1. A. HODGE, T. HELGASON, A.H. FITTER
DARIO GONZALEZ ROMERO

2. Introduction
arbuscular mycorrhizal

3. Introduction
The arbuscular mycorrhizal
symbiosis is viewed as a classic
mutualism, an interaction in
which both partners benefit.
The fungi appear to acquire
their entire carbon supply from
the plant, and although
colonisation of roots by AM
fungi (AMF) can confer a wide
range of benefits to the plant

5. Introduction
Phosphorus acquisition.
The fungal hyphae can
explore a large volume of
soil and acquire P beyond
the phosphate depletion
zone that rapidly builds up
around the root

6. Introduction
• The first land plants had
rhizomes and rhizoids,
but no root systems
• Its evidence reveals that
these early plants had
fungal structures
strikingly similar
tomodern AM structures AM in Erythronium americanum

7. Morphology of AMF colonisation
AMF
ARUM PARIS

8. Arum Type
• Series associations
where hyphae
proliferate in the cortex
by growing
longitudinally between
host cells.
• This occurs because
hyphae grow through
longitudinal
intercellular air spaces
that are present

9. Paris Type
• series where
hyphae spread by
forming coils within
cells because there
are no continuous
longitudinal air
spaces.

10. Carbon
• The major fluxes in the AM
symbiosis appear to be of C from
plant to fungus and of P, and
possibly N, from fungus to plant.
• The mechanisms of these fluxes
are not yet well understood.
• Even the location of the carbon
flux is obscure, withthe best
evidence – from activity of
ATPases – suggesting that it occurs
in the Arum type at the inter-
cellular hyphae.

11. Phosphate
• Phosphate is takenup
by high-affinity
phosphate
transporters in the
mycelium. and once
in the hyphae the
long chains are
hydrolysed,
facilitating transfer to
the plant

12. N
AMF can certainly transport N to roots:
• AM exposed to NO3 or NH4 became
highly labelled and this N was
subsequently translocated to the roots.
• N is translocated in the hyphae as
arginine but probably broken down to
urea and ultimately transferred to the
plant as NH4 with the resulting C
skeletons from arginine breakdown being
re-incorporated into the fungal.

13. Plant–fungus reciprocity: who drives
whom?
These fluxes underlie the operation
of the symbiosis, but we do not know
how the exchange is managed.
Is there some reciprocity between
the C supplied by the plant and the P
(or N). supplied by the fungus?
If so, there will have been powerful
selection on the operation of this
mechanism, with potentially
conflicting pressures on the two
partners

14. Plant–fungus reciprocity
• Plants will have been under
powerful selection to discriminate
among fungal partners on the basis
of symbiotic effectiveness.
• One possible mechanism for that
would be a set of molecular signals
specific to each fungus.
• The establishment of the symbiosis
involves just such a recognition
process

15. Nutrient acquisition by AM fungi
• Unlike other mycorrhizal
associations, where at least some of
the fungi involved can be grown in
pure culture and we can measure
the capability of the fungus acting
alone to decompose organic
materials and take up the products
of decomposition
• AMF, however, are not saprotrophic
and therefore are reliant on
saprotrophic microorganisms to
decompose organic matter and
release inorganic ions for capture by
AM hyphae
(Smith & Read 2008)

16. Nutrient acquisition by AM fungi
• The external AM
mycelium phase is the
fungal phase which is in
contact with the soil and
thus responsible for
nutrient acquisition and
transport to the internal
mycelium inside the root
before any transfer to
the plant occurs

17. • In return, triacylglycerides can be transported from the
internal to the external mycelium phase to support the
glyoxyl cycle for metabolic activity

18. Nutrient acquisition by AM fungi
• N capture by AM fungi
was previously believed
to have little ecological
relevance. An important
and influential article by
Read (1991) argued that
AM associations tend to
dominate in systems
where nitrification is
favoured and the main
form of inorganic N will
consequently be nitrate

19. • As NO3, unlike
phosphate, is highly
mobile in soils and
depletion zones
around roots can be
measured in
centimetres rather
then millimetres,
plants should not
require AM fungi in
order to enhance
capture of NO3

20. Nutrient acquisition by AM fungi
• NH4 uptake may be
less energetically
expensive for the
fungus as NO3 first
has to be reduced to
NH4 prior
tincorporation into
amino acids.

21. CONCLUSIONS
• An increased acceptance of the independent
but interacting roles of plant and fungus in
nutrient transfers allows a new and necessary
focus on the nutritional needs of the fungus
itself

22. CONCLUSIONS
• All fungi have a high
nitrogen content and
hence potentially a high N
demand.
• AMF are among the most
abundant fungi on earth,
their role in global N and
P cycles would repay close
attention.

23. CONCLUSIONS
• AMF can acquire N (and
presumably also P) from
decomposing organic material
and transfer it to the plant. The
N and P transfer to the plant
may, if the model proposed
above is correct, be a
consequence of the fungal
demand for nutrients, with
both host plant and fungus
evolving transporters to take
advantage of localised
increases in nutrients.

24. CONCLUSIONS
The mechanisms by which these
fungi actively forage in soil for
both N and P remain unclear.
What is certain is that we need to
pay much more attention to the
biology and ecology of the extra-
radical phase of these fungi if we
are to understand how they
operate in soil and the roles that
they play in ecosystems