Plants Warn One Another of Pest Attack through Mycorrhizal Fungal Network

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ISIS Report 28/10/13
Plants Warn One Another of Pest Attack through Mycorrhizal Fungal Network
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Exquisite inter-species relationships between plants and fungi threatened by
industrial farming. Dr Eva Sirinathsinghji

Underground intercom between plants

A network of the soil microorganism – arbuscular mycorrhizal fungi – act as
an underground intercommunications system between plants to warn off aphid
attacks, a recent study from the University of Aberdeen in the UK reveals
[1].

Bean plants (Vicia faba) free of pea aphids (Acyrthosiphon pisum) were found
to release aphid-deterring chemicals when neighbouring plants were under
attack.
Such a chemical response was previously only associated with plants directly
under attack. These chemicals also attracted the aphid predator – the
parasitoid wasp (Aphidius ervi Haliday) – that keeps aphid populations down.
The response was dependent on the arbuscular mycorrhizal fungus (Glomus
intraradices) that forms branching mycelial networks between the plants.

This remarkable phenomenon is an example of interspecific symbiotic
relationships, many of which researchers are yet to fully comprehend. It is
an aspect of agricultural science totally ignored by industrial farming
systems.
Pesticides and herbicides kill soil microorganisms and damage
mycorrhizal/plant associations. This may explain the effects of Monsanto’s
Roundup herbicide, which has been blamed for the epidemic spread of plant
diseases associated with the planting of Roundup tolerant GM crops (see [2] Ban GMOs Now).

Mycorrhizal fungi are among the most functionally important soil
microorganisms.
They form symbiotic relationships with a range of plants to increase plant
mineral uptake, increase tolerance to root and shoot pathogens and
redistribute water during drought stress [3]. The fungi in return, gain
carbohydrates supplied by plants to enhance their mycelial networks
underground. These networks are extensive, connecting plants of the same
species, and can even connect to plants of different species as mycorrhizae
lack host plant specificity. Common mycelial networks facilitate seedling
establishment, influence plant community composition and are the primary
pathways through which many species of non-photosynthetic plants acquire
their energy. Furthermore, they have been shown to transport signalling
molecules from plant to plant, including allelochemicals – metabolites
release d by plants to positively or negatively affect neighbouring plants.
Another example of such symbiosis is the recent finding that tomato plants
rely on mycelial networks to increase enzyme activity and defence-related
gene expression in resisting early leaf blight [4].

New tests show mycorrhizal fungi are underground agents

The new study, for the first time, tests whether these mycelial networks are
able to facilitate interplant signalling during attack by herbivores. Also
tested is whether this signalling induces the release of volatile organic
chemicals (VOCs) used to deter or attract pests or natural enemies of pests.
It is already known that plants often produce VOCs systemically, which are
transmitted through the air. VOCs are also known to be produced in the root
system that may be transmitted by mycelial networks.

To test these theories, the researchers grew pea plants in small growth
chambers that bring a small part of the natural environment under controlled
conditions.
They used ‘donor’ plants that are to be infested with aphids and ‘receiver’
plants connected via mycelial networks created by inoculating the soil with
mycorrhizal fungi. One receiver plant was connected to the donor without any
physical barrier between donor and receiver plant allowing the intermingling
of mycorrhizal fungi and roots, the other receiver plant was connected
through a 40 µm mesh penetrable by mycelial networks but not by roots. These
two conditions allow distinction between the possibility of communication
between plant root systems and the mycelial network. Control ‘receiver’
plants were blocked from receiving information: one was surrounded by 0.5 µm
mesh that prevents mycorrhizal penetration and the other was grown in a
penetrable 40 µm mesh, but immediately before aphids were added to the donor
plant the chamber was rotated to snap any connections that had been made. To
confirm the presence of mycelial networks, roots were stained with dye and
analysed under a microscope.

Five weeks into the study after mycelial networks should be well
established, bags were placed over plants before aphid infestation to
prevent aerial communication between plants. To measure whether plants
released VOCs to repel aphids and attract to the predatory parasitoid wasp,
the air or ‘headspace’
around the plant was captured and exposed to both insect species. The
researchers found that headspace from donor plants both repelled aphids and
attracted the wasps; the same result was obtained with headspace from
receiver plants connected via mycelial networks, while both types of control
plants remained attractive to aphids, as shown in Figure 1. Furthermore,
there was not a significant difference in repellent properties of receiver
plants connected to the donor plant via both roots and mycelial networks
compared to those connected by mycelial networks alone, implying that
mycorrhizal fungi and not root-to-root connectivity were responsible for the
communication.

Figure 1   Mycorrizhal fungi provide hyphal connections from aphid infested
‘donor’ plants to aphid-free ‘receiver’ plants to repel aphids and attract
their predator, the parasitoid wasp (A. ervi) (see text for further details)

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