Rhizosphere: the Place to be in Soil

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Rhizosphere the Place to be in Soil

• SOIL 7200
• Presented by
• Hassna Alkher
• Department of Plant Science

March,12th,2007
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What is the Rhizosphere

• Rhizo Rhiza root
• Sphere field or area of influence , physical
  location around the root
• Introduced by Hiltner in 1904
• the region surround plant roots
• influence by root exudates
• 1-2 mm from root
• variable region where the root production
  stimulate microbial population and activity

Lorenz Hiltner
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Rhizosphere

• Comprised of different zones
• Endorhizosphere
• multi-layered microenvironment, which includes a
  mucoid layer on the root surface, the epidermal
  layer of the root tissue including the root hairs
  and the cortical cells
• Ectorhizosphere
• rhizosphere soil

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Rhizosphere zone
http//www.personal.psu.edu/faculty/j/e/jel5/biofi
lms/rhizosphere.html
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Rhizosplane

• Its plants root surface (external surface)
  where many microorganism adhere to it.
• 1µm thick.
• hard to distinguish between Rhizo sphere/ plane.
• NonRhizosphere

Zone that not penetrated by plant root . it has
less microbial activity.
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Rhizosphere vs Nonrhizosphere
Clarholm et al., 1985
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Rhizosphere vs NonRhizosphere

• In the Rhizosphere
• More enhanced microbial growth
• More soluble C, nutrients

From the plant root
Foster (1986)
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Wheat Rhizosphere and nonrhizosphere populations
of some taxonomic and nutritional group of soil
microorganism.
Rhizosphere vs Nonrhizosphere
Rouatt et al., 1960
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Rhizosphere vs Nonrhizosphere
Dolichos lab lab
Lakshmi Kumari,1961
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• How to obtain organisms from the
• compartments of the Rhizosphere?

• Sutra et al., 2000
• Normander et al., 1999
• Nautiyal et al., 1997
• Talukdar et al., 2004
• De La Fuenta et al., 2006
• http//www.regional.org.au/au/allelopathy/

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Shaking
Non-adhering soil
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Lolium perenne
Trifolium repens
Bulk soil
Washed root (endrhizosphere/rhizoplane)
Soil adhering to the root
Purify DNA
Amplify 16S rDNA
16S rDNA cloning
16S rDNA characterized by PCR restriction analysis
OUT measurement
Calculate the diversity
Marilley et al., 1998
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What is Attractive About the Rhizosphere?

• Mucilages
• Root exudates

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Rhizosphere zone
http//biology.kenyon.edu/courses/biol272/agricult
ure/agriculture.htm
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Mucilage

• Insoluble organic compounds of four different
  origins.
• - mucilage secreted by Golgi organelles in the
  root cap cells.
• - hydrolysates of the polysaccharides of the
  primary cell wall between
• epidermal cells of the primary wall and
  sloughed root cap call.
• - mucilage secreted by epidermal cells and root
  hairs.
• - mucilage produced by bacterial degradation of
  dead epidermal cells.

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Root exudates

• Low molecular weight compounds.
• Act as a messengers
• Energy for microbes

Root exudates as drops on root surface
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Chemical composition of exudates
Felix.D and Donald. A, 2002
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Root exudates function

• Defend the rhizosphere and root against
  pathogenic microorganism
• Attract greater number of microorganism
• Keep the soil around the root moist
• Obtain nutrients
• Change the chemical properties of the soil around
  the root
• Inhibit the growth of competing plant species

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Root exudates in different plant
Vancura., 1964
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Enzymes in the rhizosphere

• Enzymes activity greater in rhizosphere than in
  bulk soil
• Wide range of enzymes ( plant- microorganism)
• -Oxidoreductases (transfer of electrons from one
  molecule to another)
• -Hydrolases (nutrient mineralization, protein
  breakdown)
• -Lyases (degrades cell wall and membrane of
  bacteria and fungi)
• -Transferases(transfer a functional group from
  one molecule to another)
• Enzymes catalyze the breakdown of
• - organic material (cellulases, dehydrogenases)
• - fertilizers (ureases)
• - organic nutrients (phosphatases, sulfatases)

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Enzymes in the Rhizosphere

• Enzymes secrete by Rhizosphere microorganism
• - Enhance plant growth
• - stimulate enzyme production by intact roots
• e.g/
• phosphatase release by Pseudomonas sp.

• Acid phosphatase activity in solution
• Root Phosphatase activity

Inoculation with Pseudomonas sp Acanthamoeba
sp.
Bouteloua gracilis
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Some function of Bacteria in rhizosphere

• Bacteria N2 fixation
• Rhizobium,Bradyrhizobium with legumes
• Azospirillum on roots surface of grass
• Frankia with nonleguminous angiosperms

Rhizobium
Frankia
Azospirillum
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Some function of Bacteria in rhizosphere

• Bacteria has role in P cycle in the rhizosphere
• Solubilization of inorganic phosphorus (P
  dissolving bacteria)
• Pseudomonas, Bacillus
• Reduce pH by secret organic acids (formic,
  acetic, lactic acid)

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Bacteria metabolism in Rhizosphere
Giri et al. 2006
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Mycorrhizal fungi

• Beneficial fungi associate with plant root
• 90 of plant count on mycorrhizal fungi root
  system
• nutrient exchange

carbohydrate
plant
mycorrhiza
nutrients
hyphae
Vesicular Arbuscular
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Microorganism in Rhizosphere

• Fauna
• Macrofauna
• - soil organisms which are at least one
  millimeter in length
• - too large to live in rhizosphere
• - feed on living or dead roots (herbivory)
• - 6 of the 26 insect orders are presented by
  active below-ground herbivores

Lavelle et al., 2001
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Microorganism in rhizosphere

• Macrofauna
• Coleoptera larvae

Gryllotalpidae
earthworms
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Microorganism in rhizosphere

• Nematodes
• Up to 83 soil nematodes are root feeders
  (herbivores)
• Nematode activity stimulate by substances in
  Rhizosphere ( CO2, ethylene- amino acids- salts-
  mucigel)

Lavelle et al., 2001
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How Rhizosphere increase the population of
nematodes in its region

• Attract nematodes to potential food supplies
• Nematodes attracted by CO2 released from roots
• Stimulates nematodes eggs to hatch by specific
  substances

Lavelle et al., 2001
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• How do microorganisms communicate ?

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Why do bacteria talk to each other?

• co-ordinate their behavior in the rhizosphere
• Survive
• Defend against other microorganisms in the
  rhizosphere
• Compete against other microorganisms in the
  rhizosphere
• avoid toxic compounds

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??????
Not enough !!
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Whats Quorum sensing (QS) ?

• Quorum sensing is the regulation of gene
• expression in response to variations in
• cell-population density

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QS forms

• Can occurs
• Plant growth-promoting .. Pseudomonas, bacilli
• at threshold cell-density level produce
  substance inhibit production of pathogens
• Beneficial bacteria .. Rhizobium spp
• Optimize nodule formation on plant roots

Sharma et al, 2003
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QS in bacteria

• complex communication mechanism
• Bacteria use signaling molecules to talk
• Molecules release into the environment
• Bacteria able to measure the concentration of the
  molecules within the population

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How does QS work?

• Bacteria produce and secret chemical signal
  molecules called autoinducers
• Autoinducers increase in concentration
• Receptor recognizes and binds with inducer
• Activates and transcripts Certain genes
• By Detection, reacting to autoinducers, bacteria
  able to sense to the surrounding cell density
• Then,,, make sure . Enough or not

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OK .. Lets go
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QS molecules
inducer

• Acyl-Homoserine Lactones (AHLs)
• Vary from microbe to another

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AHL production in rhizobia
Gonzalez et al, 2003
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QS in rhizobium sp.

• Requirements
• Inducers ( AHL)
• Receptors

Gonzalez et al, 2003
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QS in rhizobium sp.
Gonzalez et al, 2003
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• How we can use QS ?

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Thank you