permaculture

[Lawrence London <lfljvenaura@gmail.com>]

Mycorrhizal fungi: all you need to know about the Internet of Plants
https://medium.com/@dario.cortese/the-internet-of-plants-mycorrhizal-fungi-sorting-facts-from-myths-7ab5c7b73bf1

Mycorrhizal fungi: all you need to know about the Internet of Plants
Symbiotic fungi have a key role in soil ecosystems and inoculating plants
with them has been claimed to benefit their growth. But scientific evidence
shows a very complex picture; which might surprise you.
[image: Dario Cortese]
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Dario Cortese
<https://medium.com/@dario.cortese?source=post_page-----7ab5c7b73bf1---------------------->
Dec 21
<https://medium.com/@dario.cortese/the-internet-of-plants-mycorrhizal-fungi-sorting-facts-from-myths-7ab5c7b73bf1?source=post_page-----7ab5c7b73bf1---------------------->
· 15 min read
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If you are a gardener, news might have reached your ear about a supposedly
beneficial category of fungi called *mycorrhizal*. If you are an ecologist,
chances are you consider *mycorrhizae* one of the most successful
partnerships of all time. If you are a plant scientist, you know a great
deal about them, but perhaps you are perplexed about the sensational claims
that home gardeners, some regenerative farmers and most suppliers have been
making in the last ten years. Although there is a wide consensus about the
extreme importance of healthy mycorrhizal populations in thriving
ecosystems and productive soils, their effects as plant growth stimulants
constitute a controversial issue, to say the least.

Unfortunately, we are often exposed only to part of the scientific evidence
available. In this article, I will try to provide a *comprehensive*, *fully
referenced* yet *non-technical* review of:

   1. what mycorrhizal fungi are and how they work;
   2. how they interact with plants: when and how they are beneficial or
   detrimental to them;
   3. how mycorrhizal fungi can be effectively employed without incurring
   risks to plant health.

The sections that follow are numbered according to the list above, so that
you may jump straight to any one of them, should you wish to.
1. What are mycorrhizae?

The term *mycorrhiza* (plural *mycorrhizae*) refers to the close and
long-term (or *symbiotic*) association between a plant and a fungus present
in the plant’s root zone (the *rhizosphere*). Scientists have known about
these fascinating relationships for more than a century¹, while agronomists
and gardeners have started to become acquainted with mycorrhizal fungi only
in the last few decades, thanks to books such as Teaming with
microbes, Mycorrhizal
Planet <https://www.goodreads.com/en/book/show/32564954>and Mycelium Running
<https://www.goodreads.com/book/show/239395.Mycelium_Running>.

Nearly all plants in the wild rely, for their nutrition, on the exchange of
nutrients and other benefits with the complex network of soil
microorganisms that is known as the soil microbiome or the *soil food web*
<https://en.wikipedia.org/wiki/Soil_food_web>². Within this rich
underground ecosystem made up of worms, insects, bacteria, algae, and other
microscopic creatures, fungi play two crucial roles: some of them (
*saprophytic*) are unique in their ability to decompose woody material
(rich in *lignin*), while others *(mycorrhizal) are the food transportation
system of the soil*. The body of a fungus is composed by fine and
intricately branched threads called *hyphae*, which form a dense network
referred to as *mycelium*. In seasonal cycles, the mycelium gives rise to
the more recognisable mushrooms; these are fruiting structures and contain
the spores that allow a fungus to spread far and wide and populate new
ground.

A single fungus’ mycelium can extend over a surface of 4 square miles, as
in the case of the largest living being on earth — a specimen of *Armillaria
ostoyae* living in Malheur National Forest, Oregon
<http://www.bbc.co.uk/earth/story/20141114-the-biggest-organism-in-the-world>.
This gives fungi the ability to populate a vast volume of soil, scouting
for nutrients and water, and transporting them over large distances. On the
other hand, most plants can’t extend their roots anywhere as far, and thus
started to make advantage of their neighbouring fungal networks as early as
400 million years ago³ — as soon as they started colonising land. In its
own root zone, *a plant can trade the surplus of sugars produced by
photosynthesis in exchange for increased access to water, nutrients and
protection from pathogens*⁴. In some cases, plants which are located far
away from each other can exchange nutrients via the fungal network that
connects them underground. In temperate forests, young trees benefit from
the ability of older (and taller) ones to reach up to sunlight; so much so
that up to 40% of their carbon can come from their grown-up neighbours’
photosynthesis⁵ via the fungal *wood wide web*.
The wood wide web, mycorrhizal fungi in temperate forests allow the
exchange of carbon among trees. From: New Zeland Geographics
<https://www.nzgeo.com/stories/the-wood-wide-web/>.Are all mycorrhizae the
same?

But not all mycorrhizal fungi are the same. First of all, fungi can
interact with plants either from within or without their cell walls. Among
the ones that colonise the cells of the host plant and from there extend
into the soil, the most numerous are *arbuscular mycorrhizal (AMF)**, *whereas
*ectomycorrhizal (EMF) *grow on the roots’ surface and form webs around
them.
EMF are the favourite helpers of conifers and deciduous trees, with which
they sometimes form exclusive one-to-one relationships, such that only a
specific species of fungus pairs up with a given species of tree⁶.

*AMF are found in 85% of all plant families and are hosted by most
agricultural crops as well as grasses, shrubs, and some trees*⁶. For this
reason, AMF have been used in agriculture and horticulture as a
biofertilizer, and are available commercially to use in home gardens. More
than 150 species of AMF are known, and they can associate with over 200,000
plant species⁷. AMF live only for a short period of time (8.5 days on
average⁸), acting as perfect guests: they do not mix their cell contents (
*protoplasm*) with the host’s ones, but form either balloon-like or
tree-shaped structures (arbuscules — from the latin *arbor*, “tree”)
located within the host cell membrane, where nutrients are stored and the
exchange takes place⁷.
Left: Mycelium in a temperate forest, from Nationalforests.org
<http://nationalforests.org>. Right: Ectomychorrizae in Pinus sylvestris
The challenging life of an arbuscular mycorrhiza

The life span of AMF is strongly correlated with their ability to deliver
nutrients to the host plant, and is therefore regulated the plant’s
demand⁹. Plants can also remove inefficient partners, thus encouraging
another one to come forward¹⁰. Before dying, arbuscular mychorrizal
reproduce asexually by means of spores, which can germinate even without
the presence of a host, although the speed at which their hypae can spread
significantly increases in the presence of root exudates¹¹. Some
researchers have suggested arbuscular mycorrhizal might have been asexual
for millions of years¹². Once germinated, the spore inoculates the nearest
host and a new life cycle begins. In order, to survive, however host plants
must be found by the newborn fungus, or else it will starve to death.

As you might have already guessed, *mychorrizal fungi are very delicate
organisms*. After all, many of us have experienced how easy it can be to
physically remove the hyphae of molds. Any significant soil disturbance,
such as tillage, digging and compaction have been shown to reduce AM
fungi’s diversity and lifespan¹³.
Other practices, common in conventional agriculture, also have a
detrimental impact on mycorrhizal networks. For istance, the application of
any fungicide as a disease control measure also kills AMF¹⁴, as we might
expect. Furthermore, low pH conditions (acid soil) often induced by
chemical fertiliser use¹⁵.

Monocultures also have a negative impact on arbuscular populations; in
particular, *brassicacae* (plants belonging to cabbage family) do not form
associations with AMF¹⁶, and therefore can act as a biological gap, thus
slowing down their spread across larger areas. Conversely, a large variety
of crops does encourage the establishment of symbiotic relationships,
because “there may be different strains of mycorrhizae that will like
something better than another one” — as suggested by J.C. Cahill of the
University of Alberta
<https://medium.com/@info_31211/what-farmers-need-to-know-about-mycorrhizae-1ef9de47a36b>.
Cahill also points out how extreme soil conditions, such as drought, high
temperatures, waterlogging, frosts can affect AMF.
Finally, and most importantly, the absence of living vegetation over large
areas (fallow ground), starves and in the long term wipes out all
mycorrhizal life from the soil¹⁷.

Interestingly, however, recent field experiments suggested that the
negative impacts of the agricultural practices discussed above has been
overestimated by previous research; in fact, *the recovery of mycorrhizal
systems through their interaction with other soil microbes is still poorly
understood and could be more efficient than we believe*¹⁸.
Arbuscular mycorrhizal fungus in a colonised root system. Fungal hyphae (E)
increase the surface area of the root and uptake of key nutrients while the
plant supplies the fungi with fixed carbon (A=root cortex, B=root
epidermis, C=arbuscle, D=vesicle, F=root hair, G=nuclei). Credit: Amarachukwu
Ifeji
<https://commons.wikimedia.org/w/index.php?title=User:Aifeji20&action=edit&redlink=1>2.
How they impact plants and ecosystemsThe importance of finding the right
match

Associations between arbuscular mycorrhizal fungi and plants are not always
straightforward. Over a century of research has shown that, similarly to
ectomycorrhizae, arbuscular partnerships only develop between certain
combinations of plants¹⁹. Some plants are generalists, which means that
they can associate with several species of AMF. Similarly, some AMF do not
show particular preferences, and pair up with hundreds of plant genera.
However, this is not always the case, and research labs around the world
have been investigating several combinations of fungi and plants, in
diverse soil types and conditions, to test when and where mycorrhizae are
formed²⁰.

Unfortunately, a lot of conflicting information has been published, and it
is very difficult to determine with certainty whether a specific fungus
will associate with a given plant²¹. Most likely, this disagreement is due
to different methods used to determine when colonization had happened, and
when it was beneficial. Recently, several efforts have been made, and
methods proposed, to unify the body of literature and collect all the
significant and most up-to-date information available to us. A huge
database, MycoDB <https://www.nature.com/articles/sdata201628>, has been
created, and collates all the information about the effect of specific AMF
species on the productivity of single plant species⁷.

One of the latest insights from research shows that plants whose nitrogen
(N) content is higher do rely less on mycorrhizal fungi³³.
Different plant species rely differently onthe same population of AMF. The
graph shows the belowground surface area provided by roots (*grey*) and AMF
hyphae (*white*)³³From mutualism to parasitism: the full symbiotic spectrum

Knowing what happens when fungus X meets plant Y is extremely important,
not only to target specific situations. In fact, although some combinations
are destined to result in mutual benefits, others can turn out to be
harmful for the host plant, to the advantage of the mycorrhizal fungus.

Symbioses can indeed be mutualistic, commensalistic or parasitic.
Mutualistic <https://en.wikipedia.org/wiki/Mutualism_%28biology%29>
associations benefit both participants (as is the case for oxpeckers and
impalas, or clownfish and sea anemones²²), while in commenalistic
<https://en.wikipedia.org/wiki/Commensalism> ones only one member gains an
advantage while the other is neither benefited nor harmed. In the worst
case scenario, parasitism <https://en.wikipedia.org/wiki/Parasitism> takes
place, and one organism exploits the other to its own advantage, thus
harming it (the obvious example being viruses
<https://en.wikipedia.org/wiki/Virus>, which are obligate parasites).

Although most of the research so far has focused on the successful stories
typical of mutualist mycorrhizae, in the last few years some research
groups around the world have started to explore what happens when the
fungus parasitises its host. This information will prove fundamental in
order to discern whether introducing arbuscular fungi to a garden or a farm
is the right thing to do.

To complicate things, as we will see, what determines whether a fungal
symbiosis is beneficial to a plant is not only the combination of species,
because some fungus-host pairs can start as mutualist and gradually become
parasitic.
Not all plants establish beneficial partnerships with AMF. The graph shows
Mycorrhizal growth dependency (MGD) as calculated from relative biomass
differences between AM and non-mycorrhizal plants³³Mutualist mycorrhizae: a
wealth of beneficial effects

It is hardly difficult to explain why so much of the scientific literature,
as well as gardening clubs and fertiliser suppliers have been leaning
towards over-optimism when talking about mycorrhizal fungi. Indeed, the
list of documented impacts of symbiotic fungi range from stimulating plants
root growth and crop yields, to ecosystem biodiversity regeneration and
greenhouse gases reduction. Here I provide a list of the beneficial effects
generally accepted among the scientific community, along with all the
references you will need, should you wish to dig a bit deeper.

   - In numerous studies, *agricultural crop yields have been shown to
   increase* where higher densities of arbuscular mycorrhizal were
   measured²³. However, yields are not directly proportional to the extent of
   the AMF infection, but to the physiological effect they have on plants⁶
   - Due to their role in plant nutrition, symbiotic AMF could also
have a *positive
   impact on the quality of vegetables*, by increasing the concentration of
   both macro and micronutrients²⁴. One of the most inestimable benefits of
   mycorrhizae is in fact the facilitation of *phosphorous uptake*.
   Phosphrous (P) is critical macronutrient for plant growth, but it is one of
   the most difficult nutrients for plants to acquire. Even though it might be
   present in large quantities, often most of it is poorly available because
   of the very low solubility of phosphates of iron, aluminum, and calcium.
   Mycorrhizal symbiosis is the most common strategy plants evolved to access
   phosphorous in its available form, as negatively charged Pi ions²⁵. The
   uptake of phosphorous in inoculated roots can be three to five times higher
   than in non-mycorrhizal roots²⁶.
   - AMF *benefit entire ecosystems* on many levels, as they improve soil
   structure and aggregation and drive the structure of plant communities,
   biodiversity, nutrient capture and productivity²⁷
   - Mycorrhizal fungi in general are responsible for *reducing emissions
   of nitrogen oxide*, which is one of the greenhouse gases that drive
   climate change²⁸
   - Plants *tolerance to stressful soil conditions*, such as drought and
   high salinity, is improved in the presence of AMF²⁹
   - Arbucules can reduce the impact of heavy metals in the host plants³⁰.
   This can also be used for *phytoremediation*
   <https://en.wikipedia.org/wiki/Phytoremediation>, *i.e.*, the use of
   plants to *clean up soil, water and air in contaminated areas*³¹
   - *Increased resistance to soil-borne pathogens*, both fungal and
   bacterial, has been widely observed on plants colonised by AMF, although we
   still don’t know what the mechanism involved is. Some argue that the
   beneficial fungi physically inhibit the attack of parasitic ones, other
   researchers have discovered that plants refine their own defences as a
   result of the chemical interaction with the mycorrhizal fungi³²

The benefits of AM-partnerships from Jacott et al, *Agronomy* *2017*, *7*(4)
<https://doi.org/10.3390/agronomy7040075>Parasitism: the dark side of the
fungus

Already in the 1980’s, plant scientists knew very well that mycorrhizal
fungi could not only be parasitic, but also start as friendly allies and
end up as deadly enemies. Studies have shown parasitic behaviour in various
vegetable crops, and in most cases the culpable fungus was one that is also
known to form mutualistic partnerships, even with the same plants. So, when
do mycorrhizal fungi turn their backs to plant needs and start to exploit
them?

As pointed out by Johnson et al.³⁶,

Mycorrhizal fungi might be considered to be parasitic on plants when net
cost of the symbiosis exceeds net benefits. Parasitism can be
developmentally induced, environmentally induced, or possibly genotypically
induced.

In practice, this means that there are many ways in which the amount of
plant resources allocated to an AMF (the *costs*) might end up exceeding
the *benefits *it gains from associating with it.

To begin with, *environmental factors are extremely important*. For
instance, a mycorrhiza might become parasitic in a *chemically fertilised
soil*, where the plant already has enough phosphorous and water, and ends
up providing carbon to the fungus for free.
Similarly, in *reduced light conditions*, a plant host might be unable
photosynthesise quickly enough to provide carbon both for its own growth
and to satisfy the fungus needs; often, this means that the plant will slow
down its growth or general productivity.
Costly and detrimental partnerships can also occur *where* *plant densities
are low*, and AMF enter in competition with each other and the hosts’ roots.

Furthermore, at different stages of the development of a plant, things can
look differently. As a *seedling*, plants’ needs for nutrients are very low
(as most of them are drawn from the seed’s store), and therefore the *costs
*are higher; however, this short-term disadvantage is usually compensated
by the long-term advantages of having easier access to soil resources.

To complicate things even further, *genetics play an important role*, as we
already know from the fact that not all matches are successful. Genetic
predispositions interact with environmental factors, as well as complex
ecosystemic dynamics that result from the interaction of several types of
plants and life in the soil.
3. Applications

Before we explore how mycorrhizal fungi can be used in agricultural and
horticultural situations, it is important to notice that, contrary to what
is commonly believed, *where a mutually beneficial mycorrhizal symbiosis is
present, the host plant has fewer roots*, because it can make advantage of
the fungus foraging ability, thus economising on root tissue³³.
Let’s pause on this idea for a moment. Perhaps some of you have come
accross the comparative photographs that mycorrhizal suppliers use to
reinforce their claim that inoculated plants have healthier roots systems
(an example has been included below). These can be interpreted in two ways:

   1. if the supplier is trying to tell us that the roots of the inoculated
   plant are more numerous, they probably have a limited comprehension of how
   mycorrhizae work;
   2. if the intent is to show the overall biomass of roots plus
   mycorrhizal hyphae, rather than roots alone, then the supplier is just
   diplaying how a mycorrhizal and a non-mycorrhizal specimen look. However,
   having AMF hyphae around its own roots, as you now know, does not mean that
   a plant is benefiting from them²³, and even if it was, it would not ensure
   that its productivity will be increased⁶.

A comparison of the roots of inoculated and uninoculated barley plants,
from Teaming with Fungi
<https://www.goodreads.com/book/show/28009897-teaming-with-fungi>
(J.Lowenfels)A growth stimulant?

As discussed in the previous section, when mycorrhiza is mutualistic, the
productivity and general health (sometimes grouped under the term
*fitness²⁵*) of the inoculated plant are increased, alongside the
biodiversity of microorganisms in the soil.

However, researchers have started to point out that mycorrhizae can be
parasitic more often than we originally thought. J. Cahill goes as far as
saying the interaction between plants and soil life is so complex that *if
we introduce a diverse mix of mycorrhizal fungi in a generic soil, 50% of
the AMF are potential parasites*
<https://www.no-tillfarmer.com/articles/8410-podcast-how-plants-find-their-food-the-behavioral-science-underlying-plant-nutrition>
³⁴.
Therefore, if we are mainly interested in the performance of individual
plants, rather than the ecosystem’s diversity and the other potential
benefits of mycorrhizae, we should take some precautions before spreading
them indiscriminately over our garden or field. A few considerations to
make, in the light of the previous sections, are the following:

   - Verify that your soil is not already rich in available (soluble)
   phosphorous, as this might cause mycorrhizal fungi to be parasitic;
   - Although mycorrhizae improve nutrient uptake, these nutrients need to
   be present in the soil in the first place. Therefore, especially if plant
   performance is a priority, it is worth ensuring that soil is chemically
   balanced³⁵
   - *Avoid introducing non-native species* of AMF, as these are less
   likely to establish readily and healthily in a foreign soil food web;
   - If possible, you should *always test* mychorrizal action on some
   individual plants first, and compare their subsequent performance with
   other individuals of the same species or family. This will give you an
   indication of whether any beneficial effects are observed in your soil
   ecosystem.
   - Notice that AMF don’t associate with Brassicas and plants that thrive
   on acidic soils (low pH), such as Rhododendrons, Azaleas, Heathers,
   Cranberries and Blueberries;
   - In vegetable gardens or farm operations, *increase the crop diversity*
   and rotate brassicas often, as to encourage permanent establishment of a
   diverse community of AMF;
   - *Avoid tillage and digging*, as these destroy fungal hyphae;
   - If want mycorrhizae to establish permanently in a vegetable or flower
   bed, whenever plants are removed to make room to another crop, their *roots
   should be left in the soil*. This might be easier said than done,
   especially on plants with strong and branched taproots. However, what is
   crucial is to leave the finer roots in the soil, as these have a higher
   density of mycorrhizal fungi; this can be achieved by *pulling plants up
   carefully or cutting their stem below ground*;
   - *Do not leave soil uncultivated* over winter or at any given time for
   long periods, because mycorrhizal fungi need hosts to survive (incidently,
   weeds as fixing agents in disturbed soil do, among other things, provide a
   home for stranded mycorrhizal fungi.

Commercial inoculants: are they effective?

Although we may legitimately expect that inoculants contain viable spores,
some research has shown that more than 50% of the products available to
home gardeners are not viable³⁷. This is especially true of mixes that are
made up of spores only. Products consisting a clay inert substrate (white
granules) inoculated with living mycelium may be more effective.
A Regenerative tool

The soil is the great connector of lives, the source and destination of
all. It is the healer and restorer and resurrector, by which disease passes
into health, age into youth, death into life. Without proper care for it we
can have no community, because without proper care for it we can have no
life. (Wendel Berry³⁷)

Despite their potential side effects on individual plant productivity, the
benefits of mycorrhizal symbioses to ecosystems, the soil food web, plant
biodiversity and the earth’s climate cannot be stressed enough.

All those gardeners, landscapers, farmers and policy-makers who are
interested not only in a *sustainable *way of maintaning and managing
ecosystems, but in regenerating the biological balance that centuries of
naive human action have seriously impaired, have long found in mycorrhizal
fungi an invaluable ally. Regenerative agricultural and gardening
practices, such as no-dig gardening
<https://en.wikipedia.org/wiki/No-dig_gardening>, organic no-till crop
production <https://en.wikipedia.org/wiki/No-till_farming>, agroforestry
<https://en.wikipedia.org/wiki/Agroforestry>and others are heavily
dependent on the fungal network that connects plants and soil microbiology,
redistrubuting nutrients and enhancing synergistic dynamics.

In this sense, mycorrhizal fungi are a tool of the future, and we need to
understand them better in order to avoid using them improperly or
ineffectively.
Further (non-technical) reading

*Mycelium running*
<https://www.goodreads.com/book/show/239395.Mycelium_Running?from_search=true&qid=1nrE3iJzhJ&rank=1>*:
**How Mushrooms Can Help Save the World*
<https://www.goodreads.com/book/show/239395.Mycelium_Running?from_search=true&qid=1nrE3iJzhJ&rank=1>
*,Paul Stamets*
If you are after an intriguing book on fungi, Paul Stamets’ book won’t
disappoint you.

*Mycorrhizal planet* <https://www.goodreads.com/en/book/show/32564954>*, M.
Phillips *
For those interested in understanding mycorrhizal, with particular
attention to their use in permaculture and regenerative agriculture, this
is a rich and pleasant read.
References

The scientific articles and all the other references corresponding to the
numbered footnotes can be opened by clicking on the link below. They are in
a google doc format. I suggest that you open them in a separate tab (by
pressing CTRL+left-click) so that you might refer back to them while you
read the article, without the need to scroll back and forth.

https://docs.google.com/document/d/13U-RaSq_Wskh2kvjBdK-l7Uqei7I1Yy7i8bLzJxyj-w/edit

References (Mycorrhizal fungi)

   1.

   Frank, A.B., 1885. Über die auf Wurzelsymbiose beruhende Ernährung
   gewisser Bäume durch unterirdische Pilze. [link
   
<https://www.worldcat.org/title/berichte-der-deutschen-botanischen-gesellschaft/oclc/1566359>
   ]
   2.

   Scheu, S., 2002. The soil food web: structure and perspectives. European
   journal of soil biology, 38(1), pp.11–20. [link
   <https://doi.org/10.1016/S1164-5563%2801%2901117-7>]
   3.

   Brundrett, M.C., 2002. Coevolution of roots and mycorrhizas of land
   plants. New phytologist, 154(2), pp.275–304. [link
   <https://doi.org/10.1046/j.1469-8137.2002.00397.x>]
   4.

   Berruti, A., Lumini, E., Balestrini, R. and Bianciotto, V., 2016.
   Arbuscular mycorrhizal fungi as natural biofertilizers: let’s benefit from
   past successes. Frontiers in microbiology, 6, p.1559. [link
   <https://www.frontiersin.org/articles/10.3389/fmicb.2015.01559/full>]
   5.

   Klein, T., Siegwolf, R.T. and Körner, C., 2016. Belowground carbon trade
   among tall trees in a temperate forest. Science, 352(6283), pp.342–344. [
   link <https://science.sciencemag.org/content/352/6283/342>]
   6.

   Chaudhary, V.B., Rúa, M.A., Antoninka, A., Bever, J.D., Cannon, J.,
   Craig, A., Duchicela, J., Frame, A., Gardes, M., Gehring, C. and Ha, M.,
   2016. MycoDB, a global database of plant response to mycorrhizal
fungi. Scientific
   data, 3, p.160028. [link <https://doi.org/10.5061/dryad.723m1>]
   7.

   Smith, S.E. and Read, D.J., 2010. Mycorrhizal symbiosis. Academic press.
   [link
   
<https://books.google.co.uk/books?hl=en&lr=&id=qLciOJaG0C4C&oi=fnd&pg=PP1&ots=zqyWoVVEnN&sig=L9Rzap4211lETRCz7tAwodI9YSw&redir_esc=y#v=onepage&q&f=false>
   ]
   8.

   Alexander, T., Toth, R., Meier, R. and Weber, H.C., 1989. Dynamics of
   arbuscule development and degeneration in onion, bean, and tomato with
   reference to vesicular–arbuscular mycorrhizae in grasses. Canadian
   Journal of Botany, 67(8), pp.2505–2513. [link
   <https://doi.org/10.1139/b89-320>]
   9.

   Bati, C.B., Santilli, E. and Lombardo, L., 2015. Effect of arbuscular
   mycorrhizal fungi on growth and on micronutrient and macronutrient uptake
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