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
interface.
Waisel et al. 2002. Plant Roots. The Hidden Half.
2nd ed.
Rhizosphere: the root environment zone
that stimulates the growth of
microorganisms that use root-derived
compounds as sources of C, N, Energy
Spermosphere: area of increased
microbial activity around seed (imbibing,
germinating) in soil- 1 to 20mm zone
• Rhizoplane: surface of plant root with
strongly adhering soil particles; provides
microenvironment- soil-plant interface –
fro mirobial activity
Ectorhizosphere: area (soil layer)
surrounding the root
Endorhizosphere: cell layers of the root
potentially colonizable by microorganisms
From: Curl and Truelove. 1986. The Rhizosphere. SpringerVerlag.
“Rhizosphere Effect” – selective
enhancement 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
rhizosphere soil
Bacteria colonizing root surface = ‘rhizobacteria”
Factors influencing “Rhizosphere
Effect”
• Root exudates-major impact due to low
available C in “bulk soil” and organism
fractions released from roots also secretion,
mucilage, lysates (Table17-1,S1; Table 11-1,
S2)
Factors influencing “Rhizosphere
Effect”
• 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
intercellular colonization of the bacterium in shoots
(coleoptiles) of wild rice (O. officinalis W0012) (E)
when seeds were inoculated with B501gfp1. Bars l
mm
BIOFILM – assemblages of microorganisms and their
associated extracellular products at an interface and
typically attached to an abiotic (mineral particle) or biotic
(root or ‘rhizoplane’) surface. Development of biofilms
follow distinct steps (see below) and may involve cell-tocell communication. [see pp. 116-117, Sylvia, 2005]
Stages of biofilm formation (Adapted from ASM Biofilms Collection by Mark Wiencek) (http:
//www.asmusa.org/edusrc/biofilms/infopage/043i.html).
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
percentage of pore space is water-filled and
oxygen may be limiting
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 HCO3or OH- (released from the plant root)  increase
pH
- NH4+ exchanges with H+ (released from the plant
root)  decrease pH
Rhizobacteria: bacteria intimately
associated 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)
i.e., PGPR
(DRB)
-manipulating DRB for beneficial effect
-”Rhizoremediation”, phytoremediation
interactions
i.e., mycorrhizae
Rhizobacteria
• 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*
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
Influence of Plant
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
Types of rhizodeposits (Adapted from Kuzyakov 2002).
Influence of Plant (cont.)
• Stimulation effects:
1. Ammonifiers – increased availability of
organic N substrates (high immobilization
rates associated with rhizosphere
community)
2. Free-living N2-fixers (associative
N2- fixing bacteria) i.e., Azospirillum
Cereal grain crops
Forage grasses
3. Denitrification –low O2, high E, if NO3is present (2NO3-+ 5H2+2H+  N2 + 6H20)
anaerobic respiration
Influence of Plant (cont.)
• 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 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
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.)
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
Fig.A-D. Root tissue of leafy
spurge seedlings inoculated
with Flavobacterium balustinum LS105 (B) and Pseudomonas fluorescens LS102 (C)
From: Souissi et al. 1997.
Phytomorphology 47:177-193
Fig.A-D. Flavobacterium
balustinum LS105 and
Pseudomonas fluorescens
LS102 in the intercellular
spaces of leafy spurge
root tissue
From: Souissi et al. 1997.
Phytomorphology 47:177193
• 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
Disease-conductive soils
Disease-suppressive soils
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 antagonistic
activity (i.e., Fe complexation) are similar in
both plant root and human ‘environments’
• Each pathogen does have its own features
See ‘Berg et al. 2005. The rhizosphere as a reservoir for opportunistic human
pathogenic bacteria. Environmental Microbiology 7:1673-1685.’