Unit 7: Arbuscular Mycorrhizal Fungi (AMF)

I. Orientation: AMF as Soil-Borne Symbiotic Fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophic soil fungi belonging mainly to the phylum Glomeromycota. They colonize the roots of most terrestrial plant species and form highly branched arbuscules, where nutrients are exchanged between fungus and plant. AMF improve plant acquisition of phosphorus and other relatively immobile nutrients, while the host supplies photosynthetically fixed carbon. Their propagules include spores, colonized root fragments, and extraradical hyphae.

• Characteristic structures:

  • Arbuscules: Finely branched intracellular structures formed within root cortical cells; they are principal sites of nutrient exchange.
  • Vesicles: Lipid-rich storage structures produced by many, but not all, AMF taxa.
  • Extraradical hyphae: Hyphae extending into soil beyond the root-depletion zone and absorbing nutrients and water.
  • Spores: Thick-walled reproductive or resting propagules occurring singly, in clusters, or within sporocarps.

• Importance in disease management:

  • Improved nutrition: Enhanced phosphorus uptake can increase plant vigor and tolerance of infection.
  • Root competition: AMF occupy root cortical tissues and influence infection sites available to soil-borne pathogens.
  • Induced resistance: Colonization can prime jasmonate- and ethylene-associated plant defenses.
  • Rhizosphere modification: AMF alter root exudation, microbial communities, soil aggregation, and pathogen activity.
  • Qualified effect: Disease suppression depends on the AMF species, host genotype, pathogen, soil fertility, and environmental conditions; AMF are not universally protective.

• Basis of spore analysis:

  • Isolation: Spores must be separated from mineral particles, organic debris, and roots.
  • Identification: Taxa are distinguished principally through spore morphology, wall structure, subtending hyphae, and staining reactions.
  • Quantification: Spores are counted relative to a defined dry mass or volume of soil.
  • Interpretive limit: A spore count measures recoverable spores, not necessarily viable propagules, active root colonization, or symbiotic effectiveness.

II. Isolation of AMF Spores

Isolation concentrates AMF spores from soil while minimizing damage and loss. The standard procedure combines wet sieving and decanting with optional density-gradient centrifugation.

A. Isolation of arbuscular mycorrhizal fungi spores from soil samples

The purpose of isolation is to obtain a clean, representative spore fraction suitable for microscopic examination and counting.

• Representative sampling:

  • Sampling zone: Collect rhizosphere soil and fine-root-associated soil, commonly from the biologically active rooting depth of the host.
  • Subsamples: Combine multiple soil cores from the plot into a composite sample to reduce small-scale spatial variation.
  • Handling: Place samples in labelled bags and avoid prolonged heating, direct sunlight, desiccation, or waterlogging.
  • Metadata: Record host plant, location, soil depth, sampling date, management history, and field replicate.

• Sample preparation:

  • Homogenization: Mix soil gently after removing stones and large plant residues; crushing may rupture spores.
  • Defined quantity: Process a known mass, such as 50 or 100 g of field-moist soil.
  • Dry-mass correction: Determine moisture content from a separate subsample because field-moist masses are not directly comparable.
  • Dispersion: Suspend soil in water and stir thoroughly so spores and aggregates enter suspension.

• Wet sieving and decanting:

  1. Sedimentation: Allow coarse mineral particles to settle briefly after stirring; many spores and light organic particles remain suspended.
  2. Decanting: Pour the supernatant through a stack of sieves arranged from coarse to fine.
  3. Sieve selection: A coarse sieve, approximately 500–710 µm, removes roots and stones, while finer sieves, often around 45–250 µm, retain spores of different sizes.
  4. Repeated washing: Resuspend the sediment and decant several times to improve recovery.
  5. Collection: Wash material retained on each relevant sieve into separate Petri dishes or centrifuge tubes.

• Density-gradient purification:

  • Principle: Spores and light organic matter can be separated from dense sand and clay using a sucrose solution.
  • Procedure: Centrifuge the wet-sieved fraction, discard the supernatant if required, resuspend the pellet in approximately 40–60% sucrose solution, and centrifuge briefly.
  • Recovery: Pour the spore-containing supernatant onto a fine sieve and rinse immediately with clean water.
  • Critical precaution: Extended contact with concentrated sucrose can cause osmotic damage and interfere with subsequent assessment.

• Recovery for examination:

  • Transfer: Move retained material into a gridded Petri dish using a wash bottle.
  • Picking: Locate spores under a stereomicroscope and transfer them with a fine pipette, needle, or forceps.
  • Storage: Examine spores promptly or preserve them in a suitable mounting medium; water storage is appropriate only for short periods.

B. Applications and limitations

The isolation procedure supports ecological surveys and inoculum assessment, but its efficiency depends strongly on soil and spore characteristics.

• Applications: Isolated spores can be used for morphological identification, abundance estimates, viability testing, trap cultures, single-spore cultures, or molecular analysis.
• Recovery bias: Very small spores may pass through a coarse lower sieve, whereas large sporocarps may be retained with root debris.
• Soil effects: Clay-rich soils form persistent aggregates, while organic soils produce debris that can obscure spores.
• Spore damage: Vigorous stirring, prolonged centrifugation, excessive centrifugal force, and rough transfer may rupture or distort walls.
• Seasonal variation: Sporulation changes with host growth and climate; samples taken at different seasons may not be comparable.
• Quality control: Use the same soil mass, sieve sizes, washing duration, centrifugation conditions, and number of decanting cycles for all samples.

III. Identification of AMF Spores

Identification assigns isolated spores to morphotypes, genera, or species by examining stable diagnostic characters. Reliable morphological identification requires intact, mature spores and careful observation of wall layers and attachment structures.

A. Identification of arbuscular mycorrhizal fungi spores from soil samples

AMF spores are identified by combining gross appearance under a stereomicroscope with detailed wall analysis under a compound microscope.

• Preliminary sorting:

  • Size: Measure spore diameter with an ocular micrometer or calibrated imaging system and report it in micrometres (µm).
  • Colour: Record colour under standardized illumination, because apparent colour changes with mounting medium and microscope settings.
  • Shape: Note whether spores are globose, subglobose, irregular, ellipsoid, or formed in clusters.
  • Surface: Observe smoothness, ornamentation, pits, projections, adherent debris, or evidence of parasitism.

• Mount preparation:

  • PVLG mount: Polyvinyl alcohol–lactic acid–glycerol preserves spores and permits examination of wall structures.
  • Melzer’s reagent: A paired PVLG–Melzer’s mount helps reveal amyloid, dextrinoid, or other colour reactions in particular wall components.
  • Crushing: Apply gentle coverslip pressure to crack the spore and spread wall layers without completely destroying their arrangement.
  • Comparison: Examine intact and crushed spores from the same morphotype because attachment and internal structures may not be visible in one preparation.

• Diagnostic characters:

  • Spore-wall organization: Record the number, thickness, texture, flexibility, and continuity of wall layers or wall groups.
  • Subtending hypha: Examine the hyphal attachment, wall continuity, pore, septum, and shape at the spore base.
  • Germination structures: Note germination shields, germination orbs, inner flexible walls, or specialized compartments where present.
  • Developmental mode: Determine whether spores are glomoid, acaulosporoid, gigasporoid, or another recognized morphological type.
  • Contents: Oil droplets and cytoplasmic contents can support maturity assessment but are less dependable than wall characters.

• Taxonomic comparison:

  • Descriptions: Match the complete character combination with authenticated taxonomic descriptions rather than identifying from colour or size alone.
  • Multiple specimens: Examine several mature spores because age, degradation, mounting, and parasitism alter appearance.
  • Identification level: Report uncertain material as a genus-level identification or numbered morphotype instead of forcing a species assignment.
  • Molecular confirmation: Ribosomal DNA markers from spores or colonized roots can complement morphology, particularly for cryptic or damaged taxa.

B. Applications and limitations

Accurate identification reveals AMF community composition, but morphology alone does not always resolve species boundaries.

• Ecological use: Taxon records permit comparison of AMF richness, dominance, host association, and responses to fertilizers or biopesticides.
• Developmental variation: Young spores may lack final pigmentation and wall differentiation; old spores may have collapsed or degraded layers.
• Convergent appearance: Unrelated taxa can have similar size and colour, making wall architecture and germination features essential.
• Reference dependence: Identification quality depends on calibrated microscopy, taxonomic expertise, and access to validated descriptions.
• Biological limitation: Species detected as spores may differ from fungi actively colonizing roots because some AMF sporulate rarely or seasonally.

IV. Quantification of AMF Spores

Quantification expresses the abundance of recoverable AMF spores in a standardized amount of soil. Counting must use replicate samples and a clearly defined denominator.

A. Quantification of arbuscular mycorrhizal fungi spores from soil samples

AMF abundance is estimated by counting isolated spores under a stereomicroscope and converting the result to spores per unit dry soil.

• Counting procedure:

  • Distribution: Spread the extracted fraction evenly in a gridded Petri dish with enough water to prevent overlapping debris.
  • Systematic scan: Examine successive grid cells in a fixed direction so each field is counted once.
  • Counting rule: Define whether intact spores, viable-looking spores, broken spores, and sporocarps are recorded separately.
  • Replicates: Process independent soil subsamples rather than repeatedly counting only one extraction.

• Dry-mass conversion:

Dry soil mass = Fresh soil mass × (1 − M)

Spore density = N / D × U

• N: Number of spores counted in the processed sample.
• D: Dry mass of processed soil, in grams.
• M: Gravimetric moisture fraction, expressed as a decimal.
• U: Reporting unit, commonly 100 g dry soil.
• Result: Spore density is reported as spores per 100 g dry soil when U = 100.

• Worked example: If 80 g field-moist soil has a moisture fraction of 0.20 and yields 160 spores, its dry mass is 80 × 0.80 = 64 g. The density is:

Spore density = (160 / 64) × 100 = 250 spores per 100 g dry soil

• Statistical reporting:
  • Central value: Report the arithmetic mean across biological replicates.
  • Variation: Include standard deviation or standard error and state the number of replicates.
  • Transformations: Count data may require square-root transformation or count-based statistical models when variance increases with the mean.
  • Community measures: Separate morphotype counts can provide relative abundance, richness, and frequency of occurrence.

B. Applications and limitations

Spore density is a useful comparative indicator, but it is not a complete measure of AMF function or inoculum potential.

• Applications: Counts support comparisons among soils, seasons, hosts, treatments, inoculum products, and disease-management practices.
• Extraction efficiency: Reported density reflects recovered spores; sieve loss, aggregation, and observer error can cause underestimation.
• Viability: Empty, dormant, parasitized, and nonviable spores may be counted unless viability is assessed separately.
• Infectivity: A high spore count does not guarantee high root colonization because hyphae and colonized root fragments are also infective propagules.
• Complementary measurements: Interpret counts alongside percentage root colonization, most-probable-number assays, trap cultures, or molecular abundance data.
• Standardization: Meaningful comparison requires identical sampling depth, dry-mass basis, extraction method, sieve range, counting criteria, and sampling season.