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Root Meristem, Root Cap, and Root Development Lecture 18 Manongdo, Marc Julian A. 3Bio01
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Page 1: Root Meristem, Root Cap, and Root

Root Meristem, Root Cap, and Root Development

Lecture 18

Manongdo, Marc Julian A.3Bio01

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The Root Apical Meristem (RAM)Primary root growth occurs at the root apical meristem (RAM). Unlike the SAM, the RAM produces cells in two directions. The RAM produces a cap of tissue called the root cap, which covers the distal tip of roots. The root cap protects the root tip as it grows through the soil by secreting mucilage. Cells are continuously sloughed off the outer surface of the root cap. The RAM also produces cells proximally that contribute to the root proper, but unlike the SAM, the RAM produces no lateral appendages.

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Zones near the Area of Division1. Central Cylinder Meristem/ Stele Initials Zone-

Cells of this zone are the precursor of procambium.

2. Cortical Initials Zone- Parenchymatous and endodermal cells are derived here. In older roots, the cortex and endodermis probably have their own committed stem cells.

3. Central Cells Zone - Cells of this zone have a stem cell function and are essential for meristem maintenance (Quiescent Zone).

4. Columella Initials Zone- The root cap is derived from this zone.

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Histogen TheoryPlant tissue derived from the subapical meristem divided into precursors.

1. Dermatogen- the outermost layer, it gives rise to the protoderm.

2. Plerome- the central cylinder, it gives rise to the procambium.

3. Periblem- sandwiched between the dermatogen and plerome, it gives rise to the ground meristem.

4. Calyptrogen- most distal, it gives rise to the root cap.

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Roots can have open or closed organization at the root tip. How a root tip is organized can best be seen with a median longitudinal section.

Closed organization means that the files of cells that arise from the root tip can be traced back to meristematic layers, or histogens. For example, the vascular tissue in this diagram can be clearly traced back to a single layer of cells. This histogen is called the plerome. The ground tissue in this drawing originates from a layer called the periblem, and the epidermis and the root cap both share a histogen layer which is called the calyptrogen/ dermatogen complex or dermatocalyptrogen.

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In open organization, the differentiated cell files cannot be traced back to a single, distinguishable layer.

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Nicotiana tabacuma. Pleromeb. Periblemc. Calyptrogen/

Dermatogen Complex

Zea maysd. Pleromee. Periblem/

Dermatogen Complex

f. Calyptrogen

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Quiescent CenterThe center of the RAM is occupied by a quiescent center which has low mitotic activity. The quiescent center is most apparent in actively growing roots and is lost during dormancy, carbohydrate starvation or root cap removal. Evidence suggests the quiescent center does function as the zone of initials. Infrequent division of initial cells in the quiescent center is the source of cells for the root apical meristem. These initial cells and tissue patterns become established in the embryo in the case of the primary root, and in the new lateral meristems in the case of secondary roots.

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The Root CapThe root cap is a section of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statoliths which are involved in gravity perception in plants.  If the cap is carefully removed the root will grow randomly. The root cap protects the growing tip in plants.  It secretes mucilage to ease the movement of the root through soil,  and may also be involved in communication with the soil microbiota.The root cap is absent in some parasitic plants and some aquatic plants, in which a sac-like structure called the root pocket may form instead.

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Tips of roots are covered by a thimble-shaped root cap, that

has its own meristem that pushes cells forward into the cap. As they move through the cap, these cells differentiate into columella cells.

Columella cells each contain 15-30 amyloplasts that sediment in response to gravity to the lower side of the cell. Besides protecting the growing root tip and its meristem, the root cap senses light and pressure exerted by soil particles.

Within a few days, columella cells differentiate into peripheral cells. The peripheral cells of the root cap and the epidermal cells of the root produce and secrete large amounts of mucigel, a slimy substance made by their dictyosomes.

Root cap cells only last 2-3 weeks because as the cells become secretory, the middle lamella weakens, the cells separate and are sloughed off into the soil.

Mucigel is a hydrated polysaccharide containing sugars, organic acids, vitamins, enzymes, and amino acids. 

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Important Functions of MucigelProtection- It protects roots from desiccation and

contains compounds that diffuse into the soil and inhibit growth in other roots.

Lubrication- It lubricates roots as they force their way between soil particles.

Water absorption- Soil particles cling to mucigel, and increase the root's contact with the soil.

Nutrient absorption- Carboxyl groups in mucigel influence ion uptake, and organic acids in mucigel make certain ions are more available to plants.

Also, fatty acids, pectins, and sterols in mucigel may help establish beneficial symbioses with soil microbes.

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Root DevelopmentA small number of stem cells at the tip of the root generate all of the cell types through stereotyped divisions followed by cell differentiation and regulated cell expansion. Because root growth is indeterminate, these processes are continual, resulting in all developmental stages being present at all times. The radial symmetry of the root combined with a lack of cell movement means that clonally related cells are frequently found in cell files. These cell files can be traced back to their origins, which are four types of stem cells (or initial cells) at the root tip. The epidermal/lateral root cap initials give rise to the epidermis and the outer portion of the root cap known as the lateral root cap. The central portion of the root cap, the columella initials. The ground tissue cells, the cortex and endodermis, are generated by division of the cortical/endodermal initials.  In older roots, the cortex and endodermis probably have their own committed stem cells.

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Finally, the vascular tissue and pericycle have their own central cylinder initials which produces the procambium. The procambium divides to produce the protoxylem, which in turn forms the metaxylem and xylem, and protophloem, which in turn forms the metaphloem and phloem. The metaxylem enlarges and dies after depositing the secondary wall. Internal to and contacting all the initials is a small number of central cells that are mitotically inactive and are known as the quiescent center (QC).

Division of initials can be either solely anticlinal (orthogonal to the axis of growth) resulting in a single file of cells or first anticlinal then periclinal (parallel to the axis of growth) resulting in two or more cell layers. The columella initials generally divide only anticlinally and their progeny undergo rapid cell expansion and then differentiate, producing starch-containing amyloplasts that play a role in gravity sensing. The other three types of initials generally undergo both anticlinal and periclinal divisions, resulting in cell lineages that acquire different identities.

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Lateral RootsLateral 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 after the formation of the leaf primordia which are able to synthesize the hormone. This allows coordination of root development with leaf development, enabling a balance between carbon and nitrogen metabolism to be established.

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Like shoot branching, root branching can be either terminal or lateral, with the terminal mode being more common in lower plants and lateral much more common in angiosperms.

a. Terminal branching involves the division of the RAM into 2 with the subsequent production of 2 roots.

b. Lateral branching is different in roots than in shoots.

1. Lateral roots initiate from internal cells of the pericycle. Initiation occurs in the late cell elongation/early cell differentiation zone, in pericycle cells that are partially to fully differentiated. Thus there is no detached meristem.

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2. A small group of pericycle cells reorient

their axis of polarity to the radial dimension and begin growing and dividing to form a mound of cells.

3. With continued growth and division, the mound of cells becomes organized into a RAM with root cap, while still within the tissues of the main root.

4. Continued growth allows the lateral root to penetrate the endodermis, cortex and epidermis, finally reaching the exterior of the parent root.

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Secondary Vascular TissueSecondary tissues comprise the greatest volume of the root mass of woody perennial plants. Primary tissues continue to form in the feeder roots, but the supporting root structure consists of secondary tissues produced by the lateral meristems, the vascular cambium, and one or more cork cambia.  Roots produce branch roots and secondary tissues at the expense of the primary tissues. Cells in the primary tissue are broken and discarded as secondary growth proceeds. New lateral roots form endogenously (from within the root) and push outward from the pericycle, destroying cortex and epidermal tissues on their way to the soil.

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Initiation of secondary growth takes place in the zone of maturation soon after the cells stop elongating there. The vascular cambium differentiates between the primary xylem and phloem in this zone and pericycle cells divide simultaneously with the procambium initials. The result is a cylinder of cambium encircling the primary xylem.The vascular cambium almost immediately begins producing xylem cells inward and phloem cells toward the outside of the root, in the process flattening the primary phloem against the more resistant endodermis. Concomitant differentiation of cork cambia in the pericycle adds other areas of cell division in the stele.  

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The combination of periderm and vascular tissue production not only physically breaks the remaining cells of the cortex and epidermis, but the lignified and suberized new cell walls laid down by the cambia effectively isolate the outer tissues as well from their source of supplies in the interior of the root. By the end of the first year, secondary growth has obliterated all but the central core of primary xylem cells and a few fibers of primary xylem pushed against the periderm. The zones at this time, therefore, from outside to inside are: periderm, pericycle, primary phloem, secondary phloem, vascular cambium, secondary xylem, and primary xylem.

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Referenceshttp://www-plb.ucdavis.edu/labs/rost/tomato/Roots/

roottip.htmlhttp://www.public.iastate.edu/~bot.512/lectures/Roots.htmhttp://www2.mcdaniel.edu/Biology/botf99/rootuse/

rootanatomy.htmlhttp://en.wikipedia.org/wiki/

Meristem#Root_apical_meristemshttp://www.bio.mtu.edu/~hlyoungs/plant_development/

root_dev.pdfhttp://vannocke.hrt.msu.edu/plb865/Root%20structure

%20and%20development/structuredevel.htmlhttp://vannocke.hrt.msu.edu/plb865/Root%20structure

%20and%20development/lateralroot.htmlhttp://www.psb.ugent.be/images/stories/psb/root/

figure_intro_small1.jpghttp://en.wikipedia.org/wiki/Lateral_roothttp://www.cliffsnotes.com/study_guide/Secondary-Growth-

of-Roots.topicArticleId-23791,articleId-23671.html