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.

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

• 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

From: Curl and Truelove. 1986. The Rhizosphere. Springer-Ver-lag.

“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

Bacteria colonizing root surface = ‘rhi-zobacteria”

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)

• 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”

intercellular colonization of the bacterium in shoots (coleoptiles) of wild rice (O. officinalis W0012) (E) when seeds were inoculated with B501gfp1. Bars l mm

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]

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

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

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)

-”Rhizoremediation”, phytoremediation interac-tions

i.e., PGPR

(DRB)

-manipulating DRB for beneficial effect

i.e., mycorrhizae

• 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

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).

• 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.)

• 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.)

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 balust- inum LS105 (B) and Pseudo- monas 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 tissueFrom: Souissi et al. 1997. Phytomorphology 47:177- 193

• 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 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.’