Root Structure and Function

Lec 4.THE PLANT ROOTS: Its Structure and FunctionCompiled by:Nanette Hope SumayaBio 103N @2012

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ROOTS IN FLOWERING PLANTSOrigin (Radicle or Adventitious)FunctionExternal AnatomyInternal AnatomySpecialized Roots

Monocotyledonous & Dicotyledonous Flowering Plants

Worlds Biggest Seed with Embryonic Root or RadicleThe Royal Botanic Garden in Edinburgh germinated this bowling-ball-like coco de mer (Lodicea maldivica) palm. The seed weighs 35lb (16kg) and can produce a tree that will live up to 300 years.Scottish botanists put in a dark case, and now a root has developed. It will produce one leaf a year for the next few years. The tree will begin to flower in 20-30 years and produce its own seeds after another five to seven years (10-09-03).Source: http://www.crocus.co.uk/whatsgoingon/regionalscotland/

Embryonic root or radicle

Comparison of Root Systems

TapRoot: typical taproots include a prominent main root directed vertically downward with numerous smaller lateral roots.Fibrous Root: have many long slender roots of about equal size with many secondary and tertiary roots off the main roots.Adventitious Root: arise from an organ in the plant other than the root proper or its branches.

Tap root and Fibrous (Diffuse) Root Systems Both arise from radicle

Adventitious Roots: roots that arise from anything other than the radicle

Adventitious Roots: roots that arise from anything other than the radicle

Roots: FunctionRoots anchor the plant in the substratum or soil. Roots absorb water and dissolved nutrients or solutes (nitrogen, phosphorous, magnesium, boron, etc.) needed for normal growth, development, photosynthesis, and reproduction. In some plants, roots have become adapted for specialized functions.

ROOT ANATOMY Root cap Region of cell division Region of elongation Region of differentiation or maturation

Fig. 38.15

Root Capthimble-shaped mass of parenchyma cells at the tip of each root protects the root from mechanical injury Dictyosomes or Golgi bodies release a mucilaginous lubricant (mucigel) cells lasts less than a week, then these die possibly important in perception of gravity (i.e., geotropism or gravitropism) amyloplasts (also called statoliths) appear to accumulate at the bottom of cells

Cont...The outer portion is rough as cells worn away by contact with rock particles as it pushes its way through the soil. It consists of large parenchyma cells that bear the brunt of friction with the soil particles. These cells rupture and exude a slimy fluid that lubricates the root tip so that it passes more easily through the soil.

Region of Cell DivisionApical meristem - cells divide once or twice per day. The transitional meristems arise from the tips of roots and shoots. These include: the protoderm (which forms the epidermis) the ground meristem (which forms the ground tissue) the procambium (forms the primary phloem and xylem).

Apical Meristem covered and protected by the root cap. It is composed of densely packed parenchyma cells which assume active division. This is the growing point that forms the root cap cells and those cells that enlarge. This region contributes to the growth in length.

Apical meristem

Region of Elongation - cells become longer and wider

is next to the cell division region. The parenchyma cells enlarged chiefly in length. Most of the elongation is due to internal pressures built up by an increase in the amount of water within the large vacuoles that occupy most of the internal part of the cells. The elongation of these cells increases the root length and is the force pushing the tip of the root forward.

Region of Maturation or Differentiation

Region of Maturation or Differentiationcells in this region developed certain structural features.root hairs develop as protuberances from epidermal cellsincrease the surface area for the absorption of water cuticle exists on root but not on root hairs

Root hairs: They are thin and an extended portion of a single epidermal cell. These cell extensions reach out into the soil and adhere to soil particles from which they absorb moisture. All together they increase the surface area of absorption. Most water absorption is done by the root hairs. All portion of the root above the maturation region are composed of matured cells: parenchyma and sclerenchyma.

Notice how the growing zone has no root hairs or lateral roots!

Growth among soil particles would result in shear forces.Zone of Maturation - cell differentiationProtodermGround MeristemProvascular

Zone of Cell Elongation - cell expansion

Zone of Cell Division - new cells by mitosis

Root Cap - penetration, padding

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Cross section of rootVascular bundle (Stele) = contains xylem and phloemCortexEpidermisRoot hairsAbsorb water and minerals

The outer cylinder This is composed of epidermis and cortex.

The cortex is composed of loosely arranged parenchyma cells with much free space between them. This arrangement allows: high volume of soil solution to quickly move into the root; the cell membranes all together form a large surface area for absorption; and their respiration supplies the energy needed to move mineral ions into the cytoplasm of the cells. Starch is more likely to be transported and stored in the cortex cells.

The endodermis is a layer of cells forming the innermost boundary of the cortex. The radial walls may be thick because they have a layer of suberin an impermeable material concentrated in bands, called casparian strips. This forms a barrier to the passage of anything moving through the cell walls. They occur in all the walls except the tangential ones that face the cortex and vascular cylinder. A few thin walled cells may allow some movement of materials into the vascular cylinder through them.

The epidermis is composed of parenchyma cells highly permeable to soil solution.

Water of the soil solution moves into the cell interior (symplastic movement), as well as through and along the walls (apoplastic movement), but the mineral ions are thought to move only along the walls.

The inner or central vascular cylinder forms the central core of the root. It functions in the lengthwise conduction and in growth too. It is made up of pericycle, primary xylem, primary phloem, and cambium. The cambium gives rise to the secondary xylem and secondary phloem

Pericycleis a narrow zone commonly one layer of cells just within the endodermis. This is composed of cells that may resume meristematic activity that forms the branch or lateral root, and adventitious shoots (sprouts and suckers).

Primary Xylemin cross section looks like star-shaped mass of conducting sclerenchyma cells, vessels.They are open-ended, barrel shaped dead cells which are joined end to end longitudinal. The smaller cells are called protoxylem (they developed lignified walls early in their growth, this prevented further enlargement thus they are small. The large cells are called metaxylem. They grew for a long time before their walls became lignified.

Primary Phloemis composed of small semi-circular cells nested between the ridges of the xylem. Sieve tube members and companion cells make up this cluster of cells. The companion cells are small cells more dense looking and may have a nucleus which fills most of the cell. Sieve tube members are larger in diameter, lack nuclei, vertically elongate, conducting cells.

Cont...They are stacked one above the other forming sieve tubes. Strands of cytoplasm extend between them through their porous end walls, called sieve plates. Sucrose and other materials synthesized by the plant is translocated through the tubes in the connecting cytoplasmic strands. Synthesized materials are translocated up or down, depending on the changing needs of the plant.

Vascular Cambiumis between the concave part of the xylem and the cluster of phloem cells.

This may be flat, rectangular-shaped cells that may produce secondary tissues.

Herbaceous dicot/monocotIn herbaceous dicot roots the outer cylinder and inner vascular cylinder are both evident. The herbaceous monocot roots have numerous xylem ridges, and the small clusters of cells nested between the protoxylem points, are the phloem cells. A large pith occupies the center of the vascular cylinder. The pericycle is several layers of cells.

This layer is just inside the endodermis, which is the innermost layer of the cortex. The endodermis developed more suberin materials and more cellulose materials in the monocot than in the dicot root. The cortex is narrower than the inner vascular cylinder. Some monocots through may have wider cortex.

Woody Rootsare with secondary xylem.occur as regular rings. outside the xylem next to the vascular cambium is the secondary phloem. As the plant ages the cortex is replaced by the corkcells that make up the periderm, replacing the epidermis.

Root Tip Senses GravityAuxin Hormone ProducedAuxin Accumulates on LowerGrowth Inhibited on LowerRelative to UpperRoot Curves DownwardGravitropismRoot Hairs Increase Surface AreaRoot Hairs Secrete Acid (H+)H+ Cation Exchange w/MineralsMineral Uptake into RootsWater and Mineral Uptake

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1996 Norton Presentation Maker, W. W. Norton & CompanyRadish seedlings have roots with long root hairs that increase the surface area for water and mineral uptake

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Osmosis: passive movement of water from pure to polluted area

cytoplasmic solutesmore concentratedsoil solutes more dilutecell membraneWater potential lowWater potential highcell wall

water flow

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Root hairs are responsible for cation exchange

soil particles covered with capillary waterand minerals

water

voids with air space

root hair penetrates soil spaces

epidermal cell

cortex cell

H+Ca2+H+Ca2+

Ca2+intercellular gas space

to vascular cylinder

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Dicot Mature Root Structure - AnatomyEpidermisCortexVascular CylinderRanunculus acris - buttercup

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Root Vascular Cylinder and CortexRanunculus acris - buttercupCortexEndodermisMeta-xylemPhloemPericycle

Proto-xylem

Endarch: protoxylem is inside the metaxylemExarch: protoxylem is outside the metaxylem

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haplostele

siphonosteleeustelesolenosteledictyosteleatactostele

leaf gapleaf tracecortex phloem xylem

cortex phloem xylem pith

dicot stemmonocot stem

Protosteles:actinostele

plectosteleSpecialized Versions

dicot rootmonocot root

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1996 Norton Presentation Maker, W. W. Norton & CompanyApoplasticSymplastic

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xylem insidecortex outsideendodermissuberin-waxy barrierto apoplasticmovementcell membrane proteins (active transporters) determine which minerals may be taken up The endodermis is thus responsible for selective mineral uptake.

minerals cannot go between cells

minerals must go through cells

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Mineral uptake: Active transport against concentration gradient

cytoplasmic solutesmore concentratedsoil solutes more dilutecell membraneWater potential lowWater potential highcell wall

water flow

Ca2+ATPADP + PiCa2+Ca2+

Calcium transport proteinOsmosis: passive movement of water from pure to polluted areatoo expensive?

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Root Anatomy: Dicot Root Cross SectionEpidermis - root hairs, mineral and water intake

Cortex - storage, defense

Endodermis - selective mineral uptake

Pericycle - lateral root formation (periderm)

(Vascular Cambium - makes 2 tissues)

Phloem - CH2O delivery from leaves

Xylem - conduct water and mineral upwards

Casparian strips in radial wallsOne Vascular Cylinder (Phloem + (Cambium) + Xylem)Radial Xylem-Phloem ArrangementExarch Xylem MaturationTetrarch (this example)

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Monocot RootCross SectionSmilax-catbrier

PithXylem/Phloem Arr?Xylem Maturation?______arch?

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How is this section different?Smilax - catbrier

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A closer lookWhat do these features tell you?StarchCutin/SuberinMitochondriaSieve Tube ElementCompanion CellVessel with LigninXylem ParenchymaLignified Pith Parenchyma

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What is the Pericycle doing?Root Cap

Zone of Cell Division

Growing out through cortex In fibrous root systems, there is much lateral root formation.Here you can see two root apices initiating from the pericycle.Notice their connection to the ridges of xylem

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Monocot Root in Cross Section

Dicot Root in Cross Section

Dicot root in Cross Section

The Casparian Strip

Lateral Roots Arise from the Pericycle of the Stele

Secondary Growth in Dicot Roots

Secondary Growth in Dicot Roots

Primary and Secondary Growth in Roots

http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT311/PrimSec/primarysecondary4.htm

Specialized FunctionsNature of Specializations or ModificationsSupportPresence of brace roots (aerial roots arising from the main stem which penetrate the ground)Presence of prop roots (aerial roots arising from the branches which penetrate the ground)Presence of clinging roots ( aerial roots which adhere to some kind of support)Presence of buttresses (planklike extensions from the lower part of the stem)Food StorageEnlarged, fleshy succulent rootsPhotosynthesisGreen aerial rootsReproductionDevelopment of adventitious budsProtectionPresence of spinesAerationDevelopment of pneumatophores (roots with spongy tissues protruding above the ground)

Modified Roots

OthersParasitic rootsSymbiotic roots mycorrhizae or fungus rootsLegumes (e.g., pea, beans, peanuts) and bacterium form root nodules.

Support: Buttress Roots

Tropical soils are nutrient poor.Roots must traverse the surface for minerals, so roots grow on the surface (no tap root).So, to keep this tall baobab tree standing upright, the roots grow in diameter but only in the vertical dimensions to form ridge rootscalled buttress roots.These roots inspired gothic cathedral architects to design buttress walls.

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http://www.dublincity.ie/dublin/citywalls/buttress.jpg

http://www.oxc.com.hk/raoul_nathalie/gallery/images/04%20Buttress.jpg

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Pandanus utilis - screw pineProp roots such as these inspired flying buttresses.

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http://www.contrib.andrew.cmu.edu/~ajm/Pages/Graphics/flyingbuttress.JPG

http://williamcalvin.com/BHM/img/FlyingButtressND.jpg

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Food Storage Roots

Jack-o'-lanterns from Turnips

Turnip + Cabbage = Rutabaga

Aeration: Pneumatophores - black mangroove

Avicennia germinans (black mangrove)pneumatophores

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STILT ROOTSe.g.Mangrove

Cypress Knees

Symbiotic Roots Legumes (e.g., pea, beans, peanuts) form root nodules. Mutualism between a plant and bacterium which allows for the fixation of atmospheric nitrogen to form that the plant can utilized. The bacterium is reward with food and a place to live

Symbiotic Roots Mycorrhizae or "fungus roots" where a symbiotic relationship forms between a plant and a fungus. In this partnership the fungus provides protection against some types of pathogens and increase the surface area for the absorption of essential nutrients (e.g. phosphorous) from the soil. The plant in return provides food for the fungus in the form of sugar and amino acids

Photosynthetic Roots

Parasitic roots - Dodder

Propagative Roots with Adventitious Buds/Stems

MERCI!