Microbial rhizospheric products and Classification of Mycorrhizae (Lecture 19 & 20)
Types of plant materials in the rhizosphere
}Exudates: Water-soluble small molecular-weight organic materials, passive release from the roots (loss of organics from plants).
}Secreted materials: actively released organic materials from the root cells (mucigel) → better root growth, greater nutrient uptake of plants.
}Lizates: released from the dead or damaged root cells (nucleic acids, amino acids).
}Gaseous materials: plant ethylene (plant growth hormone).
Beneficial microbes in the rhizosphere, the role of micro-symbionts
Symbiosis=beneficial, synergistic interaction
}Arbuscular mycorrhizal fungi (AMF)
}Phosphorus (P) mobilizing microbes
}Nitrogen-fixers (Rhizobium, Azospirillum)
}Plant-growth-promoting rhizobacteria (PGPR),
}Siderophore producing Pseudomonas sp.
}Hormone-producing, plant-growth-regulating (PGR) microbes
A.Symbiotic relationship between plants (roots) & soil fungi
- Plant provides energy (C) to fungus
- Fungus enhances soil resource uptake
Mycorrhizae are extremely Widespread –
- Occurs ~80% of angiosperm sp.
- All of gymnosperms
- Sometimes an obligate relationship
The bi-directional movement of nutrients, carbon (C) from plant to fungus, and soil nutrients from fungus to plant,
is the essential feature of a mycorrhiza,
and is believed to be the basis for the mutualistic association.
As a rule, mycorrhizal infection enhances plant growth,
by increasing nutrient uptake in up to three ways:
- increasing the surface area of absorption within the soil;
- by mobilizing sparingly-available nutrient sources from unavailable compounds;
- by excreting chelating compounds or ectoenzymes.
Classification of Mycorrhizae
Mycorrhizas were traditionally classified into the two types:
a classification based on the location of the fungal hyphae in relation to the root tissues of the plant;
ecto means outside the root, endo means inside.
This classification is now regarded as too simplistic, and there is now a nomenclature identifying seven mycorrhizal types
- 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
-In this type of mycorrhiza, the fungal sheath, that forms around the secondary root tips, accumulate minerals from the decomposing litter.
-Fungus does obtain simple carbohydrates that are produced by the plant, but not used by the plant. So it appears that these carbohydrates may be specifically produced by the plant for the fungus.
-Usually, species from Basidiomycota and the Ascomycota form ectomycorrhizal association with plants and tress.
-Plants that are involved in ectomycorrhizae are always trees and are found only in a few families.
-They include the Betulaceae (Alders), Casuarinaceae (Ironwood), Fagaceae (Oaks), Myrtaceae (Eucalyptus) and Pinaceae (Pines) etc..
-Most of these are utilized as a source of lumber.
-When planting these trees, it is a routine practice,
in forestry, to inoculate the seedling with a mycorrhizal fungus.
-This group of mycorrhiza have also been tested as a
means of resisting fungal, root pathogens.
-The fungal sheath of the ectomycorrhizal fungus
is covering the root tips, fungal root pathogens would be
unable to gain entry into the root system of the host.
2) Endomycorrhizas, in which the fungal structure is almost entirely within the host root, comprising three major and two minor groupings:
- Arbuscular (AM) endomycorrhizas
- Ericoid endomycorrhizas
- Arbutoid endomycorrhizas
- Orchidaceous endomycorrhizas
- Monotropoid endomycorrhizas
- Fungus penetrates cells of root
- Found in both herbaceous & woody plants
- Arbuscule = exchange site
- Externally to the host the fungal hyphae produce the very large spores (often called chlamydospores).
- Spores germinate near a plant root and the germinating hyphae penetrate the root in response to root exudates.
- Hyphae grow through the root tissues and in the root cortex hyphal branches form appressoria that penetrate the plant cells. The host plasmalemma invaginates and proliferates around the fungal intrusion.
- All AM fungi are obligatory biotrophic, as they are completely dependent on plants for their survival.
- AM are found extensively in pteridophytes, gymnosperms and angiosperms of all habitats.
-These structures (arbuscules) can be recognized by their branched tree-like appearance.
-Another structure that can be frequently observed are the rounded vesicles.
-The vesicles and arbuscules contain the stored minerals that are needed by the plant.
-The group of fungi involved is always a member of the Zygomycota.
Mycorrhizal classification and N2 Fixation
Ericaceous (Ericoid) mycorrhiza:
-The mycorrhiza formed in this group is between fungi in the Ascomycota, and more rarely in the Deuteromycota, and species in the families Epacridaceae, Ericaceae.
-Ericoid mycorrhizae have evolved in association with plants that are continually under stress within the soil.
-The soil is typically extremely acid, peatland soil, low in available minerals because mineralization is inhibited.
Plants with ericoid mycorrhizae seem to have a high tolerance to these stresses.
Monotropoid mycorrhiza (ecto, endo)
-The Monotropaceae and Pyrolaceae are two families of plants that are achlorophyllous, form association with the fungi belonging to Basidiomycota
-Thus, plants in these families are more dependent upon their mycorrhizal partners which can carry out photosynthesis.
-Fungi colonizing achlorophyllous plants in the Monotropaceae family (but now included within the Ericaceae), such as Monotropa hypopitys (in Europe) and M. uniflora (North America).
Arbutoid mycorrhiza (ecto, endo)
-This group forms associations with plants that are trees and shrubs that belong to the genera Arbutus and Pyrola.
-Fungi forming this association are members of the Basidiomycota.
- Arbutoid mycorrhizas are, like those of Ericoid and Monotropoid mycorrhizas, found in the plant order Ericales, family Ericaceae and Pyrolaceae, and in the genera Arctostaphylosand Arbutus.
- Root systems of species of Pyrolado not resemble those of ectomycorrhizas as much as do Arbutusand Arctostaphylos
-Orchid mycorrhiza is an endomycorrhizal association (fungus & orchidaceae).
-Most orchids associate with saprotrophic or pathogenic species of Basidiomycota, while a few associate with ectomycorrhizal fungal species.
- The Orchidaceae is one of the largest families in the plant kingdom, with more than 20,000 species identified,
- Orchid mycorrhiza are similar to ericoid mycorrhizas but their carbon nutrition even is more dedicated to supporting the host plant as the young orchid seedling is non-photosynthetic and depends on the fungus partner utilizing complex carbon sources in the soil, and making carbohydrates available to the young orchid.
- All orchids are achlorophyllous in the early seedling stages, but usually chlorophyllous as adults, so in this case the seedling stage orchid can be interpreted as parasitizing the fungus.
- A characteristic fungus example is the Basidiomycete genus Rhizoctonia
|Summary of the characteristics of the seven types of mycorrhiza.|
|Fungal sheath||no||no||yes or no||yes||no||yes||yes or no|
|Vesicles||yes or no||no||no||no||no||no||no|
|Plant host chlorophyllous*||yes
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
- High P – plants suppress fungal growth
– taking up P directly
- high N (N saturation) results in decrease in mycorrhizae
III. N2 Fixation:
Most abundant gas on earth, but chemically inert – Triple bond
-Nitrogen gas (N2) constitutes nearly 80% of the atmosphere, but in this form nitrogen is not available to plants.
-Several groups of soil bacteria convert nitrogen gas into ammonium (NH4+), which is a form of nitrogen that can be used directly by plants.
-This transformation is catalyzed by the bacterial enzyme nitrogenase.
-Very few bacteria have the capacity to activate the nitrogenase enzyme system, and those that do occur as
-free living bacteria in soil, as loose associations with root surfaces or
-within highly specialized, symbiotic associations with plants.
Highly specific nitrogen-fixing associations are also formed
-between some tree species and bacteria (actinomycetes) in the genus Frankia.
-The more primitive gymnosperms such as cycads also form characteristic associations with cyanobacteria.
-The most widely studied of all nitrogen fixing associations are formed between legumes and bacteria from the family Rhizobiaceae.
A. Occurs only in prokaryotes:
- Bacteria (e.g. Rhizobium, Frankia)
- Cyanobacteria (e.g. Nostoc, Anabaena)
ØFree-living in soil/water – heterocysts
ØSymbiotic with plants – root nodules
ØLoose association with plants– in soil. May not release N until die
What is ‘associative’ nitrogen fixation?
-Nitrogen-fixing bacteria that live in loose associations with roots are called ‘associative’ nitrogen fixers.
-Associative nitrogen fixing bacteria grow on the surface of roots and may also colonize the outer layers of a root by entering between epidermal cells.
-A common genus of associative nitrogen fixing bacteria is Azospirillum.
-Species of Azospirillum are not all equally effective at converting atmospheric nitrogen to ammonium.
-There is conflicting evidence about the extent to which nitrogen fixed by these bacteria is available to plants. Even for the species of Azospirillum that are very effective at fixing atmospheric nitrogen, the nitrogen fixed is not immediately available to the plant. It is not until the bacteria die and are mineralized that the nitrogen becomes available to the plants.
Given that mycorrhizas are nearly always beneficial to a plant’s growth, and often health, their potential use to humans, in terms of agriculture and horticulture, could be immense.
Despite this potential inoculation of crops is rarely seen and mycorrhizas are only introduced deliberately in a few industries.
Many studies have shown that a variety of crops respond well to inoculation with arbuscular mycorrhizal (AM) fungi (Fig. 1).
The major crop plants, maize, wheat and barley, show growth rates increasing by 2.3, 3 and 4 times respectively when inoculated with AM fungi.
Onion inoculated with AM fungi show growth 6 times greater than non-AM onions.