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