Plant-Microbe Interactions

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

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

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

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

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)

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

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

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 

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

Plant-Microbe Interactions