Watercolor Pansy

1
Rhizosphere Interactions
2

• The Rhizosphere is the zone surrounding the roots
  of plants in which complex relations exist among
  the plant, the soil microorganisms and the soil
  itself. The plant roots and the biofilm
  associated with them can profoundly influence the
  chemistry of the soil, including pH, which in
  turn profoundly affects the community of
  microorganisms.

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Tree-girdling experiment in boreal pine forest,
northern Sweden. Girdling severs the phloem
flux of photosynthates from the tree canopies to
roots, mycorrhizal fungi and other soil
microorganisms). The treatment reduced total soil
respiration by 37 within five days and by 56
reduction within 14 days. (Högberg, P. et al.
(2001) Large-scale forest girdling shows that
current photosynthesis drives soil respiration.
Nature 411, 789-792)
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• Experiments like this that interrupt the flow of
  carbon from forest trees to soil indicate that
• recently-fixed carbon is rapidly transported
  below ground.
• About half of the respiration from soils is
  fueled by photosynthesis from a few hours ago (in
  grasslands) or a few days ago (in forests). The
  other half is from decomposition of litter that
  may be a few months to many years old.

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ECM ectomycorhizae
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Root exudation is reported to make up 5-20 of
total C assimilation, but in low phosphate soils
the amount can be much higher (estimates vary
widely). This is a huge cost to the plant!
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Exudates consist of a complex and variable
mixture of organic acids, sugars, vitamins, amino
acids, purines, nucleosides, inorganic ions
(HCO3-, OH-, H), gaseous molecules (CO2, H2),
enzymes and sloughed root cells.
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From Dakora and Phillips (2002) Plant and Soil
24535-47
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• Bringhurst et al 2001 (PNAS 984540-4545)
  constructed a strain of the soil bacterium
  Sinorhizobium meliloti containing a gene that
  results in fluroresence when exposed to galactose
  and galactosides. They used the bacterium as a
  biosensor to detect the release of galactosides
  from germinating seeds and from roots of alfalfa
  seedlings growing on artificial medium.

(the next few slides show images from their work.
They are mostly for your enjoyment)
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Sinorhizobium meliloti biosensor - reports
presence of galactosides
Bringhurst et al. (2001) PNAS
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Localized expression of the pmelAgfp reporter
Alfalfa
Bromus
Emerging lateral root
Root hairs
Bringhurst et al. (2001) PNAS
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Alfalfa root in non-sterile soil inoculated with
pmelAgfp and pconrfp reporters
Lateral root
Bringhurst et al. (2001) PNAS
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Galactosides are in a steep concentration
gradient near the root green fluorescence can
be observed out to 200 mm
Bringhurst et al. (2001) PNAS
14
Swarm of protozoa grazing on red fluorescent
bacteria
Bringhurst et al. (2001) PNAS
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This is an individual protozoan that has gorged
on red-fluorescing bacteria
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Why do plants release all of these complex
chemicals from their roots into the soil?
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• 1. Mobilization of chemically bound nutrients.
  Protons (H) and small organic acids exuded from
  the roots can solubilize nutrients in the
  rhizosphere by lowering pH. Organic acids also
  can solubilize iron and trace elements (e.g., Mn)
  by forming chelates. (More on this in a moment)
• Ion exchange for mineral nutrients adsorbed on
  clay and humic particles. Occurs through release
  of H and HCO3- (products of respiratory CO2
  dissolved in water).
• Enhancing activity of microbes (mycorrhizae, soil
  bacteria, etc.) through carbon feeding

See Larcher, 1995, The utilization of mineral
elements , in Physiological Plant Ecology also
Lambers, Chapin and Pons
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From Dakora and Phillips (2002) Plant and Soil
24535-47
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Chelating agents form complexes with cations like
iron, making them soluble in the soil solution
Chelating compounds also bind other cations, such
as calcium, releasing phosphates (which are
anions) from salts, like calcium phosphate, with
low solubility.
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Exudates also mediate communication
root-root e.g., allelopathy root-microbe
e.g. flavanoids in exudates from N2 fixers
activate genes in the Rhizibium melioti
bacterium that are responsible for the
nodulation process. Also, fungal and bacterial
pathogens can elicit release of antimicrobial
chemicals from roots. root-insect root
herbivores can stimulate the production of
insecticidal chemicals
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From Walker et al. (2003) Plant Physiology
13244-51
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In Ectomycorrhizae, which are normally found on
pine, spruce, fir, beech, eucalyptus, alder, oak,
and hickory, the fungi grow between rather than
within root cells. They form a structure known as
the Hartig net between the cells, as well as a
fungus mantle or cover on the surface of feeder
roots. The ectomycorrhizal feeder roots develop a
swollen appearance, and in pines they normally
have a forking habit (fig. 1-7).
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The endomycorrhizae are found on maple, sycamore,
ash, gum, walnut, cypress, some poplars, and some
other conifers. This mycorrhizal type also occurs
on all agronomic crops, such as sorghum, corn,
and grasses used as cover crops in tree
nurseries. These fungi grow into the root cells,
and a mantle or exterior cover is lacking.