The Plant Fungi Partnership — Benefits of Trade Presentation

Plant Roots Function

Roots, along with leaves, are important organs of plants.

Their functions are diverse and vary across species. Two of their best-known functions are supply of resources and mechanical support.

Chief Resources

• Carbon
• Water
• Nutrients

Importantly, the roots not only absorb the resources from the soil but are able to redistribute them if needed.

Additional Functions

• Nutrient and Water Storage
• Chemical Balancing
• Sensor Network
• Absorptive Function
• Habitat for Mycorrhizal Fungi

Plant Roots Structure 1

The outer layer of the root is termed cortex. It is mainly composed of the cells with large vacuoles used to store solutes. Its size and capacity differ across species.

Epidermis is the outmost layer of the cortex. It is in direct contact with soil and serves as a conductor for resource distribution and sensory perception. The function of epidermis obviously depends on the area of contact.

To extend the contact with soil, epidermis cells grow into long extensions called root hairs. The denser and thicker the hairs, the better the functioning capacity of the root.

Plant Roots Structure 2

Under the cortex, the root contains exodermis – a layer of cells modified with a hydrophobic compound called suberin. Its most likely function is protection of the root from adverse soil conditions and parasites (University of Western Australia, 2008b).

Endodermis is the innermost layer of cells which separates the stele from the outer layer and provides isolation allowing for water and nutrient transfer in the root.

Importantly, endodermis also regulates mycorrhiza associations by confining the penetration of fungi into the outer layers of the root (University of Western Australia, 2008b).

Fungal Symbiosis 1

The plant-fungi symbiosis is a type of relationship where fungi living on the plants’ roots absorb a portion of their nutrients.

Such setting is usually perceived as undesirable since the symbiosis is incorrectly associated with parasitic activity.

Besides, fungi are known as a reason behind several plant diseases, including smut, rust, blight, powdery mildew, and fusarium wilt, among others (The Compost Gardener, n.d.).

Fungal Symbiosis 2

In reality, the process of symbiosis requires mutual benefits for all organisms involved in the relation.

Mycorrhizal associations, in particular, are thought to be largely beneficial for the carrier (the plant).

While the understanding of the fungi-plant symbiosis is far from complete because of its extreme complexity, the current consensus of the issue suggests the following benefits:

• Increased nutrient uptake
• Assistance in nitrogen fixing process
• Drought tolerance
• Increased disease resistance
• Glomalin production
• Improved resource redistribution (The Compost Gardener, n.d.)

Forms of Symbiosis

Two types of symbiotic relations exist.

Ectomycorrhiza

• Fungi do not penetrate walls of the root’s cells.
• Use hyphae to form an intercellular interface known as Hartig Net (University of Western Australia, 2008a).

Endomycorrhiza (i.e. arbuscular mycorrhiza)

Fungi penetrate cortical cells of the root. Arbuscular mycorrhiza form arbuscules and vesicles – specialized structures which allow for better nutrient and micronutrient exchange (University of Western Australia, 2008a).

Stages of Symbiosis 1

The symbiosis is initiated by soil hyphae (e.g. external or extraradical hyphae) present in the soil or left over from previous root activity.

1. Upon detecting new root presence, hyphae start growing toward it and along the root surface.
2. Some hyphae penetrate between epidermal cells and form swellings called appressoria (University of Western Australia, 2008a).
3. Hyphae from the appressoria start penetrating cortical cells and form a network in the outer cortex.

Two main types of VAM hyphae

• Distributive (thicker, responsible for association propagation)
• Absorptive (thinner, facilitate nutrient acquisition)

Stages of Symbiosis 2

Inside the cortex, hyphae eventually spread to form a colony – an association originating from the same external hyphae.

Two types of colony morphology

• Linear (Arum) Characterized by longitudinal growth between host cells. Occurs in roots which have intercellular air spaces aligned horizontally.
• Coiling (Paris) A colony grows in the form of a coil. Occurs where the root contains no longitudinal intercellular spaces.

Stages of Symbiosis 3

In a few days after root penetration, VAM start establishing what is considered main means of exchange with host root.

Arbuscules

• Vastly branched fine hyphal structures resembling trees.
• Form a nutrient exchange interface
• Grow inside individual cells.
• Short-lived (lifespan of several days)

Vesicles

• hyphal swellings in cortical cells which have accumulative function
• Contain lipids and cytoplasm
• Intercellular or intracellular
• Have lifespan of months or years

Stages of Symbiosis 4

Spores are swellings of hyphae similar to vesicles.

Features

• Can form inside the root or on soil hyphae.
• Contain lipids, cytoplasm, and nuclei.
• Develop thick walls.
• Sometimes cluster to form a structure called sporocarp (University of Western Australia, 2008a).

Functions

• Act as propagules (form specialized germination structures or grow hyphae directly through spore walls)
• Remobilize resources (accumulate nutrients with senescence of root association)

Benefits

• The resulting hyphal network allows arbuscular mycorrhizal fungi (AMF) to take carbon directly from the root.
• In return, the network improves phosphorus intake by the plant.
• This is achieved mainly by the density of fine hyphae in the soil which contributes the area of root surface.

In economic terms, such symbiosis can be compared to international trade with two partners specializing in certain markets.

Benefits of Specialization 1

In the described relationships, the root specializes in producing sugars. It has an advantage over AMF thanks to photosynthetic mechanisms of the plant.

Phosphorus is usually present in soil in forms which are hard for plants to use.

Mycorrhizal fungi produce acids which break down mineral compounds into ones usable by plants.

The fungi, on the other hand, make it possible for the plant to obtain phosphorus in greater amounts.

Roots have a lower opportunity cost of carbon production (they sacrifice less for it) (Pearson Learning Solutions, 2014).

The AMF have a comparative advantage in supply of phosphorus (Pearson Learning Solutions, 2014).

Benefits of Specialization 2

Observations reveal that plants which exist in symbiosis with AMF are healthier and have better chances of survival.

It is thus tempting to assume that the carbon investment results in sufficient returns for plant roots.

A paper by Walder et al. (2012) measures and illustrates these benefits by studying microcosms of two plants interlinked by a common mycorrhizal network (CMN).

This fact aligns well with the principles of specialization, which, like the definition of symbiosis discussed above, suggest that these relations are beneficial for both partners.

The results indicated a return of investment of nitrogen and phosphorus sufficient for a highly facilitated growth of both plants and the overall biomass production surpassing the mean of separate monocultures (Walder et al., 2012).

Benefits of Specialization 3

Intriguingly, the results of the study also indicate strong asymmetry in returns.

One plant invested about a third of its carbon to CMN and gained massive returns (94% of phosphorus and 80% of nitrogen) (Walder et al., 2012).

• Virtually unattainable outside symbiosis
• Enormous contribution to plant’s development

Another plant made a much greater investment of about 70% of its carbon. However, the returns were much more modest (6% phosphorus and 20% nitrogen) (Walder et al., 2012).

• Ineffective when viewed in separation
• Does not affect growth
• Contributes to intercropping

In economic terms, the lower opportunity cost of carbon production of the second plant allows it to redistribute resources and benefit other trade partners without significant sacrifice.

Benefits of Specialization 4

Aside from the direct benefits of nutrient exchange, the symbiosis of mycorrhizal fungi and plant roots demonstrates several indirect benefits.

• Rhizobia requires the assistance of mycorrhizal fungi to fix nitrogen.
• The size of hyphae allows the CMN to collect water more efficiently and hydrate plants in dry periods.
• The intra-root hyphae form a barrier which protects the root mechanically from harmful organisms and facilitate growth of cultures which defend plants from disease.
• Glomalin produced by CMN prevents soil erosion and aggregates its particles, which improves air and water penetration (The Compost Gardener, n.d.).

Basically, the fungi have a competitive advantage in producing a range of products and services in exchange for carbon offered by the root.

Green Economy

The symbiosis with mycorrhizal fungi offers benefits not only on the microcosm level but poses several possibilities of economic and ecological variety.

Ecology

• Maximize efficiency of nutrient and water intake naturally present in soil.
• Decrease erosion rate.
• Redistribute resources, thereby increasing the chances of survival of an ecosystem.
• Create a self-regulating sustainable system (Green Economy, 2012).

Economy

• Decrease the need for costly hydration and fertilization.
• Minimize the disruptive effect of unwise agricultural practices.
• Equalize the yield and increase predictability of an outcome.
• Decrease number of steps in a production cycle and lower carbon imprint.

References

Green Economy. (2012). Web.

Pearson Learning Solutions. (2014). International trade and finance. Web.

The Compost Gardener. (n.d.). The plant fungi partnership – Benefits of trade. Web.

University of Western Australia. (2008). Arbuscular mycorrhizas. Web.

University of Western Australia. (2008). Roots. Web.

Walder, F., Niemann, H., Natarajan, M., Lehmann, M. F., Boller, T., & Wiemken, A. (2012). Mycorrhizal networks: common goods of plants shared under unequal terms of trade. Plant Physiology, 159(2), 789-797.