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Plant-Microbe Interactions PowerPoint PPT Presentation


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Plant-Microbe Interactions. Plant-microbe interactions diverse – from the plant perspective: Negative – e.g. parasitic/pathogenic Neutral Positive – symbiotic.

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Plant-Microbe Interactions

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Plant-Microbe Interactions

  • Plant-microbe interactions diverse – from the plant perspective:

    • Negative – e.g. parasitic/pathogenic

    • Neutral

    • Positive – symbiotic

  • This lecture  important positive interactions with respect to plant abundance and distribution – related to plant nutrient and water supply:

    • Decomposition

    • Mycorrhizae

    • N2 fixation

    • Rhizosphere

       the role of this interaction in the N cycle


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  • Input rates –

    • Generally follow rates of production

    • Deciduous = evergreen

I. Decomposition

  • Primary supplier of plant nutrients – particularly N & P

  • Raw material

  • Soil organic matter derived primarily from plants –

    • Mainly leavesandfine roots

    • Wood can be important component in old growth forests


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nematode

termites

springtail (Isotoma viridis)

B. Processes

  • 1. Fragmentation –

    • Breakdown of organic matter (OM) into smaller bits = humus

    • By soil ‘critters’ – including nematodes, earthworms, springtails, termites

      • consume and excrete OM  incomplete digestion


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For Nitrogen

energy for heterotrophic bacteria

Mineralization

Ammonium

NH4+

proteins

(insoluble)

amino

acids

proteases

Immobilization

Nitrification

Nitrite

NO2-

energy for

nitrifying bacteria*

Microbial uptake

Nitrate

NO3-

Plant uptake

  • 2. Mineralization

    • Breakdown OM inorganic compounds

    • Microbial process: accomplished by enzymes excreted into the soil

  • * In 2 steps by 2 different kinds of bacteria – (1) Nitrosomonas oxidize NH3 to nitrites + (2) Nitrobacter oxidize nitrites to nitrates


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mineralization

proteins

NH4+

NO3-

plant uptake

C. N uptake by plants – Chemical form taken up can vary

  • 1) Nitrate (NO3-)

    • Preferred by most plants, easier to take up

    • Even though requires conversion to NH4+before be used  lots of energy

    • vs. taking up & storing NH4+ problematic

      • More strongly bound to soil particles

      • Acidifies the soil

      • Not easily stored

  • 2) Ammonium (NH4+ ) –

    • Used directly by plants in soils with low nitrification rates (e.g. wet soils)


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proteins

mineralization

NH4+

amino

acids

immobilization

nitrification

microbial uptake

NO3-

Direct uptake

plant uptake

  • 3) Some plants can take up smallamino acids(e.g. glycine)

    • Circumvents the need for N mineralization

    • Facilitated by mycorrhizae


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  • Temperature –

    • Warmer is better

    • <45°C

  • 2) Moisture – intermediate is best

    • Too little  desiccation

    • Too much  limits O2 diffusion

Soil Microbial Respiration

T

Soil Moisture %

D. Controls on rates of decomposition


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3) Plant factors – Litter quality

Decomposition rate

as fn(lignin, N)

Deciduous forest spp

  • b) Plant structural material

    • Lignin– complex polymer, cell walls

      • Confers strength with flexibility

      • – e.g. oak leaves

      • Relatively recalcitrant

      • High conc.  lowers decomposition

  • a) Litter C:N ratio (= N concentration)

    • If C relative to N high  N limits microbial growth

      • Immobilization favored

      • N to plants 


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OH

R

  • Consequence of controlling soil OM chemistry and microclimate …

    • Plants important factor controlling spatial variation in nutrient cycling

c) Plant secondary compounds

  • Anti-herbivore/microbial

  • Common are phenolics – e.g. tannins

    • – Aromatic ring + hydroxyl group, other compounds

  • Control decomposition by:

    • Bind to enzymes, blocking active sites lower mineralization

    • N compounds bind to phenolics greater immobilization by soil

    • Phenolics C source for microbes greater immobilization by microbes


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II. Mycorrhizae

  • Symbiotic relationship between plants (roots) & soil fungi

    • Plant provides fungus with energy (C)

    • Fungus enhances soil resource uptake

    • Widespread –

    • Occurs ~80% angiosperm spp

    • All gymnosperms

    • Sometimes an obligate relationship


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  • Major groups of mycorrhizae:

  • 1) Ectomycorrhizae –

    • Fungus forms “sheath” around the root (mantle)

    • Grows in between cortical cells = Hartig net – apoplastic connection

    • Occur most often

    • in woody spp


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Arbuscule in plant cell

  • 2) Endomycorrhizae –

    • Fungus penetrates cells of root

  • Common example is arbuscular mycorrhizae (AM)

    • Found in both herbaceous & woody plants

    • Arbuscule = exchange site


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  • C. Function of mycorrhizae:

    • Roles in plant-soil interface –

      • Increase surface area & reach for absorption of soil water & nutrients

  • Increase mobility and uptake of soil P

  • Provides plant with access to organic N

  • Protect roots from toxic heavy metals

  • Protect roots from pathogens

  • Effect of soil nutrient levels on mycorrhizae

    • Intermediate soil P concentrations favorable

      • Extremely low P – poor fungal infection

      • Hi P – plants suppress fungal growth

        • – taking up P directly

    • N saturation


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III. N2 Fixation

  • N2 abundant – chemically inert

  • N2 must be fixed = converted into chemically usable form

    • Lightning

    • High temperature or pressure (humans)

    • Biologically fixed

  • Nitrogenase– enzyme catalyzes N2 NH3

  • Expensive process – ATP, Molybdenum

  • Anaerobic – requires special structures


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  • Free-living in soil/water – heterocysts

  • Symbiotic with plants – root nodules

  • Loose association with plants

Anabaena with heterocysts

A. Occurs only in prokaryotes:

  • Bacteria (e.g. Rhizobium, Frankia)

  • Cyanobacteria (e.g. Nostoc, Anabaena)

  • Symbiosiswith plants – Mutualism

    • Prokaryote receives carbohydrates

      • Plant may allocate up to 30% of its C to the symbiont

    • Plant provides anaerobic site – nodules

    • Plant receives N


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alpine clover

soybean

root

  • Examples of plant–N2-fixing symbiotic systems –

    • Legumes (Fabaceae)

      • Widespread

      • bacteria = e.g., Rhizobium spp.

  • Those with N2-fixing symbionts form root “nodules”

  • – anaerobic sites that “house” bacteria


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Cross-section of nodules of soybean nodules

  • Problem of O2 toxicity –

    • Symbionts regulate O2 in the nodule with leghemoglobin

      • Different part synthesized by the bacteria and legume


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  • 2) Non-legume symbiotic plants –

    • “Actinorhizal”= associated with actinomycetes (N2-fixing bacteria)

      • genus Frankia

    • Usually woody species – e.g. Alders, Ceanothus

Ceanothus velutinus - snowbrush

Buffaloberry (Shepherdia argentea)

- actinorhizal shrub (Arizona)

Ceanothus roots, with

Frankia vesicles

  • Bacteria in root or small vesicles


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B. Ecological importance of N2 fixation

  • 1) Important in “young” ecosystems –

    • Young soils low in organic matter, N


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  • 2) Plant-level responses to increased soil N conc:

  • Some plants (facultative N-fixers) respond to soil N concentration 

    • Plant shifts to direct N uptake

    • N fixation 

    • Number of nodules decreases


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  • 3) Competition: N fixers-plant community interactions

  • N2-fixing plants higher P, light, Mo, and Fe requirements

    •  Poor competitors

    • Competitive exclusion less earlier in succession

    • Though - N2 fixers in “mature” ecosystems

  • Example N-fixing plants important in early stages of succession:

    • Lupines, alders, clovers, Dryas


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PLANT

REMAINS

PLANT

Natural N cycle

N2O

  • IV. N losses from ecosystem

    • Leaching  to aquatic systems

    • Fire  Volatization

    • Denitrification  N2, N2O to atmosphere

    • – Closes the N cycle!

      • Bacteria mediated

      • Anaerobic


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Fertilizer

80

Legumes, other plants

40

Fossil fuels

20

Biomass burning

40

Wetland draining

10

Land clearing

20

Total from human sources

210

Annual release(1012 g N/yr)

NATURAL SOURCES

Soil bacteria, algae, lightning, etc.

140

ANTHROPOGENICSOURCES

Annual release(1012 g N/yr)

Altered N cycle

From - Peter M. Vitousek et al., "Human Alteration of the Global Nitrogen Cycle - Causes and Consequences," Issues in Ecology, No. 1 (1997), pp. 4-6.


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  • V. Rhizosphere interactions

    • – the belowground foodweb

Fine root

  • Zone within 2 mm of roots – hotspot of biological activity

  • Roots exude C & cells slough off = lots of goodies for soil microbes  lots of microbes for their consumers (protozoans, arthropods)

  • “Free living” N2-fixers thrive in the rhizosphere of some grass species


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Summary

  • Plant–microbial interactions play key roles in plant nutrient dynamics

    • Decomposition –

      • mineralization, nitrification …

      • immobilization, denitrification …

    • Rhizosphere – soil foodweb

    • Mycorrhizae – plant-fungi symbiosis

    • N fixation – plant-bacteria symbiosis

  • Highly adapted root morphology and physiology to accommodate these interactions

  • N cycle, for example, significantly altered by human activities


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