Biology · Fungal Biology & Symbioses

Mycorrhizae, lichens and other fungal partnerships

A fungal branch is visible inside the outer wall of a root cell. It is tempting to imagine fungal and plant cytoplasm freely mixed. In an arbuscular mycorrhizal interface, however, a plant-derived membrane surrounds the fungal structure and separates it from the plant cytoplasm. 'Inside a cell' needs a more precise boundary description.

Phylogeny, Classification & the Tree of Life introduces clades and the difference between similarity and ancestry. Energy, Enzymes & Metabolism supplies enzyme action, fermentation and separate material and energy accounts. Meiosis & Patterns of Inheritance supplies chromosome sets and DNA-copy accounting for the optional nuclear-cycle calculation. Qualitative explanations come before those calculations; the model's units and counting rules are introduced where they are used. Cells, Membranes & Organisation is a useful return path for membrane boundaries, and Microbial Eukaryotes & Algal Diversity introduces photosynthetic partners.

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By the end, you can

Locate plant–fungal and lichen interfaces, trace an assigned carbon exchange, and evaluate benefits for partners under stated conditions.

Two different root interfaces

Mycorrhiza names a fungal association with plant roots. In arbuscular mycorrhiza, branching fungal structures called arbuscules form within root cells while remaining outside direct contact with the plant cytoplasm. A plant-derived interface membrane surrounds each structure. The fungus also retains its own wall and plasma membrane. Exchange occurs across an organised interface, rather than by combining two cytoplasms.

The local cross-section model places fungal cytoplasm inside a fungal membrane of radius 1 drawing unit. Its wall extends outward to radius 1.2. An interface space lies between that wall and the surrounding plant-derived membrane at radius 1.5. Plant cytoplasm lies outside the latter boundary in this selected view. The drawing is a slice through one branch, not a whole plant cell or a scale measurement.

In the ectomycorrhizal comparison, a selected fungal strand lies in the gap between two root cells. Such intercellular hyphae form a Hartig net; many ectomycorrhizal roots also have an outer fungal mantle. The model's local gap view omits the mantle and the rest of the root. It contrasts an intercellular route with the arbuscular interface without claiming that all root associations have either one of these exact geometries.

Intracellular location and separate cytoplasm

The first slice preserves a plant-derived boundary around the fungus; the second preserves an intercellular route. Exchange arrows describe potential directions without assigned rates.

Read the diagram in words

The arbuscular branch has fungal cytoplasm inside radius 1, wall from 1 to 1.2, interface space from 1.2 to 1.5, and a plant-derived membrane at 1.5 separating it from surrounding plant cytoplasm. The second panel places a strand centred at x = 4, half-width 0.25, in the gap between root cells ending at x = 3 and starting at x = 5.

Selected arbuscular interface
Boundary or compartment	Inner radius	Outer radius
Fungal plasma membrane	1	1
Fungal wall	1	1.2
Plant-derived interface membrane	1.5	1.5

Selected arbuscular interface
Boundary or compartment	Inner radial boundary	Outer radial boundary
Fungal cytoplasm	0	1
Fungal wall	1	1.2
Interface space	1.2	1.5
Plant cytoplasm in this local slice	1.5	2.2

Selected arbuscular interface
From	To	Relationship
Fungal cytoplasm	Plant cytoplasm in this local slice	Nutrient exchange crosses distinct boundaries
Plant cytoplasm in this local slice	Fungal cytoplasm	Organic carbon exchange crosses distinct boundaries

Selected intercellular ectomycorrhizal strand
Local component	Drawing x	Drawing y	Drawing width	Drawing height
Root cell 1	0	0	3	6
Root cell 2	5	0	3	6

Selected intercellular ectomycorrhizal strand
Containing root cell	Drawing x	Drawing y	Drawing radius
Root cell 1	1.5	3	0.3
Root cell 2	6.5	3	0.3

Selected intercellular ectomycorrhizal strand
Local component	Drawing x	Lower coordinate	Upper coordinate	Tube half-width
Fungal strand within the intercellular gap	4	1	5	0.25

Selected intercellular ectomycorrhizal strand
Local component	Starting x	Ending x	Starting y	Ending y
Intercellular space	3	5	0	6

Inside the plant wall does not mean within plant cytoplasm

Partners retain distinct membranes

Drawing units; no measured root dimensions

Organic carbon

Nutrients

Local interfaces; whole-root structures omitted

Assigned local cross-sections rather than whole roots, measured microscopy or universal mycorrhizal geometries. The arbuscular slice omits the outer plant-cell boundary; its interface membrane is plant-derived and continuous with the plant membrane system outside this plane. The ectomycorrhizal local gap omits the outer mantle.

Exchange can have costs as well as returns

Mycorrhizal fungi can contribute to a plant's acquisition of nutrients such as phosphorus, while plants supply organic carbon to fungal partners. Studies of arbuscular mycorrhiza have demonstrated plant-to-fungus lipid transfer as well as the contribution of sugars. The materials differ chemically, but each can carry carbon into the fungal account.

Acquiring additional nutrients and providing carbon both affect a partner's balance. Under some conditions, a nutrient gain can support growth more than the carbon cost restricts it; under others, the outcome can differ. Calling an association mutualistic describes beneficial effects for the participants under the relevant conditions. Finding an interface alone does not measure either participant's benefit.

Compare a plant with and without a particular association under a defined nutrient environment, and measure a stated outcome. Stem extension, nutrient content, survival and reproductive output answer different questions. A larger nutrient concentration can result from more uptake or from less tissue diluting the same amount. Amount, concentration and growth therefore need distinct measurements.

A lichen's producer and fungal structures

A lichen combines a lichen-forming fungus with one or more photosynthetic partners, such as green algae or cyanobacteria. The photosynthetic partner is called the photobiont. Its carbon fixation can supply organic material to the association. The fungal partner remains a heterotroph; proximity to photosynthetic cells does not give its own cells a photosynthetic pathway.

The selected layered lichen model places fungal hyphae around a photobiont-rich layer. A dense upper fungal region and a more open lower region show one possible arrangement, with no claim that all lichens share this tissue plan. Partner organisation can create a persistent composite form whose properties are not described simply by counting loose cells.

Additional fungi and bacteria can occur in lichen communities. A primary study identified particular basidiomycete yeasts in the cortex of sampled ascomycete macrolichens. This shows why an account limited to exactly two organisms can omit participants; it does not establish that every lichen universally requires a third fungal partner. Presence, abundance, function and necessity are different claims.

Some cyanobacterial photobionts can contribute fixed nitrogen. A green-algal photobiont alone does not establish that capability. Specify the participant performing the process. The same discipline applies to carbon: the model follows photosynthetic input to different destinations without assigning photosynthesis to all cells in the lichen.

Partners within a selected lichen structure

The additional yeast is a possible participant, not a universal lichen requirement. The carbon account has constructed allocations and measures no partner's net benefit.

Read the diagram in words

The layered slice has an open fungal region at y = 0–2, photobionts at y = 2.5 within the 2–3 layer, and an upper fungal region at y = 3–4. An optional additional yeast lies at y = 3.5. Separately, 100 fixed carbon units are assigned as 60 retained by the photobiont and 40 transferred to the fungus; the latter becomes 10 retained and 30 in CO2. Total retained organic carbon is 70.

Selected arrangement; other structures omitted
Boundary or compartment	Lower coordinate	Upper coordinate
Selected open fungal region	0	2
Photobiont-rich region	2	3
Selected dense upper fungal region	3	4

Photosynthetic cells
Model identity	Drawing x	Drawing y	Drawing radius
p1	2	2.5	0.25
p2	4	2.5	0.25
p3	6	2.5	0.25

Lichen-forming fungus
Model identity	Path point order	Drawing x	Drawing y
open-1	1	0.5	0.4
open-1	2	2	1.3
open-1	3	4	0.5
open-1	4	6	1.4
open-1	5	7.5	0.4
open-2	1	0.5	1.6
open-2	2	2	0.8
open-2	3	4	1.6
open-2	4	6	0.7
open-2	5	7.5	1.6
cortex-1	1	0.5	3.2
cortex-1	2	3	3.3
cortex-1	3	5	3.2
cortex-1	4	7.5	3.3
cortex-2	1	0.5	3.6
cortex-2	2	3	3.7
cortex-2	3	5	3.6
cortex-2	4	7.5	3.7

Possible additional participant; no universal requirement
Meaning	Drawing x	Drawing y	Drawing radius
Additional yeast shown as a possible participant	7	3.5	0.12

Selected arrangement; other structures omitted
Assigned quantity	Value
Attachment-side boundary; lower structures omitted	0

Assigned drawing geometry
Starting x	Ending x	Starting y	Ending y
0	8	0	4

Carbon-account units
Model identity	Meaning	Drawing x	Drawing y
new-carbon	New fixed carbon: 100	0	0
photobiont	Photobiont retains 60	3	-2
fungus	Fungus receives 40	3	2
fungal-material	Fungal material: 10	6	1
co2	CO2: 30	6	3

Carbon-account units
From	To	Relationship	Element-account amount
New fixed carbon: 100	Photobiont retains 60	Ending retention	60
New fixed carbon: 100	Fungus receives 40	Partner carbon transfer	40
Fungus receives 40	Fungal material: 10	Ending retention	10
Fungus receives 40	CO2: 30	Respired carbon	30

Carbon-account units
Assigned quantity	Value
Initial inventory	100
Photobiont retains 60	60
Partner carbon transfer	40
Fungal material: 10	10
Respired carbon	30
Ending organic carbon across both partners	70

Lichen-forming fungus

Photosynthetic cells

Possible additional participant; no universal requirement

Selected arrangement; other structures omitted

Carbon-account units

Drawing units; no measured anatomy

One assigned interval

Original partial layered lichen arrangement and independent carbon-only teaching allocation; not a source micrograph, universal lichen anatomy, measured exchange fraction or proof that a third fungus is required.

A partner can disperse with or without the association

A fungal spore released from a lichen can disperse the fungal partner, which must encounter a suitable photobiont to establish that kind of lichen again. A fragment carrying both partners can transport more of the association together. These are different establishment problems, even if both propagules leave the same parent thallus.

The distinction helps interpret a survey. Finding fungal DNA in a new location does not alone show that the composite lichen structure has established there. Evidence of the partner, organised association and persistence would address further steps. Likewise, discovering a root-associated fungus does not by itself demonstrate a functioning nutrient exchange with every nearby root.

Worked example

The carbon account of a selected lichen

The model assigns 100 newly fixed carbon units to a photobiont during one interval. It retains 60 and transfers 40 to the fungus. The fungus retains 10 in new fungal material and releases 30 as CO2. Its carbon balance is 40 = 10 + 30.

Across both partners, retained organic carbon is 60 + 10 = 70, and released CO2 carbon is 30. The total is 100. Adding the transfer of 40 again would count an internal movement as another external input. The allocations are deliberately constructed and do not estimate a measured exchange fraction or net benefit.

Check your understanding

Why is the fungal branch in the arbuscular model not directly exposed to plant cytoplasm?
Show reasoning
Its own membrane and wall are enclosed by an interface space and a plant-derived membrane. Plant cytoplasm is outside that latter boundary in the local slice. Crossing the plant wall during establishment does not eliminate the surrounding membrane interface.
How much new organic carbon remains in both partners at the end of the lichen model?
Show reasoning
The photobiont retains 60 and the fungus retains 10, for 70 units. Thirty units are in CO2, so 70 + 30 = 100. The 40 transferred between partners is part of those destinations, not an additional input.
Does finding an additional yeast in a lichen establish that every lichen requires that yeast?
Show reasoning
No. It establishes presence in the studied context. Universal requirement would need different evidence across the relevant lichens, including evidence about function and necessity. The fungal and photosynthetic partners should still be identified, with additional participants described by their observed roles.

What to carry forward

Locate the actual boundaries and participants, balance internal exchange separately from new inputs, and measure benefits rather than inferring them from association.

References

Resources supporting this lesson’s explanations and examples.