Common arbuscular mycorrhizal networks : trade of carbon and soil nutrients between different plant species and their shared fungal symbiont

Plants commonly live in symbiotic associations with arbuscular mycorrhizal fungi (AMF). They invest substantial amounts of photosynthetic products to feed their fungal partners, which, in return, provide mineral nutrients foraged in the soil by their hyphal networks. AMF may supply up to 90 % of the host plant's nitrogen (N) and phosphorus (N) requirements. Moreover, AMF are important determinants of plant community structure and ecosystem productivity. Typically, AMF exhibit little host-specificity; a single individual may simultaneously colonize multiple plants, even from different species and thereby it forms far-reaching common mycorrhizal networks (CMNs). Hence, the interconnected plants share their C investments and nutritional benefits of the common fungal partner. This fact arises the question about the terms of trade between plants and their shared fungal partners. Or in other words, what is the C investment of a given plant into a CMN, and what is the return of this investment in terms of mineral nutrients provided by the CMN? However, up to now, the relationship between carbon investment and nutritional benefit of different plants engaged in a CMN has never been assessed.
To address the terms of trade in a CMN experimentally, we set up microcosms containing a pair of test plants, interlinked by a CMN of Glomus intraradices or G. mosseae. The plants were flax (Linum usitatissimum) and sorghum (Sorghum bicolor) grown either in "monocultures", as pair of identical plant species, or in a "mixed culture", as pair of different plant species. The microcosms were compartmented by nylon mesh screens to separate the CMN physically and functionally from the plant roots. Flax (a C3-plant) and sorghum (a C4-plant) display distinct C isotope compositions. This allowed us to differentially assess the C investment of the two plants into the CMN through the analysis of the C isotopic signature of isolated AMF hyphae or spores, or, with higher precision, of the AMF-specific fatty acid C16:1ω5. In parallel, we determined the plants' return of investment by measuring the acquisition of nutrients via CMN, using 15N and 33P as tracers only accessible for AMF hyphae. Plant growth response was determined by assessing the biomass of the plants.
Interestingly, we found a strong asymmetry in the terms of trade: when the CMN was formed by G. intraradices, flax invested only little C but gained up to 94 % of the CMN-mediated N and P while sorghum invested massive amounts of C without receiving a corresponding nutritional gain. The acquisition of nutrients was more balanced with a CMN formed by G. mosseae. However, sorghum still contributed the lion’s share of C to the CMN. Nonetheless, in both cases sorghum was barely affected in growth, probably because it had a surplus of C. Excitingly, the growth of flax was highly increased due to the facilitated nutrient uptake via the CMN, which increased the overall biomass production in the mixed culture compared to the mean of the monocultures.
Many mycorrhizal plants are highly dependent on AMF for P acquisition; moreover the mycorrhizal P uptake usually dominates the plant’s P acquisition. The mycorrhizal P uptake pathway starts in the soil far away from the roots, where AMF hyphae forage for immobile inorganic phosphate (Pi). The AMF hyphae take up Pi and translocate it to the roots. Inside the root, Pi is transferred from fungus to plant with the help of specific Pi transporters induced by the AMF. Remarkably, these AM-inducible Pi transporters are crucial for symbiotic Pi acquisition. In order to characterize the plants P acquisition via the CMN in our model system, we described for the first time Pi transporter genes belonging to the Pht1 gene family in flax and sorghum. We found that the expression of these Pi transporter genes was highly dependent on the presence and identity of the AMF. Surprisingly, the plant’s mycorrhizal Pi uptake appeared to be independent of the expression levels of AM-inducible Pi transporters in the roots. The genes showed very similar expression levels, even if the Pi uptake was dramatically different between the treatments. Nevertheless, AM-inducible Pi transporters showed different expression levels depending on culture system indicating that interconnected plants can influence the neighboring plant’s gene expression.
An extreme example of terms of trade in CMNs displays achlorophyllous mycoheterotrophic (MH) plants. Most MH plants obtain, besides nutrients, their entire C from CMNs and thus indirectly exploit neighboring autotrophic plants. While temperate MH plants associated to ectomycorrhizal fungi are well described, tropical MH plants often associated to AMF are overlooked due to difficulties of examining AMF tissue. By analyzing AMF spores, we were able to investigate C and N isotopic signature of MH plants, green plants and AMF in Caribbean rainforests. These organisms displayed similar C and N isotopic signatures, while temperate MH plants, fungi and green plants differ in their isotopic signature, suggesting differences in C and N exchange between the two MH systems. Furthermore, the isotopic analysis revealed canopy trees as main resource provider for AMF and MH plants in the Caribbean forests. Thus, we provide a first description of the autotrophic – AMF – MH continuum in tropical forests.
In order to track the C source of the CMN in our model system, we used different methods to make use of the C isotopic signature of AMF. In the last part of this thesis, we compare three methods to analyze the C isotopic signature in the AMF. Bulk C isotope analysis of washed extraradical mycelium is possible, but has the drawback of potential contamination from non-mycorrhizal organic sources. Bulk C isotope analysis of isolated AMF spores yields more reliable results but is rather tedious and most applicable for field studies. Therefore, we explain, in detail, a more refined analysis based on the extraction of lipids from soil, followed by analysis of the AMF biomarker fatty acid C16:1ω5.
In summary, this PhD thesis describes for the first time terms of trade in a CMN shared by two plants. The nutritional return provided by the fungus differed greatly between the examined plants and was not related to the extent of C investment, but dependent on the involved AMF species. However, the huge differences in nutrient uptake were not reflected in the expression levels of AM-inducible Pi transporters. In MH plants the use of the CMN is not only asymmetric, but even unidirectional. The investigation of both systems, MH and mixed culture, revealed that plant growth can be promoted by asymmetric use of CMNs. We propose that thanks to an exchange of surplus resources this can occur without impairment of the donor plant. Finally, the herein described mechanisms may help to understand the great impact of AMF on plant community structure and productivity.