Transcript Document

Plant-Microbe Interactions
interactions diverse – from the plant perspective:
• Negative – e.g. parasitic/pathogenic
• Neutral
• Positive – symbiotic
 Plant-microbe
lecture  important positive interactions with respect to plant
abundance and distribution – related to plant nutrient and water
supply:
 This
Decomposition
Mycorrhizae
N2 fixation
Rhizosphere
 the role of this interaction in the N cycle
I. Decomposition
Primary supplier of plant nutrients – particularly N & P
A. Raw material
Soil organic matter derived primarily from plants –
• Mainly leaves and fine roots
• Wood can be important component in old growth forests
Input rates –
• Generally follow
rates of production
• Deciduous =
evergreen
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
nematode
springtail (Isotoma viridis) termites
2. Mineralization
• Breakdown OM  inorganic compounds
• Microbial process: accomplished by enzymes excreted into the soil
For Nitrogen
energy for heterotrophic bacteria
proteins
(insoluble)
amino
proteases acids
Mineralization
Ammonium
NH4+
Nitrification
Nitrite
NO2-
Microbial uptake
Plant uptake
energy for
nitrifying
bacteria*
Nitrate
NO3-
* In 2 steps by 2 different kinds of bacteria – (1)
Nitrosomonas oxidize NH3 to nitrites + (2)
C. N uptake by plants – Chemical form taken up can vary
1) Nitrate (NO
proteins
3)
mineralization
NH4+
• 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
NO3-
plant uptake
2) Ammonium (NH4+ ) –
• Used directly by plants in soils with low nitrification rates (e.g. wet soils)
3) Some plants can take up small amino acids (e.g. glycine)
• Circumvents the need for N mineralization
• Facilitated by mycorrhizae
mineralization
proteins
amino
acids
NH4+
immobilization
nitrification
microbial uptake
Direct uptake
NO3plant uptake
D. Controls on rates of decomposition
1) Temperature –
• Warmer is better
• <45°C
Soil Microbial Respiration
2) Moisture – intermediate is best
• Too little  desiccation
• Too much  limits O2 diffusion
T
Soil Moisture %
3) Plant factors – Litter quality
a) Litter C:N ratio (= N concentration)
• If C relative to N high  N limits microbial growth
• Immobilization favored
• N to plants 
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
c) Plant secondary compounds
• Anti-herbivore/microbial
• Common are phenolics – e.g. tannins
– Aromatic ring + hydroxyl group, other compounds
OH
R
• 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
Consequence of controlling
soil OM chemistry and
microclimate …
Plants important factor
controlling spatial
variation in nutrient
cycling
II. Mycorrhizae
A. 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
B. 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
2) Endomycorrhizae –
• Fungus penetrates cells of root
• Common example is arbuscular mycorrhizae
(AM)
• Found in both herbaceous & woody plants
• Arbuscule = exchange site
Arbuscule in plant cell
C. Function of mycorrhizae:
1) Roles in plant-soil interface –
a) Increase surface area & reach for absorption of soil water & nutrients
b) Increase mobility and uptake of soil P
c) Provides plant with access to organic N
d) Protect roots from toxic heavy metals
e) Protect roots from pathogens
2) 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
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
A. Occurs only in prokaryotes:
• Bacteria (e.g. Rhizobium, Frankia)
• Cyanobacteria (e.g. Nostoc, Anabaena)



Free-living in soil/water – heterocysts
Symbiotic with plants – root nodules
Loose association with plants
Anabaena with heterocysts
Symbiosis with 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
Examples of plant–N2-fixing symbiotic systems –
1) Legumes (Fabaceae)
• Widespread
• bacteria = e.g., Rhizobium spp.
• Those with N2-fixing symbionts form root “nodules”
– anaerobic sites that “house” bacteria
alpine clover
soybean
root
Problem of O2 toxicity –
• Symbionts regulate O2 in the nodule with leghemoglobin
• Different part synthesized by the bacteria and legume
Cross-section of nodules of soybean nodules
2) Non-legume symbiotic plants –
• “Actinorhizal”= associated with actinomycetes (N2-fixing bacteria)
• genus Frankia
• Usually woody species – e.g. Alders, Ceanothus
• Bacteria in root or small vesicles
Ceanothus velutinus - snowbrush
Ceanothus roots, with
Frankia vesicles
Buffaloberry (Shepherdia argentea)
- actinorhizal shrub (Arizona)
B. Ecological importance of N2 fixation
1) Important in “young” ecosystems –
• Young soils low in organic matter, N
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
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
Natural N cycle
N 2O
PLANT
IV. N losses from ecosystem
• Leaching  to aquatic systems
• Fire  Volatization
• Denitrification  N2, N2O to
atmosphere
– Closes the N cycle!
• Bacteria mediated
• Anaerobic
REMAINS
PLANT
Annual release
(1012 g N/yr)
NATURAL SOURCES
Soil bacteria, algae, lightning, etc.
ANTHROPOGENIC
SOURCES
140
Annual release
(1012 g N/yr)
Fertilizer
Legumes, other plants
80
40
Fossil fuels
Biomass burning
Wetland draining
Land clearing
20
40
10
20
Total from human sources
Altered N cycle
210
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.
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
• 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