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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.

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slide1

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

slide2

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
slide3

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
slide4

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
slide5

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)
slide6

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
slide7

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

slide8

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 
slide9

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
slide10

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
slide11

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
slide12

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
slide13

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
slide14

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
slide15

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
slide16

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
slide17

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
slide18

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
slide19

B. Ecological importance of N2 fixation

  • 1) Important in “young” ecosystems –
    • Young soils low in organic matter, N
slide20

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
slide21

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
slide22

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
slide23

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.

slide24

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
summary
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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