Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Biology and Microbiology

First Advisor

Senthil Subramanian


Carbon allocation, Legume, Mycorrhiza, Nutrient exchange, Rhizobia, Tripartite


Mutualism is a very common phenomenon among living organisms on earth. Legumes because of their high protein content, serve as a great nutrient resource for animals. This group of plants can form a mutualistic symbiosis with beneficial microbes. For example, Alfalfa (Medicago) and soybean (Glycine max) can get colonized with arbuscular mycorrhizal fungi (AMF) and rhizobia bacteria simultaneously forming a complex tripartite interaction for nutrient benefits. Most of the previous research evaluated individual symbionts, either rhizobia bacteria or AMF, but not both. There are only a few reports which discuss the nutrient exchange mechanisms in a tripartite interaction. Thus, there is a lack of fundamental understanding of how the resources are exchanged in tripartite interactions. Nitrogen (N) and phosphorus (P) are essential nutrients for plant growth; AMF can supply both P and N, while rhizobia bacteria can only supply N to their host plant. Both root symbionts can provide other benefits like abiotic and biotic stress tolerance. In return, the host plant distributes a substantial amount of its photosynthetic carbon (C) produced in the leaves to its root symbionts. However, the regulation mechanisms on C resources allocation by the host plant to its root symbionts is not well understood. In my first experiment, I hypothesized that the N-fixing capability of the rhizobia bacteria affects the C allocation pattern in a tripartite system with AMF. I evaluated C allocation to the symbionts under in a tripartite interaction with various nutrient access scenarios including the use of a rhizobial strain that lacks biological nitrogen fixation (BNF) capability and AMF having access to a labeled N source. The dual inoculation of N fixing rhizobia (Fix+) and AMF results in a synergistic increase in shoot biomass, enhanced N and P uptake in the sink (roots) but low delivery toward the source (leaves). On the other hand, tripartite interactions of Fix- rhizobia that lack biological N fixation activity and AMF lead to a significant increase in N uptake and delivery towards the source but a significant drop in carbon allocation towards Fix- rhizobia root. Consistent with these findings, we found changes in SUCROSE UPTAKE TRANSPORTER (SUT) and SUGAR WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET) genes. These results provide substantial new information about how host plants control their carbon allocations under the different status of N demand in presence of rhizobia and AMF inoculation. During tripartite interactions, rhizobia bacteria are restricted to the host roots but extraradical mycelia (ERM) of AMF can go beyond, colonizing another host root. This leads to the development of common networks among two or more plants which are known as the common mycelial Network (CMN), creating a biological market for nutrient transport. The nitrogen-fixing capability of rhizobia bacteria can affect the transport of nitrogen (N) by AMF to host plants connected by CMNs. In the second experiment, I hypothesized that access of exogenous 15N to AMF would allocate more N to host plants colonized by Fix- rhizobia that lack BNF capability than those colonized by Fix+ rhizobia. We found that co-inoculation with Fix- rhizobia with AMF or non-mycorrhizal control plants resulted in elevated 15N enrichment in the shoot of the host plant. This suggests that AMF allocates most of the N they uptake from the soil to the host plant with a greater N demand due to the lack of access to fixed nitrogen. As expected, we found that AMF does not transfer as much N with host plants colonized by Fix+ rhizobia because their N demand can be fulfilled by the rhizobia bacteria. Plant diseases can be managed in various ways, including the use of disease-resistant and/or tolerant crop varieties, chemical controls, and biological controls. A diseaseresistant variety can lose its resistance due to the development of a new variant of the pathogen. Chemicals used in agriculture and other systems can have a very adverse effect on the environment. The use of Microbes for controlling plant diseases is safer and offers environmental sustainability compared to chemical pesticides. In my third experiment, I evaluated if AMF could mitigate the destructive effect of Soybean cyst nematode (SCN: Heterodera glycines), one of the most dreadful pests in soybean. Soybean plants infested with SCN do not show any aboveground symptoms in most of the cases, so the field gets unrecognized for a long time. Through the AMF symbiosis, plant hosts receive protection from pathogens as well among other benefits. In this experiment, we evaluated the effects of a commercially available AMF soil additive called MycoApply® (consists of an equal ratio of Glomus mossaea, Rhizophagus irregulare, G. etunicatum, G. aggregatum) under greenhouse and field conditions on the reproduction of SCN and the soybean growth and yield increase. We observed increased shoot weight for AMF-treated SCN susceptible variety (Williams-82) infested with SCN but no effect on the resistant variety, Jack (PI88788) in a greenhouse but no differences were found in SCN egg number. However, soybean seed yield was increased up to 40 % in mycorrhizal treated plots than nonmycorrhizal plots (they do have a natural community of AMF). Our results show that commercially available AMF inoculum can be used to increase soybean production even in the field infested with SCN. However, further investigation should be conducted to know the actual mechanism of how these fungi are able to increase soybean production without any change in AM colonization rate and reduction in SCN egg population in the soil. In summary, tripartite interactions of legumes with AM fungi and rhizobia bacteria led synergistically increase in plant growth independent of N fixing capability of rhizobia. However, delivery of N by AMF towards shoot increased when plants only have AMF for N source. Consistent with the biological market model, the host plant allocates a significant amount of C to benefit root symbionts. Similar trends were found when plants were interconnected via CMNs. On the other hand, AMF does not provide nutritional benefits but also can provide biotic stress tolerance such as enhanced SCN tolerance. All these indicated a bigger potential role for beneficial microbes in sustainable agriculture.

Number of Pages



South Dakota State University

Included in

Microbiology Commons



Rights Statement

In Copyright