PLANT MICROBE INTERACTIONS
MYCORRHIZAE
What is mycorrhizae
➢Mutualisms between plant roots and fungi in which nutrients are transferred in
both directions
➢Associations are formed with plants of all sizes ranging from trees, shrubs, fobs
and grasses.
➢The fungus transfers nutrients in particular, phosphorus and nitrogen from the
soil to the plant
➢The plant in turn transfers carbohydrates to the fungus.
➢These mutualisms are harnessed in agriculture where soil inoculants are
produced to enhance plant growth.
Types of mycorrhizae
Ectomycorrhizae
➢Characterized by hyphae that grow in an extra and intracellular fashion
▪ However, with extensive fungal growth outside the root
➢The structure of an ectomycorhizae is made up of;
▪ a sheath of fungal tissue, Hartig net, outward growing hyphal elements
➢Mainly found on roots of forest trees, especially conifers, beeches and oaks.
➢Association formed by Basidiomycota and Ascomycota
▪ Examples woodland mushrooms, such as Amanita spp., Boletus spp. and Tricholoma spp.
Types of mycorrhizae
Endomycorrhizae
➢The most common type of is arbuscular
mycorrhizae (AM)
➢Form a loose network on the root
➢Grows extensively within the root.
➢Associated with roots of about 80% of plant
species
➢Structure;
▪ arbuscules.
▪ vesicles.
▪ Fungal hyphae in the soil
➢Formed with members of phylum Glomeromycota
➢These are obligately biotrophic
Types of mycorrhizae
Ectomycorrhizae Endomycorrhizae
Root colonisation
➢Root colonization stimulated by the root exudates in the rhizosphere
➢Colonization of root arises from several sources of inoculum collectively
termed propagules
▪ Spores
▪ Infected root fragment
▪ Extraradical mycelium (ERM)
Root colonisation by AM fungi
Root colonisation by AM fungi
• Begins with germination of soil borne spore, producing a short germination
mycelium
• This recognised the host plant through reciprocal chemical signalling
• Plant-derived strigolactones are perceived by the AM fungi
• Fungi then produces a mixture of chitooligosaccharides and lipo-
chitooligosaccharides (LCOs)
• Attachment of fungal hyphae occurs on root epidermal cells
• hyphal tips differentiate to form hyphopodia, before hyphal entry of the root
• Penetrating hyphae then extends into the plant until inner cortex where arbuscles
are formed.
• Host plasma membrane then expands to form periarbuscular membrane
Benefits for the plant
• The beneficial effect of the mycorrhizal fungus on the plant is best
observed in poor soils
• Mycorrhizal plants can absorb nutrients from the environment more
efficiently and thus have a competitive advantage.
• The improved nutrient absorption is due to the greater surface area
provided by the fungus
• The association confers the following to the plant
▪ Increased nutrient uptake – phosphorus, nitrogen, zinc and copper
▪ Increased water uptake
▪ Protection from root pathogens
Nutrient uptake - Phosphorus
• Inorganic phosphate (Pi) is required in relatively large amounts by
the plant but often limiting in the environment
• More than 80% of the soil P becomes immobile and unavailable for
plant uptake because of adsorption by soil particles or conversion to
the organic form
• Pi is mainly supplied to the root absorbing surfaces by diffusion
• This results with a P depletion zone around the plant, hence no
phosphorus for the plant.
• The fungal hyphae play an important role in the acquisition of P for
the plant
Nutrient uptake - Nitrogen
• Nitrogen as nitrate (NO-3) predominates in agricultural soils
where it is readily available – AM of less importance in these soils
• Mycorrhizal associations are more beneficial in water deficient
soils where mobility of NO-3 is affected.
Other benefits
• Water uptake
• Enhanced by increasing the absorbing surface of the root system and by
accessing the smallest soil pores
• Mycorrhizae therefore contribute significantly to increase plant resistance to
drought and can also relieve other abiotic stresses
• Plant Protection
• AM fungi also help plants to resist and overcome pathogen infections
• Bioprotection by AM fungi resulted from a combination of local and systemic
effects
Plant protection
Mechanisms
• Morphological alterations
➢Thickening of the plant cell wall
➢Stronger vascular system
• Improved nutrient status of the host plant
• Competition for host photosynthates
• Activation of the plant defences
✓plant defence compounds
✓Higher levels of phenols and amino acids
Benefits to Mycorrhizae
• Carbon Source
• Habitat Stability
• Protection Against Stress
