The Rhizosphere and Spermosphere

1. 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.

3. 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

4. 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

5. From: Curl and Truelove. 1986. The Rhizosphere. Springer-Verlag.

6. “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

7. Bacteria colonizing root surface = ‘rhizobacteria”

8. 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)

10. 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

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

13. 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-to-cell 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).

14. 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

15. 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

16. 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)

20. i.e., PGPR (DRB) -manipulating DRB for beneficial effect i.e., mycorrhizae -”Rhizoremediation”, phytoremediation interactions

21. 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*

22. Rhizosphere Ecology • Influence of Plant (Table 17-1,S1; 11-1,S2) • Influence of Microorganisms (Table 17-4, S1; 11-4, S2) • Rhizosphere Competence – ability of microorganisms to colonize the rhizosphere indicates potential effects of rhizobacteria on plant growth; potential as inoculant

23. Influence of Plant

24. 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

25. Types of rhizodeposits (Adapted from Kuzyakov 2002).

26. 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 NO3- is present (2NO3-+ 5H2+2H+ N2 + 6H20) anaerobic respiration

27. 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)

28. 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

29. 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.)

30. 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

31. 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

32. 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 47:177- 193

35. 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

36. Disease-conductive soils

37. Disease-suppressive soils

38. 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.’