plant-microbe interactions plant-microbe interactions diverse – from the plant perspective:...
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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
I. DecompositionPrimary 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. Processes1. 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
Microbial uptakeIm
mob
iliza
tionPlant uptake
NitriteNO2
-
NitrateNO3
-
energy fornitrifying bacteria*
Nitrification
For Nitrogen
proteins(insoluble)
aminoacids
energy for heterotrophic bacteria
proteases
AmmoniumNH4
+
Mineralization
* In 2 steps by 2 different kinds of bacteria – (1) Nitrosomonas oxidize NH3 to nitrites + (2)
Nitrobacter oxidize nitrites to nitrates
NH4+proteins
mineralization
NO3-
plant uptake
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
C. N uptake by plants – Chemical form taken up can vary
2) Ammonium (NH4+ ) –
• Used directly by plants in soils with low nitrification rates (e.g. wet soils)
proteins NH4+
mineralization
microbial uptake
immobilization
NO3-
nitrification
plant uptake
aminoacids
3) Some plants can take up small amino acids (e.g. glycine)• Circumvents the need for N mineralization• Facilitated by mycorrhizae
Direct uptake
D. Controls on rates of decomposition
1) Temperature – • Warmer is better• <45°C
2) Moisture – intermediate is best • Too little desiccation • Too much limits O2 diffusion T
Soil Moisture %
Soil Microbial Respiration
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 rateas 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
Consequence of controlling soil OM chemistry and microclimate …
Plants important factor controlling spatial variation in nutrient cycling
c) Plant secondary compounds
• Control decomposition by:
Bind to enzymes, blocking active sites lower mineralizationN compounds bind to phenolics greater immobilization by soilPhenolics C source for microbes greater immobilization by microbes
• Anti-herbivore/microbial• Common are phenolics – e.g. tannins
– Aromatic ring + hydroxyl group, other compounds
OH
R
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
II. Mycorrhizae
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
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
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
• Those with N2-fixing symbionts form root “nodules”– anaerobic sites that “house” bacteria
soybeanroot
alpine clover
Examples of plant–N2-fixing symbiotic systems –
1) Legumes (Fabaceae)
• Widespread• bacteria = e.g., Rhizobium spp.
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
Buffaloberry (Shepherdia argentea)
- actinorhizal shrub (Arizona)
2) Non-legume symbiotic plants –
• “Actinorhizal”= associated with actinomycetes (N2-fixing bacteria)• genus Frankia
• Usually woody species – e.g. Alders, Ceanothus
Ceanothus velutinus - snowbrush
Ceanothus roots, withFrankia vesicles
• Bacteria in root or small vesicles
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
IV. N losses from ecosystem
• Leaching to aquatic systems
• Fire Volatization
• Denitrification N2, N2O to atmosphere
– Closes the N cycle!• Bacteria mediated• Anaerobic
Natural N cycle
PLANT
PLANT
REMAINS
N2O
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.
ANTHROPOGENICSOURCES
Annual release(1012 g N/yr)
Fertilizer 80
Legumes, other plants 40
Fossil fuels 20
Biomass burning 40
Wetland draining 10
Land clearing 20Total from human sources
210
Altered N cycle
NATURAL SOURCESSoil bacteria, algae, lightning, etc.
140
Annual release(1012 g N/yr)
V. Rhizosphere interactions– the belowground foodweb
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
Fine root
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