A plasma membrane zinc transporter from Medicago truncatula is up-regulated in roots by Zn fertilization,yet down-regulated by arbuscular mycorrhizal colonization

Here we present a Zn transporter cDNA named MtZIP2 from the model legume Medicago truncatula. MtZIP2 encodes a putative 37 kDa protein with 8-membrane spanning domains and has moderate amino acid identity with the Arabidopsis thaliana Zn transporter AtZIP2p. MtZIP2 complemented a Zn-uptake mutant of yeast implying that the protein encoded by this gene can transport Zn across the yeast's plasma membrane. The product of a MtZIP2-GFP fusion construct introduced into onion cells by particle bombardment likewise localized to the plasma membrane. The MtZIP2 gene was expressed in roots and stems, but not in leaves of M. truncatula and, in contrast to all other plant Zn transporters characterized thus far, MtZIP2 was up-regulated in roots by Zn fertilization. Expression was highest in roots exposed to a toxic level of Zn. MtZIP2 expression was also examined in the roots of M. truncatula when colonized by the obligate plant symbiont, arbuscular mycorrhizal (AM) fungi, since AM fungi are renowned for their ability to supply plants with mineral nutrients, including Zn. Expression was down-regulated in the roots of the mycorrhizal plants and was associated with a reduced level of Zn within the host plant tissues.     

Contribution of the VA mycorrhizal hyphae in acquisition of phosphorus and zinc by maize grown in a calcareous soil

An investigation was carried out to test whether the mechanism of increased zinc (Zn) uptake by mycorrhizal plants is similar to that of increased phosphorus (P) acquisition. Maize (Zea mays L.) was grown in pots containing sterilised calcareous soil either inoculated with a mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe or with a mixture of mycorrhizal fungi, or remaining non-inoculated as non-mycorrhizal control. The pots had three compartments, a central one for root growth and two outer ones for hyphal growth. The compartmentalization was done using a 30-m nylon net. The root compartment received low or high levels of P (50 or 100 mg kg^–1 soil) in combination with low or high levels of P and micronutrients (2 or 10 mg kg^–1 Fe, Zn and Cu) in the hyphal compartments.Mycorrhizal fungus inoculation did not influence shoot dry weight, but reduced root dry weight when low P levels were supplied to the root compartment. Irrespective of the P levels in the root compartment, shoots and roots of mycorrhizal plants had on average 95 and 115% higher P concentrations, and 164 and 22% higher Zn concentrations, respectively, compared to non-mycorrhizal plants. These higher concentrations could be attributed to a substantial translocation of P and Zn from hyphal compartments to the plant via the mycorrhizal hyphae. Mycorrhizal inoculation also enhanced copper concentration in roots (135%) but not in shoots. In contrast, manganese (Mn) concentrations in shoots and roots of mycorrhizal plants were distinctly lower, especially in plants inoculated with the mixture of mycorrhizal fungi.The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorrhizal plants. The minimal hyphae contribution (delivery by the hyphae from the outer compartments) to the total plant acquisition ranged from 13 to 20% for P and from 16 to 25% for Zn.     

The symbiotic recapture of nitrogen from dead mycorrhizal and non-mycorrhizal roots of tomato plants

Aims

The aim was to quantify the nitrogen (N) transferred via the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices from both a dead host and a dead non-host donor root to a receiver tomato plant. The effect of a physical disruption of the soil containing donor plant roots and fungal mycelium on the effectiveness of N transfer was also examined.

Methods

The root systems of the donor (wild type tomato plants or the mycorrhiza-defective rmc mutant tomato) and the receiver plants were separated by a 30 μm mesh, penetrable by hyphae but not by the roots. Both donor genotypes produced a similar quantity of biomass and had a similar nutrient status. Two weeks after the supply of ¹⁵?N to a split-root part of donor plants, the shoots were removed to kill the plants. The quantity of N transferred from the dead roots into the receiver plants was measured after a further 2 weeks.

Results

Up to 10.6 % of donor-root ¹⁵N was recovered in the receiver plants when inoculated with the arbuscular mycorrhizal fungus (AMF). The quantity of ¹⁵N derived from the mycorrhizal wild type roots clearly exceeded that from the only weakly surface-colonised rmc roots. Hyphal length in the donor rmc root compartments was only about half that in the wild type compartments. The disruption of the soil led to a significantly increased AMF-mediated transfer of N to the receiver plants.

Conclusions

The transfer of N from dead roots can be enhanced by AMF, especially when the donor roots have been formerly colonised by AMF. The transfer can be further increased with higher hyphae length densities, and the present data also suggest that a direct link between receiver mycelium and internal fungal structures in dead roots may in addition facilitate N transfer. The mechanical disruption of soil containing dead roots may increase the subsequent availability of nutrients, thus promoting mycorrhizal N uptake. When associated with a living plant, the external mycelium of G. intraradices is readily able to re-establish itself in the soil following disruption and functions as a transfer vessel.     

Characterization of zinc transport by divalent metal transporters of the ZIP family from the model legume <Emphasis Type="Italic">Medicago truncatula</Emphasis>

To understand how plants from the Fabaceae family maintain zinc (Zn) homeostasis, we have characterized the kinetics of three Zn transporting proteins from the ZIP family of divalent metal transporters in the model legume Medicago truncatula. Of six ZIP’s studied, MtZIP1, MtZIP5 and MtZIP6 were the only members from this family determined to transport Zn and were further characterized. MtZIP1 has a low affinity for Zn with a K_m of 1 μM as compared to MtZIP5 and MtZIP6 that have a higher affinity for Zn with K_m of 0.4 μM and 0.3 μM, respectively. Zn transport by MtZIP1 was more sensitive to inhibition by copper (Cu) concentrations than MtZIP5 and MtZIP6, because 3 μM Cu inhibited Zn transport by 80% in MtZIP1 while 5 μM Cu was required to achieve the same inhibition of Zn transport in MtZIP5 and MtZIP6. Cadmium (Cd) had a greater effect on the ability of MtZIP1 to transport Zn than MtZIP5 and MtZIP6, because at a concentration of 3 μM Cd, the Zn transport by MtZIP1 was inhibited 55% and the transport of Zn by MtZIP5 and MtZIP6 was inhibited by 20–30%. However, only MtZIP6 transported Cd at higher rates than those observed in the control plasmid pFL61, demonstrating a low affinity for Cd based on a K_m of 57 μM. These results suggest that Medicago truncatula has both high and low affinity Zn transporters to maintain Zn homeostasis and that these transporters may function in different compartments within the plant.     

Absence of carbon transfer between Medicago truncatula plants linked by a mycorrhizal network, demonstrated in an experimental microcosm

Carbon transfer between plants via a common extraradical network of arbuscular mycorrhizal (AM) fungal hyphae has been investigated abundantly, but the results remain equivocal. We studied the transfer of carbon through this fungal network, from a Medicago truncatula donor plant to a receiver (1) M. truncatula plant growing under decreased light conditions and (2) M. truncatula seedling. Autotrophic plants were grown in bicompartmented Petri plates, with their root systems physically separated, but linked by the extraradical network of Glomus intraradices. A control Myc-/Nod- M. truncatula plant was inserted in the same compartment as the receiver plant. Following labeling of the donor plant with 13CO2, 13C was recovered in the donor plant shoots and roots, in the extraradical mycelium and in the receiver plant roots. Fatty acid analysis of the receiver's roots further demonstrated 13C enrichment in the fungal-specific lipids, while almost no label was detected in the plant-specific compounds. We conclude that carbon was transferred from the donor to the receiver plant via the AM fungal network, but that the transferred carbon remained within the intraradical AM fungal structures of the receiver's root and was not transferred to the receiver's plant tissues.     

丛枝菌根根外菌丝网络形成过程中的时间效应及植物介导作用

以豆科植物紫花苜蓿为试验材料,应用三室(供体室-间隔室-受体室)培养系统,研究在供体和受体紫花苜蓿根系之间菌丝网络形成的时间效应以及间隔室中不同植物对菌丝网络建成的介导作用.第一个试验在供体和受体植物生长8、10、12、14周之后进行收获以检验菌丝网络形成的时间效应;第二个试验则在间隔室分别种植紫花苜蓿、羊草和独行菜,以考察菌根依赖性不同的植物对菌丝网络形成的介导作用.试验结果显示:(1)接种丛枝菌根真菌的供体紫花苜蓿根系能够形成良好的菌根共生,其外延菌丝可穿过尼龙网和间隔室侵染受体植物根系;植物生长8周后,在受体植物根系检测到菌根侵染,证实供体和受体植物间形成了根间菌丝网络;10周后,尽管供体室和受体室植物的侵染率已无差异,但二者的生物量和地上部磷浓度差异却加大,表现出菌丝网络对植物种内竞争影响的不对称性.(2)试验条件下,不同介导植物对受体植物的菌根侵染及生物量均无明显影响,但显著降低了供体植物生物量和地上部磷浓度;间隔室无介导植物或种植独行菜时,受体植物地上部和根系生物量显著低于供体植物,而当介导植物为紫花苜蓿和羊草时,受体和供体植物生物量无显著差异.研究表明,植物根间菌丝网络的形成受时间和介导植物的影响,同时也具有调节植物间资源分配和植物相互作用的功能.     

The role of the external mycelial network of vesicular-arbuscular mycorrhizal fungi: a study of carbon transfer between plants interconnected by a common mycelium

The transfer of ¹⁴C from Lolium perenne (the donor) to Plantago lanceolata (the receiver), mediated by vesicular-arbuscular (VA) mycorrhizal fungi, was examined when the two species were grown together or separately. The VA mycorrhizal infection led to a significant increase, relative to that in uninfected plants, in the ¹⁴C transferred from donor to receiver plants, not only when the roots of the two plants were growing in intimate mixture, but also when they were separated by a root-free zone of 2.33 cm. The majority of isotope transfer between the two plant species was along the direct pathway via VA mycelium.     

Radiocesium transfer between Medicago truncatula plants via a common mycorrhizal network

Common mycorrhizal networks of arbuscular mycorrhizal fungi have been reported to transfer cesium between plants. However, a direct hyphae-mediated transfer (via cytoplasm/protoplasm) cannot be distinguished from an indirect transfer. Indeed, cesium released by the roots of the donor plant can be taken up by the receiver plant or fungal hyphae. In the present study, Medicago truncatula plants were connected by a common mycorrhizal network and Prussian Blue (ammonium-ferric-hexacyano ferrate) was added in the growth medium to adsorb the released radiocesium. A direct transfer of radiocesium to roots and shoots of the receiver plant was clearly demonstrated for the first time. Even though this transfer was quantitatively low, it suggested that shared mycorrhizal networks could contribute to the redistribution of this radionuclide in the environment, which otherwise would be restricted both in time and space. This finding may also help to understand the behaviour of its chemical analogue, potassium.     

The effect of aluminium on the distribution of calcium,magnesium and phosphorus in mycorrhizal and non-mycorrhizal seedlings of Eucalyptus rudis: a cryo-microanalytical study

The distribution of Al, Ca, Mg and P in the lateral roots and leaves of mycorrhizal and non-mycorrhizal seedlings of Eucalyptus rudis grown with and without Al was analysed using energy-dispersive X-ray microanalysis on a cryo-scanning electron microscope. Al accumulated in all tissues of nonmycorrhizal plants: the endodermis was not a barrier to the translocation of Al. In mycorrhizal roots, Al was concentrated within the sheath. The presence of Al reduced the levels of Ca and Mg in both mycorrhizal and non-mycorrhizal roots and shoots in comparison with control plants. The presence of mycorrhizas increased the levels of Ca and Mg in plants grown with Al in comparison with non-inoculated plants, although there was no evidence that mycorrhizas increased the levels of P in plants grown in Al-amended soils. P levels were higher in the mycorrhizal sheath of plants grown with Al than the controls.     

Vesicular arbuscular mycorrhizal mediation of grass response to acidic and heavy metal depositions

Summary The perennial bunchgrassEhrharta calycina was grown with and without V.A.M. fungal infection (Glomus fasciculatum) in a sandy loam exposed to a range of acidic and heavy metal depositions. Heavy metals (Cu, Ni, Pb, Zn, Fe, and Co) were applied in simulated rain (pH 3.0, 4.0, and 5.6) at deposition rates approximating those observed to result from smelter efluents. Metal concentrations in the roots and shoots of mycorrhizal plants were greater than those of non-mycorrhizal plants. Mycorrhizal enhancement of plant metal uptake increased with greater acidity and higher heavy metal content of treatment. The growth of mycorrhizal plants was reduced compared to non-mycorrhizal plants when metal depostion was combined with simulated acid rain. We propose that mycorrhizal enhancement of heavy metal uptake caused reduced growth in plants exposed to acidic and heavy metal depositions.     

Differential effects of AM fungal isolates on Medicago truncatula growth and metal uptake in a multimetallic (Cd, Zn, Pb) contaminated agricultural soil

Toxic metal accumulation in soils of agricultural interest is a serious problem needing more attention, and investigations on soil–plant metal transfer must be pursued to better understand the processes involved in metal uptake. Arbuscular mycorrhizal (AM) fungi are known to influence metal transfer in plants by increasing plant biomass and reducing metal toxicity to plants even if diverging results were reported. The effects of five AM fungi isolated from metal contaminated or non-contaminated soils on metal (Cd, Zn) uptake by plant and transfer to leachates was assessed with Medicago truncatula grown in a multimetallic contaminated agricultural soil. Fungi isolated from metal-contaminated soils were more effective to reduce shoot Cd concentration. Metal uptake capacity differed between AM fungi and depended on the origin of the isolate. Not only fungal tolerance and ability to reduce metal concentrations in plant but also interactions with rhizobacteria affected heavy metal transfer and plant growth. Indeed, thanks to association with nodulating rhizobacteria, one Glomus intraradices inoculum increased particularly plant biomass which allowed exporting twofold more Cd and Zn in shoots as compared to non-mycorrhizal treatment. Cd concentrations in leachates were variable among fungal treatments, but can be significantly influenced by AM inoculation. The differential strategies of AM fungal colonisation in metal stress conditions are also discussed.     

The magnitude and control of carbon transfer between plants linked by a common mycorrhizal network

Various claims have been made about the ecological significance of plant-to-plant carbon movement through common mycorrhizal networks (CMNs). Most suggest that resource competition among interconnected plants should be less important than previously thought. If true, that would profoundly alter our perception of how plants interact among themselves and with their environment. However, there are difficulties in quantifying the amounts of resource transferred via CMNs, ensuring that transfer is genuinely through hyphae, not soil, and understanding its control. Carbon movement has not been quantified in many of the published studies. Where it has, its likely functional role has not been clarified. Some recent, well-publicized research suggests that carbon transferred to trees via an ectomycorrhizal (EcM) network may be physiologically and ecologically important. Our view, however, is that the evidence for this remains equivocal. Appropriate controls for the possibility of carbon transfer via soil were not used under field conditions. In laboratory experiments, controls failed to clarify the role of EcM links in carbon transfer. To resolve some areas of uncertainty, abundances of ¹³C have been measured to estimate carbon transfers via an arbuscular mycorrhizal (AM) network connecting grasses and forbs of the same or different species. Permeable barriers to roots and hyphae allowed any direct carbon transfer via soil to be detected. Large amounts of carbon (typically 10% of that in roots) were transferred between linked plants via the CMN. Transferred carbon was never transported into shoots of 'receiver' plants. It remained in roots, probably inside fungal structures and, therefore, unavailable to the plants into which it was apparently transferred. Carbon transfer via an AM network does not allow 'resource sharing' among linked plants. It is probably irrelevant to the botanical components of a community, but it may be fundamental for fungal members. The 'mycocentric' view is that fungal structures within roots are parts of extended mycelia through which fungi move carbon according to their own carbon demands, not those of their autotrophic hosts.     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

Contribution of an arbuscular mycorrhizal fungus to the uptake of cadmium and nickel in bean and maize plants

Two experiments were carried out in pots with three compartments, a central one for root and hyphal growth and two outer ones which were accessible only for hyphae of the arbuscular mycorrhizal fungus, Glomus mosseae ([Nicol. and Gerd.] Gerdemann and Trappe). In the first experiment, mycorrhizal and nonmycorrhizal bean (Phaseolus vulgaris L.) plants were grown in two soils with high geogenic cadmium (Cd) or nickel (Ni) contents. In the second experiment, mycorrhizal and nonmycorrhizal maize (Zea mays L.) or bean plants were grown in a non-contaminated soil in the central compartment, and either the Cd- or Ni-rich soil in the outer compartments. In additional pots, mycorrhizal plants were grown without hyphal access to the outer compartments. Root and shoot dry weight was not influenced by mycorrhizal inoculation, but plant uptake of metals was significantly different between mycorrhizal and nonmycorrhizal plants. In the first experiment, the contribution of mycorrhizal fungi to plant uptake accounted for up to 37% of the total Cd uptake by bean plants, for up to 33% of the total copper (Cu) uptake and up to 44% of the total zinc (Zn) uptake. In contrast, Ni uptake in shoots and roots was not increased by mycorrhizal inoculation. In the second experiment, up to 24% of the total Cd uptake and also up to 24% of the total Cu uptake by bean could be attributed to mycorrhizal colonisation and delivery by hyphae from the outer compartments. In maize, the mycorrhizal colonisation and delivery by hyphae accounted for up to 41% of the total Cd uptake and 19% of the total Cu uptake. Again, mycorrhizal colonisation did not contribute to Ni uptake by bean or maize. The results demonstrate that the arbuscular mycorrhizal fungus contributed substantially not only to Cu and Zn uptake, but also to uptake of Cd (but not Ni) by plants from soils rich in these metal cations. Deceased 21 September 1996 Deceased 21 September 1996     

Mycorrhizal colonization and drought stress affect Δ13C in CO2‐labeled lettuce plants

We studied the role of different arbuscular‐mycorrhizal (AM) fungi on lettuce (Lactuca sativa L.) plant carbon metabolism under drought stress. Plants were grown in pots maintained at two levels of soil moisture and labeled during photosynthesis with CO₂. P‐fertilized plants were used as a non‐mycorrhizal control. Well‐watered mycorrhizal plants showed similar growth to that of P‐fertilized plants. The level of mycorrhizal root infection was not significantly affected by fungal species or by water treatment. In contrast, important differences in Δ¹³C between P‐fertilized and AM plants were found in shoot and root tissues as a consequence of both water limitation and fungal presence. Δ¹³C in shoots and roots increased in non‐mycorrhizal treatment as compared with the well‐watered plants, whereas this parameter decreased significantly in mycorrhizal plants. Photosynthetic activity was increased in AM plants in well‐watered and droughted plants. G. deserticola was the most beneficial endophyte for water use efficiency in both water treatments. Transpiration rate was not affected by any of the treatments. On the basis of total C in plant tissues, in AM plants the newly fixed C seemed to be preferentially utilized for fungal activity rather than being stored in roots.     

Genetic variation for root architecture, nutrient uptake and mycorrhizal colonisation in Medicago truncatula accessions

Sustainable agriculture strives for healthy, high yielding plants with minimal agronomic inputs. Genetic solutions to increase nutrient uptake are desirable because they provide ongoing improvements. To achieve this it is necessary to identify genes involved in uptake and translocation of nutrients. We selected Medicago truncatula L. as a model because of its: i) close genetic relationship to food legumes, ii) use as a pasture legume in southern Australia and iii) availability of mapping populations generated from genetically diverse accessions. We discovered statistically significant differences between eight accessions for: root architecture in growth pouches, % root colonisation with the arbuscular mycorrhizal (AM) fungus Glomus intraradices, and plant tissue concentration of most macro- and micronutrients. Mycorrhizal colonisation had a significant effect on P concentration in roots but not shoots, Mg concentration in both roots and shoots, and the concentration of various micronutrients in shoots including Fe, Ca, but not Zn. Comparison of micronutrient uptake between root and shoot tissues showed that some M. truncatula accessions were more efficient at mobilisation of nutrients from roots to shoots. We are now in a position to use existing mapping populations of M. truncatula to identify quantitative trait loci important for human health and sustainable agriculture.     

Acquisition of phosphorus and copper by VA-mycorrhizal hyphae and root-to-shoot transport in white clover

White clover (Trifolium repens L.) plants were grown in a calcareous soil in pots with three compartments, a central one for root growth and two outer ones for growth of vesicular-arbuscular (VA) mycorrhizal (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe) hyphae (hyphal compartments). Phosphorus (P) was applied at three levels (0, 20 and 50 mg kg⁻¹ soil) in the outer compartments in mycorrhizal treatments. Root and shoot dry weight were increased in mycorrhizal plants with hyphal access to outer compartments. Growth of the mycorrhizal hyphae in the outer compartments was not significantly affected by variation in P level in these compartments. However, both concentration and amount of P in roots and shoots sharply increased with increasing P supply in the outer (hyphal) compartments. With increasing P levels the calculated delivery of P by the hyphae from the outer compartments increased from 34% to 90% of total P uptake. Hyphal access to the outer compartments also significantly increased both concentration and quantity of Cu in the plants. The calculated delivery of Cu by the hyphae from the outer compartments ranged from 53% to 62% of total Cu uptake, irrespective of the P levels and the amounts of P taken up and transported by the hyphae. However, the distribution of Cu over roots and shoots was largely dependent on P levels. With increase in P level in the outer compartments the calculated hyphal contribution to the total amount of Cu in the shoots increased from 12% to 58%, but decreased in the roots from 75% to 46%. In conclusion, uptake and transport by VA-mycorrhizal hyphae may contribute substantially not only to P nutrition, but also to Cu nutrition of the host.     

Priming and filtering of antiherbivore defences among Nicotiana attenuata plants connected by mycorrhizal networks

Arbuscular mycorrhizal fungi (AMF) establish symbiotic associations with a majority of terrestrial plants to form underground common mycorrhizal networks (CMNs) that connect neighbouring plants. Because Nicotiana attenuata plants do not respond to herbivory‐elicited volatiles from neighbours, we used this ecological model system to evaluate if CMNs function in interplant transmission of herbivory‐elicited responses. A mesocosm system was designed to establish and remove CMNs linking N. attenuata plants to examine the herbivory‐elicited metabolic and hormone responses in CMNs‐connected “receiver” plants after the elicitation of “donor” plants by wounding (W) treated with Manduca sexta larval oral secretions (OS). AMF colonization increased constitutive jasmonate (JA and JA‐Ile) levels in N. attenuata roots but did not affect well‐characterized JAs‐regulated defensive metabolites in systemic leaves. Interestingly, larger JAs bursts, and higher levels of several amino acids and particular sectors of hydroxygeranyllinalool diterpene glycoside metabolism were elevated in the leaves of W + OS‐elicited “receivers” with CMN connections with “donors” that had been W + OS‐elicited 6 hr previously. Our results demonstrate that AMF colonization alone does not enhance systemic defence responses but that sectors of systemic responses in leaves can be primed by CMNs, suggesting that CMNs can transmit and even filter defence signalling among connected plants.     

Medicago truncatula Mtha1-2 mutants loose metabolic responses to mycorrhizal colonization

Bidirectional nutrient transfer is one of the key features of the arbuscular mycorrhizal symbiosis. Recently we were able to identify a Medicago truncatula mutant (mtha1-2) that is defective in the uptake of phosphate from the periarbuscular space due to a lack of the energy providing proton gradient provided by the symbiosis specific proton ATPase MtHA1¹ In order to further characterize the impact of fungal colonization on the plant metabolic status, without the beneficial aspect of improved mineral nutrition, we performed leaf ion analyses in mutant and wildtype plants with and without fungal colonization. Although frequency of fungal colonization was unaltered, the mutant did not show a positive growth response to mycorrhizal colonization. This indicates that nutrient transfer into the plant cell fails in the truncated arbuscules due to lacking expression of a functional MtHA1 protein. The leaves of wildtype plants showed clear metabolic responses to root mycorrhizal colonization, whereas no changes of leaf metabolite levels of mycorrhizal mtha1-2 plants were detected, even though they were colonized. These results show that MtHa1 is indispensable for a functional mycorrhizal symbiosis and, moreover, suggest that fungal root colonization per se does not depend on nutrient transfer to the plant host.     

Bacterial effects on arbuscular mycorrhizal fungi and mycorrhiza development as influenced by the bacteria, fungi, and host plant

Bacterial strains from mycorrhizal roots (three belonging to Comamonadaceae and one to Oxalobacteraceae) and from non-mycorrhizal roots (two belonging to Comamonadaceae) of Medicago truncatula and two reference strains (Collimonas fungivorans Ter331 and Pseudomonas fluorescens C7R12) were tested for their effect on the in vitro saprophytic growth of Glomus mosseae BEG12 and on its colonization of M. truncatula roots. Only the Oxalobacteraceae strain, isolated from barrel medic mycorrhizal roots, and the reference strain P. fluorescens C7R12 promoted both the saprophytic growth and root colonization of G. mosseae BEG12, indicating that they acted as mycorrhiza helper bacteria. Greatest effects were achieved by P. fluorescens C7R12 and its influence on the saprophytic growth of G. mosseae was compared to that on Gigaspora rosea BEG9 to determine if the bacterial stimulation was fungal specific. This fungal specificity, together with plant specificity, was finally evaluated by comparing bacterial effects on arbuscular mycorrhizal symbiosis when each of the fungal species was inoculated to two different plant species (M. truncatula and Lycopersicon esculentum). The results obtained showed that promotion of saprophytic growth by P. fluorescens C7R12 was expressed in vitro towards G. mosseae but not towards G. rosea. Bacterial promotion of mycorhization was also expressed towards G. mosseae, but not G. rosea, in roots of M. truncatula and L. esculentum. Taken together, results indicated that enhancement of arbuscular mycorrhiza development was only induced by a limited number of bacteria, promotion by the most efficient bacterial strain being fungal and not plant specific.