Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)


Biology and Microbiology

First Advisor

Heike Bücking


Under natural conditions, legumes, such as alfalfa (Medicago) and soybean (Glycine max) are colonized with arbuscular mycorrhizal (AM) fungi and rhizobial bacteria forming tripartite interactions. Legumes are important crop species due to their high nutritional and economic values. Most of the previous literatures focused on experiments with an individual symbiont: either AM fungi or rhizobial bacteria, but not with both symbionts at the same time, thus our current understanding of resource exchange in tripartite interactions is limited. It has been reported that AM fungi primarily provide phosphate (P), nitrogen (N), and other nutritional and non-nutritional benefits while rhizobial bacteria solely supply N to their host plant. In return for the nutritional benefits conferred by root symbionts, the host plant reciprocally allocates a significant proportion of its photosynthetic carbon (C) resources to its root symbionts. In tripartite interactions, AM fungi and rhizobial bacteria facilitate synergistically for plant growth and nutrient acquisition. However, how the host plant allocates its C resources to both symbionts in tripartite interactions is still poorly studied. More attention has been paid to AM fungal benefits in terms of nutrient acquisition and growth response for soybean plants under the controlled conditions using laboratory produced AM fungal inoculum. Due to technical difficulties to produce in a large quantity of AM inoculum, it is not pragmatic to apply this lab-based AM fungi for agronomic purpose in a larger area in the field conditions. However, effects of commercially available AM fungal additives on soybean cultivars in the greenhouse and field conditions have not well reported before despite importance of AM fungi on soybean. To address these questions, we conducted different experiments in a pot system, split root system with and without fungal access to exogenous N in a hyphal compartment. Medicago truncatula was kept either non-inoculated as control (none), or with only AM fungi, or with only rhizobial bacteria, or with dual symbionts (both with AM fungi and rhizobial bacteria) with different nutrient supply conditions. To tract the C allocation to different symbiotic partners, we labelled/exposed the host shoot with 13CO2. Similarly, to test how does host plant change its strategy for C allocation to symbionts if AM fungus has an exogenous source of N, we provided 15NH4Cl in the hyphal compartment to which only AM fungus had access not to host root. Moreover, in association with C allocation to symbiotic root halves, we examined gene expression of several plant transporters of Sucrose Uptake Transporter (SUT) and Sugars Will Eventually be Exported Transporter (SWEET) family. We also analysed P and N acquisition of host tissues in association with plant growth response. We used four different soybean cultivars in separate experiments that usually use by farmers for the seed production in this region of Upper Midwest. These soybean cultivars were either non-inoculated control (none), or inoculated with only commercially available AM additives, or with only rhizobial bacteria, or with both AM fungi and rhizobial bacteria (dual inoculation). Soybean plant growth response in association with plant nutrient uptake, and seed yield was compared between control and AM plants of greenhouse and field condition experiments. Tripartite interactions favor the growth response in association with higher P and N uptake of the host plant in nutrient limited soil conditions. We found that the nutrient demand of the host, and the fungal access to nutrients are important factors that control the carbon allocation to individual root symbionts in tripartite interactions. Plant allocated more carbon to rhizobia under nitrogen demand, but more carbon to the fungal partner when exogenous nitrogen was available. The expression of genes for several SUTs and SWEETs transporters was consistent with the observed changes in carbon allocation. Exploring the full yield potential of legumes will require insights in how host plants regulate the substantial carbon costs of these interactions as host plant invest substantial amount of energy and resources to produce carbon during photosynthetic process. We observed soybean plant growth and seed yield was significantly higher with only AM inoculation than either control or only rhizobial alone inoculation. Moreover, the difference in seed yield of AM additives plants was notably higher in limited supply of P and N both in greenhouse and field conditions. Interestingly, seed yield of AM inoculated soybean was similar with or without fertilizer application in the field conditions. Different soybean cultivars had different response to AM fungal inocula for plant growth and seed yield. Among commercial AM fungal additives, MycoApply outperformed other two commercial inocula for plant growth and seed yield. Taken together, tripartite interactions of legumes with AM fungi and rhizobial bacteria facilitate for the plant growth and seed yield in limited soil nutrient conditions indicating tripartite interactions may have a bigger potential role to maintain sustainable agriculture.


Includes bibliographical references



Number of Pages



South Dakota State University


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