the number of different cells. Three main types exist: 1. the basic epidermis cell 2. the cells of the stoma complexes and 3. the trichomes (gr.: trichoma = hair), epidermal attachments of varying shape, structure and function 

The basic epidermis cells, i.e. the least specialized cells constitute the largest group of dermal cells. They seem either polygonal or elongated in top view. Their walls are often wavy or sinuate. It is unknown, what induces this shape during development, since the explanations given by the existing hypotheses seem insufficient. Elongated epidermis cells can be found at organs or parts of organs that are elongated themselves, like stems, leaf petioles, leaf veins or leaves of monocots. The epidermises of the leaf's upper- and undersurface may have different structures. The shape of the cells, the thickness of the walls as well as the distribution and number of specialized cells (guard cells and trichomes) per area may all vary. Wide varieties of different cell shapes may even exist in species of a single family, e. g. in the Crassulaceae family 

The wall of epidermal cells that constitute the leaf's surface is often thicker than the other walls. This can be particularly well observed with the epidermis of conifer needles and that of xerophytes (plants living in dry habitats). Aquatic plants have usually thin walls. The wall of many seeds becomes stronger during ripening and may fill nearly all of the cell's lumen so that the protoplast is driven out and degenerates. The basic epidermal cells of most species contain no chloroplasts. Some ferns and several aquatic or shade plants are exceptions. 

The epidermis is more often than not built from a single cell layer, though multi-layered, water-storing epidermises that evolved from initially single-layered tissues by periclinal division have been shown among the species of several families (Moraceae: most Ficus-species, Piperaceae: Peperonia, Begoniaceae, Malvaceae and others). Epidermis cells secrete a cuticle, that covers all epidermal surfaces like an uninterrupted film. It may either be smooth or structured by bulges, rods, filaments, folds, or furrows. Although leaves are green most of the epidermal cells do not have functional chloroplasts. In most plants guard cells are the only cells within a leaf's epidermis that contain chloroplasts It does not function as a photosynthesizing tissue since it has various other primary functions thus in the process of evolution to increase its efficiency it may have lost its chloroplast Although it should be noted that it is not entirely true that epidermal cell do not have chlp there are al

The lack of chloroplast is due to the function of the epidermis. The epidermis is transparent; to allow light to enter, it's waxy to prevent water loss due to evaporation and it is the first barrier for the leaf from the external environment very much like human skin. The epidermis protects, regulates and keeps the leaf airtight.

The absence of chloroplast in the epidermal layers clearly indicate the function of epidermis is not to prepare food but to protect the internal tissues.

The epidermis cells (from the Greek "επίδερμίδα", meaning "over-skin") is a single-layered group of

cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and

the external environment. The epidermis serves several functions, it protects against water loss,

regulates gas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and

mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and

lower (abaxial) surfaces have somewhat different construction and may serve different functions.

Woody stems and some other stem structures produce a secondary covering called the periderm that

replaces the epidermis as the protective covering.

Contents

  [hide] 

1   Description

2   Guard cells

3   Cell differentiation in the epidermis

4   See also

5   References

6   External links

Description[edit]

The epidermis is the outermost cell layer of the primary plant body. In some older works the cells of

the leaf epidermis have been regarded as specialised parenchyma cells,[1] but the established modern

preference has long been to classify the epidermis as dermal tissue, whereas parenchyma is

classified as ground tissue.[2] The epidermis is main component of the dermal tissue system of leaves

(diagrammed below), and also stems, roots, flowers, fruits, and seeds; it is

usually transparent (epidermal cells have a lower number of chloroplasts or lack them completely,

except for the guard cells.)

The cells of the epidermis are structurally and functionally variable. Most plants have an epidermis

that is a single cell layer thick. Some plants like Ficus elastica and Peperomia, which have periclinal

cellular division within the protoderm of the leaves, have an epidermis with multiple cell layers.

Epidermal cells are tightly linked to each other and provide mechanical strength and protection to the

plant. The walls of the epidermal cells of the above ground parts of plants contain cutin, and are

covered with a cuticle. The cuticle reduces water loss to the atmosphere, it is sometimes covered

with wax in smooth sheets, granules, plates, tubes or filaments. The wax layers give some plants a

whitish or bluish surface color. Surface wax acts as a moisture barrier and protects the plant from

intense sunlight and wind.[3] The underside of many leaves have a thinner cuticle than the top side,

and leaves of plants from dry climates often have thickened cuticles to conserve water by reducing

transpiration.

The epidermal tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary

cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least

specialized. These are typically more elongated in the leaves of monocots than in those of dicots.

Trichomes or hairs grow out from the epidermis in many species. In root epidermis, epidermal hairs,

termed root hairs are common and are specialized for absorption of water and mineral nutrients.

In plants with secondary growth, the epidermis of roots and stems is usually replaced by a periderm

through the action of a cork cambium.

Guard cells[edit]

Stoma in a tomato leaf (microscope image)

Main article: Stoma

The leaf and stem epidermis is covered with pores called stomata (sing., stoma), part of a stoma

complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and

two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of

gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are

more numerous over the abaxial (lower) epidermis of the leaf than the (adaxial) upper epidermis. An

exception is floating leaves where most or all stomata are on the upper surface. Vertical leaves, such

as those of many grasses, often have roughly equal numbers of stomata on both surfaces. The stoma

is bounded by two guard cells. The guard cells differ from the epidermal cells in the following aspects:

The guard cells are bean-shaped in surface view, while the epidermal cells are irregular in shape

The guard cells contain chloroplasts, so they can manufacture food by photosynthesis (The

epidermal cells do not contain chloroplasts)

Guard Cells are the only epidermal cells that can make sugar. According to one theory, in

sunlight the concentration of potassium ions (K+) increases in the guard cells. This, together with

the sugars formed, lowers the water potential in the guard cells. As a result, water from other cells

enter the guard cells by osmosis so they swell and become turgid. Because the guard cells have

a thicker cellulose wall on one side of the cell, i.e. the side around the stomatal pore, the swollen

guard cells become curved and pull the stomata open.

At night, the sugar is used up and water leaves the guard cells, so they become flaccid and the

stomatal pore closes. In this way, they reduce the amount of water vapour escaping from the leaf.

Cell differentiation in the epidermis[edit]

Scanning electron microscope image ofNicotiana alata leaf's epidermis, showingtrichomes (hair-like appendages)

andstomata (eye-shaped slits, visible at full resolution)

The plant epidermis consists of three main cell types: pavement cells, guard cells and their subsidiary

cells that surround the stomata and trichomes, otherwise known as leaf hairs. The epidermis of petals

also form a variation of trichomes called conical cells. These cells all develop from the pavement cells,

which make up the majority of the plants surface cells. In short, cellular differentiation of the epidermal

cells is controlled by two major factors:genetics and environmental conditions.

Trichomes develop at a distinct phase during the actual leaf development, under the control of two

major trichome specification genes: TTG and GL1. The process may be controlled by the plant

hormones gibberellins, and even if not completely controlled, gibberellins certainly have an effect on

the development of the leaf hairs. GL1 causes endoreplication, the replication of DNA without

subsequent cell division as well as cell expansion. GL1 turns on the expression of a second gene for

trichome formation, GL2, which controls the final stages of trichome formation causing the cellular

outgrowth.

Arabidopsis thaliana uses the products of inhibitory genes to control the patterning of trichomes, such

as TTG and TRY. The products of these genes will diffuse into the lateral cells, preventing them from

forming trichomes and in the case of TRY promoting the formation of pavement cells.

As previously mentioned, conical cells are a form of trichome that occurs on the petals of flowers.

Expression of the gene MIXTA, or its analogue in other species, later in the process of cellular

differentiation will cause the formation of conical cells over trichomes. MIXTA is a transcription factor.

Stomatal patterning is a much more controlled process, as the stoma effect the plants water retention

and respiration capabilities. As a consequence of these important functions, differentiation of cells to

form stomata is also subject to environmental conditions to a much greater degree than other

epidermal cell types.

Stomata are holes in the plant epidermis that are surrounded by two guard cells, which control the

opening and closing of the aperture. These guard cells are in turn surrounded by subsidiary

cells which provide a supporting role for the guard cells.

Stomata begin as stomatal meristemoids. The process varies between dicots and monocots. Spacing

is thought to be essentially random in dicots though mutants do show it is under some form of genetic

control, but it is more controlled in monocots, where stomata arise from specific asymmetric

divisions of protodermal cells. The smaller of the two cells produced becomes the guard mother cells.

Adjacent epidermal cells will also divide asymmetrically to form the subsidiary cells.

Because stomata play such an important role in the plants survival, collecting information on their

differentiation is difficult by the traditional means of genetic manipulation, as stomatal mutants tend to

be unable to survive. Thus the control of the process is not well understood. Some genes have been

identified. TMM is thought to control the timing of stomatal initiation specification andFLP is thought to

be involved in preventing further division of the guard cells once they are formed.

Environmental conditions affect the development of stomata, in particular their density on the leaf

surface. It is thought that plant hormones, such as ethylene and cytokines, control the stomata’s

developmental response to the environmental conditions. Accumulation of these hormones appears to

cause increased stomatal density such as when the plants are kept in closed environments.

Stomatal cells only occur on the leaf epidermis, and it is thought that inhibitory signals must occur on

other parts of the plants epidermis to prevent stomatal formation there. These signals could be

hormonal, or perhaps gene products transmitted from underlying tissues via the plasmodesmata

Why onion epidermal cells do not contain Chloroplast?

They don't need it. An onion plant has two parts- 1. Onion bulbs (the part which is pink in color) and 2.onion leaves. Onion bulbs grow 'under' the soil while onion leaves are slender and straight and grow above the ground towards the sunlight just like other plants do. The onion 'leaves' contain chloroplasts not the 'bulbs'. You can see in the following picture how onion leaves look like and remember that the onion bulbs are growing just underneath the soil.

Do the epidermal cells of a leaf contain chloroplast?

No, regular epidermal cells do not have chloroplasts. The guard cells that surround the stoma do have chloroplasts, but they are not the regular epidermal cells.

Why doesn't the upper epidermis of the leaf have chloroplasts?The epidermis serves several functions: protection against water loss, regulation of gas exchange (although, this is mostly served at the lower epidermis), secretion of metabolic compounds, and (in some species) absorption of water. 

The epidermis is usually transparent (as you said, epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss.

Epidermis

The epidermis is the outermost cellular layer which covers the whole plant structure, i.e. it covers roots, stem, leaves, flowers and fruit. It is composed of a single layer of living cells, although there are exceptions. Epidermis is usually closely packed, without intercellular spaces or chloroplasts. The outer walls, which are exposed to the atmosphere and usually thickened, and may be covered by a waxy, waterproof cuticle which are made up ofcutin. Apart from the normal epidermal cells there are also stomata in the epidermis of leaves and stem. A stoma is an opening (pore) which is bounded by two beanshaped cells called guard cells . The guard cells differ from normal epidermal cells in that they have chloroplasts and the cell walls are thickening unevenly; the outer wall is thin and the inner wall (nearest the opening) is thick. The thin-walled epidermal cells of roots give rise toroot hairs. Hair- like outgrowths may also be found in the epidermis of leaves and stems.

Functions:

o the epidermal cells protect the underlying cells,o the waxy cuticle prevents the loss of moisture from the leaves and stems,o the transparent epidermal cells allow sunlight (for photosynthesis) to pass

through to the chloroplasts in the mesophyll tissue,o the stomata of leaves and stems allow gaseous exchange to take place

which is necessary for photosynthesis and respiration,o water vapour may be given off through the stomata during transpiration,o the root-hairs absorb water and dissolved ions from the soil

Root epidermis not covered with cuticle

Roots take in water and the stem tries to conserve water. That simple.

o The cuticle is water proof - the root epidermis eneeds to be semi-permiable for

o osmosis to occur

EpidermisThe root hairs of the young epidermal cells vastly increase the surface area through

which movement of materials can occur. The thread-like hairs are simply enlargements

of the protoplast that extend outward into the soil. They have little wall material and are

extremely fragile and easily broken. The root epidermis of some plants is covered by a

thin, waxy cuticle, which apparently isn't thick enough to impede movement of

substances through the epidermis.

CortexThe cortex, composed primarily of parenchyma cells, is the largest part of the primary

root, but in most dicots (eudicots) and in gymnosperms that undergo extensive

secondary growth, it is soon crushed, and its storage function assumed by other tissues.

Three layers of cortex are recognized: the hypodermis (also called exodermis),

the endodermis and, between them, the storage parenchyma. The outer and inner

layers of the cortex, the hypodermis and endodermis, are cylinders of tightly packed cells

with heavily suberized walls and no intercellular spaces. (Suberin is the fatty substance

that gives cork its distinctive attributes.) In contrast, the storage parenchyma cells are

thin-walled and loosely packed with many intercellular spaces among them.

Hypodermis (exodermis). Just under the epidermis forming the outermost layer of the

cortex is a layer one or two cells in width called the hypodermis. Since its cell walls are

heavily suberized and impermeable to water its apparent function is to keep the water

and nutrients (which are absorbed in the root zone further down the root) from leaking

out through the cortex. The hypodermis is especially well developed in plants of arid

regions and in those with shallow root systems. It also deters the entrance of soil

microorganisms.

Endodermis. The innermost layer of the cortex is the endodermis, which is readily

identifiable by the presence of Casparian strips, bands of suberin present on

transverse and radial walls of its cells—the walls perpendicular to the surface of the root.

The endodermis regulates the passage of water and dissolved substances by forcing

them to move through living plasma membranes and plasmodesmata and not simply

diffuse through the porous cell walls. The absorption and translocation of materials is

thus selective; not everything in the surrounding soil gets through and into the plant

body. An endodermis almost always is present in roots and generally never in stems.

Storage parenchyma. The bulk of the cortex consists of thin-walled, living parenchyma

cells, which store starch and other substances. The cells expand or shrink as materials

move in and out of their protoplasts. The large volume of air present in the intercellular

spaces of this tissue provides important aeration for roots.

Stele (vascular cylinder)The stele includes all of the tissues inside of the cortex: the pericycle, the vascular

tissues—xylem and phloem—and, in some plants, a pith. Most dicot (eudicot) roots have

a solid core of xylem in their center whereas most monocots have a pith composed of

parenchyma.

Pericycle. The pericycle is a cylinder of parenchyma, one or at most a few cells in width,

which lies in the stele immediately inside the endodermis. The cells retain their ability to

divide throughout their lives, and localized divisions in the pericycle give rise to lateral

(branch) roots. When secondary growth occurs in roots, the vascular cambium and

usually the first cork cambium originate in the pericycle. Other cell divisions in the

pericycle produce additional pericycle cells.

Vascular tissues. Most dicot (eudicot) roots differ from eudicot stems in having a lobed

column of primary xylem as their core with phloem tissue occurring as strings of cells

between the lobes. This arrangement is called a protostele. The primary xylem of

monocots, on the other hand, forms a cylinder around a central mass of pith

parenchyma, a siphonostele. The way in which the vascular tissues develop is useful in

tracing ancestral relationships in the plant kingdom.

Taiz and Zeiger on line

Topic 1.4Plant Tissue Systems: Dermal, Ground, and Vascular

Dermal Tissue

The epidermis is the dermal tissue of young plants undergoing primary growth (see textbook Figure 1.3). It is generally composed of specialized, flattened polygonal cells that occur on all plant surfaces. Shoot surfaces are usually coated with a waxy cuticle to prevent water loss and are often covered with hairs, or trichomes, which are epidermal cell extensions.

Pairs of specialized epidermal cells, the guard cells, are found surrounding microscopic pores in all leaves (see textbook Figure 1.3A). The guard cells and pores are called stomata (singular stoma), and they permit gas exchange (water loss, CO2 uptake, and O2 release or uptake) between the atmosphere and the interior of the leaf.

The root epidermis is adapted for absorption of water and minerals, and its outer wall surface typically does not have a waxy cuticle. Extensions from the root epidermal cells, the root hairs , increase the surface area over which absorption can take place (see textbook Figure 1.3C).

Ground tissue. Making up the bulk of the plant are cells termed the ground tissue. There are three types of ground tissue: parenchyma, collenchyma, and sclerenchyma.

Parenchyma, the most abundant ground tissue, consists of thin-walled, metabolically active cells that

carry out a variety of functions in the plant, including photosynthesis and storage (see textbook Figure 1.3B)

Collenchyma tissue is composed of narrow, elongated cells with thick primary walls (see textbook Figure 1.3C). Collenchyma cells provide structural support to the growing plant body, particularly shoots, and their thickened walls are nonlignified, so they can stretch as the organ elongates. Collenchyma cells are typically arranged in bundles or layers near the periphery of stems or leaf petioles.

Sclerenchyma consists of two types of cells, sclereids and fibers (see textbook Figure 1.3D) Both have thick secondary walls and are frequently dead at maturity. Sclereids occur in a variety of shapes, ranging from roughly spherical to branched, and are widely distributed throughout the plant. In contrast, fibers are narrow, elongated cells that are commonly associated with vascular tissues. The main function of sclerenchyma is to provide mechanical support, particularly to parts of the plant that are no longer elongating.

In the stem, the pith and the cortex make up the ground tissue (see textbook Figure 1.1B). The pith is located within the cylinder of vascular tissue, where it often exhibits a spongy texture because of the presence of large intercellular air spaces. If the growth of the pith fails to keep up with that of the surrounding tissues, the pith may degenerate, producing a hollow stem. In general, roots lack piths, although there are exceptions to this rule. In contrast, the cortex , which is located between the epidermis and the vascular cylinder, is present in both stems and roots (see textbook Figure 1.1B and C).

At the boundary between the ground tissue and the vascular tissue in roots, and occasionally in stems, is a specialized layer of co rtex known as the endodermis (see textbook Figure 1.1C). This single layer of cells originates from cortical tissue at the innermost layer of the root cortex and forms a

cylinder that surrounds the central vascular tissue, or stele. Early in root development, a narrow band composed of the waxy substance suberin is formed in the cell walls circumscribing each endodermal cell (see textbook Figure 1.1). These suberin deposits, called Casparian strips, form a barrier in the endodermal walls to the intercellular movement of water, ions, and other water-soluble solutes to the vascular cells.

Leaves have two interior layers of ground tissue that are collectively known as the mesophyll (see textbook Figure 1.1A). The palisade parenchyma consists of closely spaced, columnar cells located beneath the upper epidermis. There is usually one layer of palisade parenchyma in the leaf. Palisade parenchyma cells are rich in chloroplasts and are a primary site of photosynthesis in the leaf. Below the palisade parenchyma are i rregularly shaped, widely spaced spongy mesophyll cells. The spongy mesophyll cells are also photosynthetic, and the large spaces between these cells allow diffusion of carbon dioxide. The spongy mesophyll also contributes to leaf flexibility in the wind, and this flexibility facilitates the movement of gases within the leaf.

Vascular tissues: xylem and phloem. The vascular tissue is composed of two major conducting systems: the xylem and the phloem. The xylem transports water and mineral ions from the root to the rest of the plant. The phloem distributes the products of photosynthesis and a variety of other solutes throughout the plant (see textbook Figure 1.1B and C).

The tracheids and vessel elements are the conducting cells of the xylem (see textbook Figure 1.3E). Both of these cell types have elaborate secondary-wall thickenings and lose their cytoplasm at maturity; that is, they are dead when functional. Tracheids overlap each other, whereas vessel elements have open end walls and are arranged end to end to form a larger unit called a vessel. Other cell types present in the xylem include parenchyma cells, which are important for the storage of energy-rich molecules and phenolic compounds, and sclerenchyma fibers.

The sieve elements and sieve cells are responsible for sugar translocation in the phloem (see textbook Figure 1.3E). The former are found in angiosperms; the latter perform the same function in gymnosperms. Like vessel elements, sieve elements are often stacked in vertical rows, forming larger units called sieve tubes, whereas sieve cells form overlapping arrays. Both types of conducting cells are living when functional, but they lack nuclei and central vacuoles and have relatively few cytoplasmic organelles.

Substances are translocated from sieve cell to sieve cell laterally through circular or oval zones containing enlarged pores, called sieve areas. In contrast, sieve tubes translocate substances through large pores in the end walls of the sieve elements, called sieve plates. Sugar movement through sieve tubes is more efficient and rapid than through sieve cells and represents a more evolutionarily advanced mechanism.

Sieve elements are associated with, and depend on, densely cytoplasmic parenchyma cells calledcompanion cells. The analogous cells adjacent to the sieve cells of gymnosperms are called albuminous cells. Companion cells provide proteins and metabolites necessary for the functions of the sieve tube elements. In addition, the phloem frequently contains storage parenchyma and fibers that provide mechanical support.

Chapter 46 - Plant Structure

Monocots Vs Dicots

Monocots Dicots

Number of Cotyledons one two

Vascular Tissue - Roots arranged in a ring phloem between arms of xylem

Vascular Tissue - Stems bundles scattered bundles form a ring

Veins in Leaves parallel net-like pattern

Number of Flower Parts 3 or multiples of 3 4 or 5 or multiples of 4 or 5

 

Roots and Shoots

Roots anchor, absorb H2O and minerals, and store starch.

The shoot is the above-ground portion of the plant. The stem transports water and minerals to the leaves and sugar to the roots.

 

Tissue types

Plants have four tissue types.

Vascular tissue transports, dermal tissue protects, meristematic tissue grows (cells divide), and ground tissue forms the rest of the plant.

Vascular tissue

Xylem

transports water and minerals from roots to leaves

composed of hollow, nonliving cells

tracheids- elongated with tapered ends, pits or depressions

vessel elements- larger, forms a continuous pipeline

Phloem

Transports organic nutrients, usually from leaves to roots

Phloem cells are living.

sieve-tube cells- no nucleus; connected to each other by plasmodesmata

Sieve plates are found at the ends of the cells.

Companion cells contain a nucleus and are located in close proximity to sieve-tube cells. They are connected to sieve tube cells by plasmodesmata.

 

Meristematic tissue

Areas within the plant that are capable of growth (cell division) are called meristems.

Primary Growth

Primary growth occurs only at the shoot and root tips in areas called apical meristems. Primary growth is responsible for elongating the plant. In areas that contain only primary growth, stem thickness increases by cell enlargement, not by the production of new cells.

Secondary Growth

Lateral meristems produce new cells that make the stems and roots thicker. This type of growth is called secondary growth. Secondary growth occurs only during the second and subsequent years and only in woody species.

There are two kinds of lateral meristems, the vascular cambium and the cork cambium. These lateral meristems form as rings within the plant body as the stem increases in thickness. The diagrams below illustrate how the vascular cambium divides to produce new xylem cells toward the inside of the vascular cambium and new phloem cells toward the outside.

The vascular cambium cells divide longitudinally.

One of the new cells remains vascular cambium and the other becomes xylem. The cells can be seen enlarging in this diagram.

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

 

Dermal tissue

outer covering of plant

It consists of closely packed cells that function to protect.

The epidermis covers the plant but is replaced by cork (periderm) in the stems and roots of woody plants.

The epidermis has a waxy covering called a cuticle that protects the plant from desiccation.

The periderm is the outer part of the bark.

 

Ground tissue

Ground tissue fills interior of plant. It contains parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma

thin-walled

least specialized of the three cell types

found in all organs

usually functions in photosynthesis or storage

photosynthetic parenchyma have chloroplasts

parenchyma that function for storage have colorless plastids

Parenchyma can divide to produce more specialized types of cells.

Collenchyma

Collenchyma cells have thicker primary cell walls, especially at the corners.

A primary cell wall is one that is produced while the cell is growing.

Collenchyma often forms bundles just beneath the epidermis for flexible support of immature parts of the plant body.

Sclerenchyma

Sclerenchyma cells have thick secondary cell walls, usually toughened with lignin.

A secondary cell wall is one that is produced after the cell is mature. It is produced inside the primary cell wall.

Most sclerenchyma cells are nonliving. They function to support mature regions and produce hard parts (example: nut shells).

omit: contain fibers  (long and slender) and sclereids  (shorter, varied shape)

example: sclereids make nut shells hard

Primary growth of stem

Primary growth occurs only in apical meristems which are located at the tips of the stems and roots.

Meristems in stems are protected by newly formed leaves within a bud.

 

Axillary buds

usually dormant

in the axes of mature leaves

develop into branches

 

Herbaceous stems (nonwoody

Herbaceous stems are produced by primary growth.

The outermost tissue is epidermis and is covered by waxy cuticle to prevent water loss.

The vascular tissue is found in bundles that are arranged in a ring (dicots) or scattered (monocots).

In dicots, the xylem is toward inside; the phloem is toward the outside.

cortex- In dicot stems, the cortex is located in the area between the vascular bundles and the epidermis. In monocot stems, it occupies the area surrounding the vascular bundles.

The center of the stem is pith and may function as storage.

 

Secondary Growth of Stems

Secondary growth occurs in plants that live > 1 year.

primary growth occurs for a short distance behind the apical meristem, then secondary growth   occurs.

It begins with the formation of a vascular cambium and a cork cambium.

 

ebVascular cambium

Initially, vascular cambium is found between the xylem and phloem in the vascular bundles of dicots.

After one years growth, it joins to form a continuous ring.

Cell division toward the inside and outside form xylem and phloem.

Seasonal climates produces growth rings because cells grow faster and are larger in the spring than later in the growing season.

 

Cork cambium

Cortex cells beneath the epidermis produce the cork cambium.

The cork cambium produces cork.

Cork is waterproof because the cell walls are impregnated with of suberin.

Pockets of cells lack suberin. These are called lenticels and function to allow gas exchange.

Cork replaces the epidermis on woody stems and roots.

 

Bark

The bark of trees consists of cork, cork cambium, cortex, and phloem.

 

Summary of Stem Growth

Primary Growth Lateral Meristems Secondary Growth

Dermal Tissue epidermis cork

Ground Tissue cortex

pith

Meristem Tissue vascular cambium cork cambium

vascular cambium

Vascular Tissue primary phloem

primary xylem

secondary xylem and phloem

 

Stem External Structure

Stems support, conduct, store water and photosynthate (products of photosynthesis).

nodes- where leaves attach

internodes   - between nodes

bud - contains apical meristem and newly-forming leaves

leaves - photosynthetic organs

 

Types of stems

stolons (runners)- horizontal, aboveground- strawberries

rhizomes- horizontal, underground; responsible for rapid spread of many weeds

tubers- enlarged tips of rhizomes; food storage- potato

corms- underground, short, thick, vertical; food storage- gladiolus

bulbs- underground with thick, fleshy leaves- onion

tendrils- assist plant in climbing

 

Leaves

Leaves usually function in photosynthesis, so they are flattened to increase the surface exposed to light.

blade, petiole

simple, compound

pinnate, palmate

opposite,alternate, whorled

vary according to environmental conditions

broad in shade

reduced in dry areas (ex: spines in cacti)

succulent leaves hold water

can be adapted for food storage (onions)

climbing leaves can be modified as tendrils

Monocot leaves have parallel veins; dicot leaves have a net-like pattern.

The top layer is the epidermis, a type of dermal tissue. It often has protective hairs and/or glands that produce irritants.

always a waxy cuticle

mesophyll: parenchyma cells w/ chloroplasts

Stomata

Stomata (sing. stoma) are openings in the epidermis of leaves and stems that allow gas exchange.  Guard cells surround the opening and function to open or close it.  Guard cells that contain chloroplasts, other epidermal cells do not contain chloroplasts

When K+ is pumped into the guard cell by active transport (requires ATP), water follows by osmosis. This causes the cells to bend and open. When K+ (and water) leaves the guard cells, they close.

 

C3 plants

palisade layer and spongy layer

The loss of water from the leaves by evaporation is called transpiration. It accounts for more than 90% of water taken up by the roots.

 

Dicot roots

zone of cell division- root apical meristem; just behind the root cap

zone of elongation- area where cells elongate; become more specialized

zone of maturation (differentiation)

cells mature and become fully differentiated (specialized)

The epidermal cells form root hairs in this zone. Root hairs increase the absorptive surface area.

 

Specialized tissues of roots

Epidermis

outer layer consisting of rectangular-shaped cells

Root hairs are extensions of epidermal cells that project 5-8 mm into the soil. They increase the surface area of the root for absorption.

Cortex

interior to epidermis

large thin-walled parenchyma; loosely packed

water can move through cortex without entering cells

starch granules in cortex function for storage

Vascular tissue

xylem- star-shaped

phloem- between rays of xylem

Endodermis

The endodermis is a single layer of cells that forms a boundary between the cortex and the inner vascular cylinder.

The endodermis is lined on 4 sides by the Casparian strip. The casparian strip is a coating that prevents water from seeping between the cells and thus forces water to enter the endodermal cells before passing through to the vascular cylinder.

The Casparian strip surrounds cells of the endodermis and prevents water and minerals from seeping between the cells. In order to get to the vascular cylinder, water and minerals must pass through the cell membrane.

Without the casparian strip, water and minerals would be able to enter the vascular cylinder by going between cells.

Pericycle

The pericycle is the layer just inside the endodermis.

It retains the capacity to divide and form branch roots.

 

Monocot roots

Unlike monocot stems, the vascular tissue in monocot roots is arranged in a ring.

Monocot roots are like dicot roots in that they contain pericycle, endododermis, cortex (outside of vascular tissue), and an epidermis.

The central portion of the root is called pith. It is composed of parenchyma and functions in storage.

Monocot roots typically have no secondary growth.

 

Secondary growth in roots

Secondary growth in roots is similar to stems; annual growth rings are formed.

Vascular cambium forms between xylem and phloem.

The pericycle produces the cork cambium.

Primary Growth in a dicot root.

As the root increases in diameter, the vascular cambium becomes circular.

 

Summary of Root Growth

Primary Growth Lateral Meristems Secondary Growth

Dermal Tissue epidermis cork

Ground Tissue cortex

pith

Meristem Tissue

pericycle

vascular cambium

cork cambium

vascular cambium

Vascular Tissue primary xylem and phloem secondary xylem and phloem

 

Root Systems

Dicots

the primary (first) root grows straight down; called a taproot

often fleshy and stores food; ex: carrots, beets, turnips, radishes

 

Monocots

fibrous root system, no main root

Adventitious roots- new roots that arise from an aboveground structure; example- prop roots on corn

 

Stems Roots

 

 

Primary

Growth

(Monocots)

 

 

Primary

Growth

(Dicots)

 

 

Secondary

Growth

(Dicots)

 

A plant cuticle is a protective film covering the epidermis of leaves, young shoots and other

aerial plant organs without periderm. It consists of lipid and hydrocarbon polymers impregnated with

wax, and is synthesized exclusively by the epidermal cells.[1] A cuticle is present in

the sporophytegeneration of hornworts, in both sporophyte and gametophyte generations

of mosses [2] and in the sporophytes of all vascular plants. Inangiosperms the cuticle tends to be

thicker on the top of the leaf (adaxial surface), but is not always thicker in xerophytic plants living in

dry climates than in mesophytic plants from wetter climates, despite a persistent myth to that effect.

The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with

soluble waxes. Cutin, a polyester polymercomposed of inter-esterified omega hydroxy acids which are

cross-linked by ester and epoxide bonds, is the best-known structural component of the cuticular

membrane.[3][4] The cuticle can also contain a non-saponifiable hydrocarbon polymer known as Cutan.[5] The cuticular membrane is impregnated with cuticular waxes[6] and covered with epicuticular waxes,

which are mixtures of hydrophobic aliphatic compounds, hydrocarbons with chain lengths typically in

the range C16 to C36.[7]

The plant cuticle is one of a series of innovations, together with stomata, xylem and phloem and

intercellular spaces in stem and later leafmesophyll tissue, that plants evolved more than 450 million

years ago during the transition between life in water and life on land.[8] Together, these features

enabled upright plant shoots exploring aerial environments to conserve water by internalising the gas

exchange surfaces, enclosing them in a waterproof membrane and providing a variable-aperture

control mechanism, the stomatal guard cells, which regulate the rates of transpiration and

CO2exchange.

In addition to its function as a permeability barrier for water and other molecules, the micro and nano-

structure of the cuticle confer specialised surface properties that prevent contamination of plant

tissues with external water, dirt and microorganisms. Aerial organs of many plants, such as the leaves

of the sacred lotus (Nelumbo nucifera) have ultra-hydrophobic and self-cleaning properties that have

been described by Barthlott and Neinhuis (1997).[9] The lotus effect has uses in biomimetic technical

materials.

"The waxy sheet of cuticle also functions in defense, forming a physical barrier that resists penetration

by virus particles, bacterial cells, and the spores or growing filaments of fungi".

DO NOT PRINT THIS PAGE. Click HERE for your print-friendly copy.

The Root

This organ is the first to emerge from a seed.

Its functions are

anchor the plant to its substrate absorb water and inorganic substances

from the substrate conduct the above upwards to the rest of

the plant production (in meristems) of certain

hormones that are transported other parts of the plant

production of secondary metabolites (e.g., nicotine in tobacco, which is transported to the leaves for deposition as an herbivore deterrent)

storage of nutrients as carbohydrates and/or lipids

Origin of the Root

The primary root is the first root that forms in and emerges from the seed.

In most other plants, the primary root develops into a taproot, a large, central root from which lateral roots emerge.

Root depth depends on environment

Porous soil - deep roots; rocky soil - shallower roots

Temperature and soil water content also affect root depth.

And of course, different species have different root characteristics

spruce, beech, poplar, tend to have relatively shallow roots oaks, most pines, maples, sycamores, etc, tend to have taproots almost as tall as the

aboveground portion of the plant. What about southern Florida root depth? The dilemma of hurricanes! record root depth is held by the mesquite: just over 53 METERS. (Why mesquite?)

The main "feeder roots" spread in the topmost soil layers, usually no deeper than about a meter. Roots tremendously increase plant surface area where the plant needs it: for water absorption.

Root surface area is much greater than shoot surface area.

Root and shoot are balanced: If the roots are damaged, the shoot may die back.If the shoot is damaged, there's less photosynthate available for new root growth. In the most derived plants (monocots), the taproot is replaced by a system of fibrous roots that all emerge in a bunch at the base of the stem.

Root Growth and DevelopmentLet's have a look at where it all begins: The Root Tip.

Again, note the relative locations of the apical and three primary meristems.

Can you see the quiescent center? Labeled with a radioactive nucleoide, the tip shows up on an autoradiograph with actively dividing nuclei showing dark (taking up the marker), and relatively inactive cells with invisible nuclei (not taking up marker).

The meristematic quiescent center

gives rise to the root cap cells, replacing them as the old ones are sloughed off organizes the growth pattern of the three primary meristems which also arise from it

Root Cap

The root cap is a protective cap of live parenchyma cells

it is produced by the apical meristem behind it it protects the root meristem as it grows through the soil outermost cells are sloughed off as the root tip "pushes" through the soil root cap cells produce a slimy lubricant (mucilage or mucigel)

o it's the plant equivalent/analog of mucuso hydrophilic polysaccharide (a type of pectin)o also contains sugars, organic acids, vitamins, enzymes, and amino acids.o produced in the Golgi and packaged in vesicles that attach to the cell wall and empty

their contents.o the goo passes through the cell wall and oozes out onto the root cap surface.o mucigel allows greater ease of transporting ions from interstitial soil water to the

root surface, where it can be absorbed.o mucigel is friendly to nitrogien-bacteria, and may help them colonize roots of

nitrogen-fixing plant species (primarily in the Pea Family, Fabaceae)o may provide protection against desiccation.o in some plants, mucigel contains inhibitors that prevent the growth of roots from

competing plants. (allelopathy)o aids in water and nutrient absorption by increasing soil/root contact.o mucigel can chelate soil ions, making them available to the root.o mucigel components can aid in the establishment of mycorrhizae and symbiotic

bacteria. The root cap cells last 4-9 days, depending on plant species and growth rate before they become root cap cells, root cap meristem cells differentiate into the cells of

the columella a central column of cells that contain starchy amyloplasts. In response to gravity, the amyloplasts fall to the bottom of the cells, attracting hormones

that promote growth in the direction of the amyloplasts. Collumella cells are also light-sensitive, and respond to pressure from soil particles, further

signalling to the plant which way is down. By the time columella cells are pushed to the periphery of the root cap by other developing

cells behind them, they have differentiated into peripheral root cap cells. Peripheral root cap cells secrete mucigel that is manufactured in their dictyosomes

(modified Golgi apparatus).

Root MeristemsThe apical meristem (from the Greek merismos, which means "division") is found at the very tip of the root, just behind the root cap.As in all meristems, the apical meristem contains some cells that will always remain meristematic: one daughter cell remains in the meristem (the initial) and continues to divide, whereas its sister cell (the derivative) stays behind as the meristem grows out. The derivative differentiates into some type of cell, depending on its gene expression. This is known as primary growth.

The very end of the root tip contains the initials and the immediate derivatives, and is known as the promeristem. Note the relative locations of the apical and three primary meristems.

(What's different about this quiescent center? Why?)

Region of Cell Division - the apical meristem

Region of Elongation - growing cells elongating (primary meristems)

Region of Maturation - cells mature, no longer elongating (mature tissues)

Root Primary StructureRecall that any plant organ has three main layers:

epidermis (and derivatives)

vascular tissue

ground tissue

Root Anatomy: Cross Sectional View

From outermost layer to innermost:

epidermis cortex endodermis - selectively permeable layer; unique to roots

pericycle - a secondary/lateral meristem that gives rise only to side branch roots; unique to roots

vascular cylinder (a.k.a. stele)

Root Epidermis

Root epidermis is the surface that meets the environment, and it is the first selectively permeable membrane the plant uses to filter uptake.

Surface area is increased by trichomes that form root hairs:

These are found primarily in the Region of Maturation, and die off once the cells age. Although the cell walls contain suberin, water and minerals can pass easily between the cells of the epidermis, so further filtration is needed down the line.

Exam I material ends here.Exam II material begins here.

Mycorrhizae

This is a symbiotic relationship between a fungus and a plant root. (What does each partner get out of the relationship?)

Vesicular Arbuscular Mycorrhizae (V.A.M.) - association between a zygomycete fungus ("Black Bread Mold") and a plant

Ectomycorrhizae - association between ascomycete (Sac Fungus) or basidiomycete (Club Fungus) and a conifer or flowering plant (usually large trees).

Some of the most valuable edible organisms in the world are TRUFFLES, various species of mycorrhizal (ascomycete and basidiomycete) fungi that partner with plants.

In mycorrhizal plants, root hair surface area is negligible compared to that provided by the interface of mycorrhiza, plant and fungus. Most absorption is done via the mycorrhizal hyphae.

Recent research suggests that mycorhizzal associations may be a symbiotic partnership between not two, but three species, including special bacteria that live inside the gungus and are essential to the establishment of the symbiosis.

The Cortex

Just internal to the epidermis lies the cortex, composed primarily of parenchyma.

Cortex plastids are primarily for storage (fats, carbs). Only in some species with photosynthetic roots (which types of plants would you expect these to be?) are there chloroplasts in these cells.

In woody plants, the cortex is shed off once woody growth begins. In herbaceous plants, the cortex is maintained throughout the life of the plant.

Most of the cortex is airy, with a lot of space (filled with fluid and or air) between the cells.

Fluids travel via:

symplast - the connection formed by plasmodesmata apoplast - the continuous surface formed by adjoining cell walls

(recall: the tonoplast is the continuous fluid pathway formed by the plasma membrane of the vacuoles)

The innermost layer of the cortex is the endodermis, the main "filtration" surface of the root.

The Casparian strips banding each endodermal cell are made of suberin (sometimes lignin, as well), and prevent interstitial entry of water into the stele (central core of vascular tissue). Thus, water cannot travel via the apoplast, and must pass through the selectively permeable plasma membrane of the endodermal cells before it reaches the vascular system.

Pericycle

This is a layer of pluripotent parenchyma cells located just inside the endodermis. Pericycle gives rise to side branch roots.

The Stele

Root morphology is fairly well conserved across plant taxa. Therefore, differences in the morphology of the stele can be an important tool for classifying plants and determining evolutionary relatedness.

The stele consists (from outermost to innermost layers):

phloem from three to many bundles, alternating with xylem xylem - central core

The number of xylem "arms" in the stele determines its classification as a...o diarch - two arms (uncommon)o triarch - three armso tetrarch - four armso pentarch - five armso hexarch - six armso polyarch - more than six arms ...stele.

In young roots, the phloem and xylem are nested in a central core of parenchymal pith In monocot roots only the xylem core surrounds a central cylinder of parenchymal pith. This

is the simplest way to tell the difference between a typical root and the highly derived monocot root.

The Variety of Roots

Roots of various plant species have evolved various specializations.

food storage roots - used by the plant to store starch for metabolic activities later in the season. Typical examples: carrot, beet, sweet potato.

water storage roots - found in arid regions, these are roots that collect large amounts of water during rainy season for the plant to use during dry season. These are most often found in xeriphytes (sometimes spelled xerophyte). Local examples include the East Indian Rosewood and the Starburst.

propagative roots - have meristematic regions from where new, genetically identical plantlets can grow. These regions are not the same as nodes: they do not contain a true apical meristem.Local examples include the East Indian Rosewood and the Starburst.

pneumatophores - gas exchange surfaces on root tips protruding from water-logged soil. Certain species of mangrove have these. But, contrary to popular myth, cypress "knees" apparently have no gas exchange function. (Cypress trees with knees removed do not suffer from any apparent lack of oxygen.

prop roots - These grow from the lower part of a stem or trunk down to the ground, and providing extra support for the plant. These tend to be more common in plants with a tall, soft stem structure, as well as in plants that live in softer soils.Common examples include corn (Zea mays), Screw "Pine" ( Pandanus tectorius ) , various species of palms, and red mangroves ( Rhizophorus mangle .

aerial roots - typical of epiphytes such as orchids (in which these roots are called velamen, with a spongy outer surface very good at absorbing and holding water) and bromeliads.

buttress roots - wall-like extensions off the base of the trunk which provide support against physical assault from high winds.Our local Ficus spp. and the Royal Poinciana (Delonix regia tend to develop these in certain environments.

contractile roots - these specialized roots, usually found at the base of an underground organ (e.g., a bulb) actually contract to perform such functions as getting a bulb to its proper soil dept for growth

haustoria - parasitic plant roots that invade the tissues of a host plant and transfer nutrients from host to parasite.Examples of plants that have haustoria are dodder and mistletoe

adventitious roots - are roots that grow anywhere they are not "expected." Examples are the adventitious roots that grow so prodigiously from some of our native and introduced species of Ficus trees. Several of the root types listed above (e.g., prop roots, aerial roots) can also be considered adventitious.

Life-sustaining Root Symbiosis: Nitrogen FixationThe Nitrogen Cycle is the pathway by which nitrogen moves through living and non-living components of the ecosystem.

Nitrogen is one of the four main elements most common in biological macromolecules, and yet no eukaryotes are capable of fixing atmospheric nitrogen , N2, into its usable forms, such as ammonium (NH4

+) with other species changing it into nitrite (NO2-) and nitrate (NO3

-).

Certain nitrogen-fixing bacteria, however, are capable of converting gaseous nitrogen into its biologically useful forms, and some of these have formed symbiotic relationships with plants, notably in the Fabaceae (Pea Family), commonly called legumes.

The roots of legumes are covered with swellings called nodules within which reside symbiotic bacteria that fix nitrogen. Various strains of a bacterial species named Rhizobium form this association.

Nitrogen fixation into ammonium requires an anaerobic environment such as that found in the root nodules. The root nodule surfaces are highly lignified, helping to prevent gas exchange. Also, root nodules often contain leghemoglobin, a hemoglobin-like molecule with high affinity for free oxygen. This protein provides a sort of "buffer" for oxygen, allowing the bacteria enough oxygen to produce ATP for the very energy-expensive reactions of nitrogen fixation without allowing too much oxygen to build up in the nodule tissues and interfere with nitrogen fixation itself.

The figure below shows the sequence of events leading to nodule formation.

How does this symbiosis develop? It's amazing...

The plant root emits flavonoids into the soil. Certain species of Rhizobium take up these flavonoids (the strain of Rhizobium colonizing

each plant species is different, and determined by the exact structure of the flavonoid messenger.)

The flavonoid activates a transcription factor protein, the activity of which results in the activation of a bacterial operon known as nod(for "nodule").

The genes in the nod group produce enzymes that catalyze Nod proteins, specific to the bacterial strain.

The Rhizobium secrete the Nod molecules into the soil, and these signal to the plant root to elongate root hairs and form the infection thread that the bacteria will use to enter the root.

There is some evidence to suggest that early mycorrhizal fungus/plant communication pathways (which also employ flavonoids) led to the evolution of the bacteria/plant communications resulting in nitrogen fixation symbiosis.

Crop Rotation

Most agricultural crops severely deplete soil nitrogen. Hence, good farming technique usually includes crop rotation, in which the farmer will grow a non-legume crop in a field for one or more years, and then plants a legume crop for a year to help restore soil fixed nitrogen.