Unravelling the importance of mycorrhiza for plant nitrogen nutrition and transfer into the networks

Abstract

Most terrestrial plants establish symbiotic associations with fungi called mycorrhiza, like ectomycorrhizas (EM) and arbuscular mycorrhizas (AM), for accessing limiting plant nutrients. For improving N nutrition, some plant species may establish EM-AM dual-mycorrhizal associations, either within the same root system or at different plant’s ontogenetic stages. Furthermore, EM and AM associations may interconnect plants via a common mycorrhiza network (CMN) for N exchange. However, available studies fail to provide univocal evidence on the advantage to a host plant of exploiting a dual association, compared to a single one for N acquisition, as well as to demonstrate the potential effects of the CMN networks mediating resource partitioning between connected plants, since transfer can occur via several routes simultaneously. In addition, quantification of the amount of possible nutrients transferred has also shown to be challenging, leading to doubts regarding the importance of the CMN in inter-plant partitioning. With this in mind, I have developed two novel experiments to prove and distinguish the contribution of the CMN for N transfer between connected plants as well as to evaluate nutritional advantages of dual vs single mycorrhizal plants. The first experiment aimed to respond two main objectives: (A) to evaluate N nutrition benefits in plants associated with single EM or AM versus dual associations and (B) to evaluate the potential of a dually associated plant as N donor via a CMN with receiver plants bearing single EM or AM associations. For this purpose, I have designed a novel multi-chamber mesocosm where a central split-rooted donor, able to associate with both AM and EM simultaneously, shares an AM or EM network with one neighbour plant simultaneously. Since only dually mycorrhized donor plants had access to both fungi type, it is possible to access different N nutrition of single colonized neighbours compared to central dual mycorrhizal plant. In addition, by applying 15N labeled solution to central dual mycorrhizal plant, I could track the preferential N allocation via AM vs EM network. At my knowledge, such evaluation was never made before. I hypothesized that (1) host plants establishing dual mycorrhiza associations will exhibit an enhanced N nutrition, compared to those depending on single associations. I further hypothesized (2) that dual mycorrhizal plants will preferentially share more N to plants bearing an EM association, due to its larger mycelium proliferation. Lastly, I hypothesized that (3) such mycelium proliferation might act as a sink for C, requiring higher C allocation from host plant. The data obtained demonstrate a nutritional advantage regarding N uptake for host plants holding dual mycorrhizal association, compared to single colonized plants. However, no transfer of N occurred between donor and receiver plants. Therefore, I concluded that CMN functioning for N transfer might occur only under specific situations, such as for particular plant–fungus combination, the characteristics of connected plants or abiotic conditions. For the second experiment, I aimed to quantify the direct transfer of N via the mycelial network in comparison to indirect pathways. I hypothesized that: (1) N transfer between connected plants occurs genuinely through hyphal connection rather than indirect pathways; (2) the proportion of N allocated from donor to receiver plants through mycorrhiza hyphae connections is significant and may improve neighboring plant nutrition and (3) by shading donor plant, N transferred to receiver plants is increased, once it might be able to produce more C to be exchanged by transported N. The data demonstrated a higher 15N transfer to ram1-1 receiver plants. The highest 15N found in the ram1-1 plant summed with the highest root biomass observed in this plant which increasing its area of nutrients absorption, highlighting the importance of indirect pathways for resources allocations in our system. Also in opposite to what was previously hypothesized, shading treatment did not increased 15N transfer. With this, it is possible to conclude that CMN are important, but most likely by other means than discussed in the literature