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The Rhizosphere and Spermosphere
Sylvia, Chap. 17 (S1); Chap. 11(S2)Pinton et al. 2001. The Rhizosphere. Biochemistry and Organic Substances of the Soil-Plant inter-face.Waisel et al. 2002. Plant Roots. The Hidden Half. 2nd ed.
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Rhizosphere: the root environment zone that stimulates the growth of micro-organisms that use root-derived com-pounds as sources of C, N, Energy
Spermosphere: area of increased micro-bial activity around seed (imbibing, germinating) in soil- 1 to 20mm zone
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• Rhizoplane: surface of plant root with strongly adhering soil particles; provides microenvironment- soil-plant interface – fro mirobial activity
Ectorhizosphere: area (soil layer) surround-ing the root
Endorhizosphere: cell layers of the root po-tentially colonizable by microorganisms
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From: Curl and Truelove. 1986. The Rhizosphere. Springer-Ver-lag.
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“Rhizosphere Effect” – selective en-hancement of bacterial/fungal species by root
Factors: -Root exudate quantities and composition -Chemotaxis and signal compounds -Atmospheric concentration, i.e., CO2 levels -Moisture microsites -pH variations – “bulk” soil environment vs rhi-
zosphere soil
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Bacteria colonizing root surface = ‘rhi-zobacteria”
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Factors influencing “Rhizosphere Effect”
• Root exudates-major impact due to low avail-able C in “bulk soil” and organism fractions re-leased from roots also secretion, mucilage, lysates (Table17-1,S1; Table 11-1, S2)
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• Chemotaxis - oriented movement of a motile organism with
reference to a chemical agent. - may be positive (toward) or negative (away)
with respect to the chemical gradient. - may guide rhizobacteria to infection sites in
plant roots up to several centimeters away
Factors influencing “Rhizosphere Effect”
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intercellular colonization of the bacterium in shoots (coleoptiles) of wild rice (O. officinalis W0012) (E) when seeds were inoculated with B501gfp1. Bars l mm
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Stages of biofilm formation (Adapted from ASM Biofilms Collection by Mark Wiencek) (http://www.asmusa.org/edusrc/biofilms/infopage/043i.html).
BIOFILM – assemblages of microorganisms and their associated extracellular products at an interface andtypically attached to an abiotic (mineral particle) or biotic (root or ‘rhizoplane’) surface. Develop-ment of biofilms follow distinct steps (see below) and may involve cell-to-cell communication. [see pp. 116-117, Sylvia, 2005]
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Factors influencing “Rhizosphere Effect”
• Moisture microsites - at low soil water potential, greatly influencing
microbial growth, as motility and diffusion of nutrients can be reduced.
- at higher soil water potentials, a large per-centage of pore space is water-filled and oxygen may be limiting
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Factors influencing “Rhizosphere Effect”
• pH variations- “bulk” soil environment vs rhizosphere soil - H+, HCO3
-, or organic compounds (root-induced production) and their subsequent release into the rhizosphere affect ion uptake and thus pH
- NO3- (supplied to the plant) exchanges with HCO3
- or OH- (released from the plant root) increase pH
- NH4+ exchanges with H+ (released from the plant
root) decrease pH
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Rhizobacteria: bacteria intimately associ-ated with plant roots
PGPR – “plant-growth-promoting rhizobacteria” Enhance plant growth or seed germination via
several mechanisms (plant-growth promoting compounds, antagonize pathogens, etc.)
DRMO – “deleterious rhizosphere microorganisms”
DRB – “deleterious rhizobacteria” Inhibit/suppress plant growth via several mechanisms
(inhibitory or toxic compounds, enzymes, over-production of growth promoters)
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-”Rhizoremediation”, phytoremediation interac-tions
i.e., PGPR
(DRB)
-manipulating DRB for beneficial effect
i.e., mycorrhizae
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• Endorhizal (or endophytic) microorganisms – microorganisms colonizing inner cellular layers of plant root
• Representative Types: Table 17-6 (11-6, S2) for PGPR
• Primarily Pseudomonas spp. representing both PGPG and DRB
* Rhizobacteria composition may be distinctive for specific plant species*
Rhizobacteria
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Rhizosphere Ecology
A. Influence of Plant (Table 17-1,S1; 11-1,S2)B. Influence of Microorganisms (Table 17-4, S1;
11-4, S2)C. Rhizosphere Competence – ability of
microorganisms to colonize the rhizosphere indicates potential effects of rhizobacteria on plant growth; potential as inoculant
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Influence of Plant
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Influence of Plant• Provide excretion products and sloughed tissues –
Rhizodeposition; C, N, Energy, growth factors for microbe• Assimilation of inorganic (mineralized) nutrients• Root respiration - influence pH, CO2
• Root penetration - soil structure effects, ** microhabitat** effect
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Types of rhizodeposits (Adapted from Kuzyakov 2002).
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• Stimulation effects: 1. Ammonifiers – increased availability of
organic N substrates (high immobilization rates associated with rhizosphere com-munity)
2. Free-living N2-fixers (associative N2- fixing bacteria) i.e., Azospirillum
Cereal grain crops Forage grasses 3. Denitrification –low O2, high E, if NO3-
is present (2NO3-+ 5H2+2H+ N2 + 6H20)
anaerobic respiration
Influence of Plant (cont.)
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• Stimulation effects (cont.): 4. Cellulolytic Bacteria – availability of substrates
5. Fungal spore germination – AMF, pathogens
germinate due to stimulating compounds released by roots
(e.g., Fusarium, Verticillium)
6. Production of antimicrobial agents (phenolic compounds, phytoalexins) – selective effect on rhizosphere microbial community (generate toxic compounds to fungi called fungitoxins)
Influence of Plant (cont.)
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Influence of Microorganisms• Produce growth - promoting substances (Auxins,
gibberellins, cytokinins)
• Phosphorus availability - high phosphatase activity, H2CO3 production,
organic acids, AMF
• Assimilation of Mn, Fe, Zn and transfer to plant - chemoautotrophic bacteria oxidize reduced
inorganic compounds to extract electrons for use in ATP production
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Influence of Microorganisms (cont.)
• Availability or toxicity of S – i.e., Desulfovibrio can be rhizosphere inhabitant
S-oxidizing bacteria may provide S in rhizosphere of canola
• Enzymatic Activity – urease, proteases – mineralized N for plant uptake
• Antibiotic Formation – defense against root pathogens (actinomycetes produce more than 50 different types: streptomycin, neomycin, etc.)
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Influence of Microorganisms (cont.)
• Siderophore production – both PGPR and DRB 1. Nutrient deprivation of root pathogens 2. Competition with Fe uptake system of plant
root• Phytotoxin Production – DRB Suppress seedling development, plant growth
(HCN, herbicidal compounds, complex phytotoxins)
- xylem occlusions formed by DRB: suppression of growth in Citrus
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Fig.A-D. Root tissue of leafy spurge seedlings inoculated with Flavobacterium balust- inum LS105 (B) and Pseudo- monas fluorescens LS102 (C)From: Souissi et al. 1997. Phytomorphology 47:177-193
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Fig.A-D. Flavobacterium balustinum LS105 and Pseudomonas fluorescens LS102 in the intercellular spaces of leafy spurge root tissueFrom: Souissi et al. 1997. Phytomorphology 47:177- 193
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• Peudobactin is known as a Siderophore: microbial Fe-chelating compounds solubilizes Fe2O3 to make Fe plant available yet deprives root pathogens, therefore, reducing growth of pathogens
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Disease-conductive soils
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Disease-suppressive soils
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The rhizosphere as a reservoir for opportunistic human pathogens?• Many bacteria can interact (colonize) both plant roots and human hosts
– Pseudomonas– Enterobacter– Burkholderia (CF pathogen)
• Mechanisms for colonization and antag-onistic activity (i.e., Fe complexation) are similar in both plant root and human ‘environments’• Each pathogen does have its own fea-turesSee ‘Berg et al. 2005. The rhizosphere as a reservoir for oppor-tunistic human pathogenic bacteria. Environmental Microbiology 7:1673-1685.’