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Research
For approximately 80 % -
90 % of all known plant species, mycorrhizal roots dominate nutrient
uptake at the soil/plant interface. The mycorrhiza, a close association
between plants and various fungal species is extremely important
for the uptake of P and N, but also contributes to the uptake of
various trace elements such as Cu and Zn (Bücking and Heyser
1994). Besides this positive effect of mycorrhizal fungi on nutrient
uptake, the mycorrhizal infection can also improve the tolerance
of plants to different stress factors like pathogen infections,
drought and heavy metals. To maintain the mycorrhizal fungal structures,
around 20 % of the assimilated carbon from the plant is translocated
to the fungal symbiont in both arbuscular- and ectomycorrhizal associations.
Ectomycorrhizal roots in particular can be entirely dependent on
the nutrient supply by the fungal symbiotic partner as they are
completely surrounded by fungal hyphae. The fungal sheath can form
an effective apoplastic barrier for the entry of water and nutrients
from the soil into the root cortex (Bücking et al. 2001, Bücking
et al. 2002). If the fungal mantle is impermeable to nutrient ions,
the underlying root tissue is isolated from the soil solution and
this necessitates the uptake of nutrients from the soil into the
fungal symplast. In this case the water and nutrient uptake from
the soil solution and the transfer of these nutrients to the mycorrhizal
plant are regulated by the mycorrhizal fungus.
The mutualistic interaction between mycorrhizal fungi and plants
is based on a bidirectional transfer of nutrients and carbohydrates
across an interface, whose structure and development vary between
different types of mycorrhizal associations. Since there is no symplastic
contiunity between both symbiotic partners, nutrients always have
to pass an interfacial apoplast before then can be absorbed. In
the ectomycorrhizal associations the interface regions is formed
by the
- fungal plasma membrane
- fungal cell wall
- interfacial matrix
- plant cell wall
- plant plasma membrane.
The normal flows of P and carbohydrates
through the plasma membranes into the interfacial apoplast are calculated
to be insufficient to maintain the symbiosis. Therefore, conditions
in the interface, which cause an enhanced efflux into the apoplast
or a decrease in the level of competing uptake systems has been
proposed. Relating
to this aspect, it was shown, that the permeability of fungal membranes
and therefore the efflux of P into the interface can be stimulated
by carbohydrates as well as different cations (Bücking, in
press). Plants can obviously promote the P transfer across the mycorrhizal
interface by their carbohydrate supply and have an influence on
the distribution of P among various fungal P pools (Bücking
and Heyser, 2003; Bücking, Heyser and Shachar-Hill, 2005).
The following model
system shows a possible interaction between the carbohydrate and
phosphate flux across the mycorrhizal interface.
Presently, we have little information
about the regulation of the transfer processes across the specialized
interface in a mycorrhizal symbiosis and the mechanisms involved
in polarizing the transfer between the symbionts. We need to know
more about:
• the regulation of the invertase activity.
Prerequisite for an uptake of glucose by the mycorrhizal fungus
from the interface is the presence and function of an acid invertase
of plant origin, which catalyzes the cleavage of sucrose into the
hexoses glucose and fructose. It is known, that the host cell invertase
activity increases substantially in response to pathogenic infection.
However, localization and regulation of the invertase activity in
mycorrhizal interfaces are still entirely unknown. Relating to this
aspect, we need to know, how biotrophic fungi induce metabolic sinks
at the infection site to ensure carbohydrate flux from the host
plant and if the carbon metabolism of the fungi is involved in these
processes.
• the gene expression of transporters
All the transporters that have been shown or postulated to be involved
in uptake from the apoplast are proton symporters, which emphasizes
the importance of H+-ATPases and the pH conditions in the interface
for nutrient uptake in mycorrhizal or in pathogenic interactions.
We know, that the carbohydrate flux from the mycorrhizal host plant
stimulates uptake, translocation from the absorbing to the releasing
hyphae in the mycorrhiza and transfer of P across the interface
to the host plant (Bücking and Heyser, 2003; Bücking,
Heyser and Shachar-Hill, 2005). The carbohydrate supply of the host
plant might also have an effect on the gene expression of P transporters:
(1) an upregulation of transporters in P absorbing hyphae at the
soil/hyphal interface and (2) a downregulation of transporters in
P releasing hyphae at the interface.
• the regulation of membrane permeability
The normal flows of nutrients through the membranes are thought
to be insufficient to explain the high exchange rates observed at
plant-fungal interface and it was assumed that conditions might
exist in the interface which promote a higher net efflux or a decrease
in competing uptake systems. It was shown, that the P efflux through
fungal plasma membranes of axenic cultures can be stimulated by
an external supply of sucrose (Bücking, 2004). However, we
have only little information about mechanisms involved in an increased
membrane permeability at the interface and how metabolites and minerals
are exported across the membranes of donor cells.
• the contribution of other exchange processes
The consideration of an exchange of P alone against carbohydrates
at the plant/fungal interface will give an incomplete view of the
transfer processes occurring in a mycorrhizal symbiosis. It is known
that mycorrhizal associations are also important for the supply
of plants with N and the processes involved in this transfer are
still entirely unknown. K and Mg, which have been shown to be the
most important counterions in polyphosphates (Bücking and Heyser
1999) will be released into the fungal symplast during the breakdown
of polyphosphates in the fungal hyphae and then a transfer of these
elements via the plant fungal interface is also possible. However,
knowledge of the effect of a mycorrhizal infection on the uptake
of nutrients such as K, Mg and Ca is limited and not consistent.
The primary goal of these investigations will be to contribute to
a better understanding of processes involved in the uptake of nutrients
from the soil/plant interface and in the transfer of nutrients across
the plant/fungal interface, which is responsible for the efficiency
of this mycorrhizal symbiosis for the host plant.
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