Multiple mutualist effects on genomewide expression in the tripartite association between Medicago truncatula, nitrogen‐fixing bacteria and mycorrhizal fungi

Abstract

While all species interact with multiple mutualists, the fitness consequences and molecular mechanisms underlying these interactions remain largely unknown. We combined factorial ecological experiments with genomewide expression analyses to examine the phenotypic and transcriptomic responses of model legume Medicago truncatula to rhizobia and mycorrhizal fungi. We found synergistic effects of these mutualists on plant performance and examined unique features of plant gene expression responses to multiple mutualists. There were genomewide signatures of mutualists and multiple mutualists on expression, with partners often affecting unique sets of genes. Mycorrhizal fungi had stronger effects on plant expression than rhizobia, with 70% of differentially expressed genes affected by fungi. Fungal and bacterial mutualists had joint effects on 10% of differentially expressed genes, including unexpected, nonadditive effects on some genes with important functions such as nutrient metabolism. For a subset of genes, interacting with multiple mutualists even led to reversals in the direction of expression (shifts from up to downregulation) compared to interacting with single mutualists. Rhizobia also affected the expression of several mycorrhizal genes, including those involved in nutrient transfer to host plants, indicating that partner species can also impact each other's molecular phenotypes. Collectively, these data illustrate the diverse molecular mechanisms and transcriptional responses associated with the synergistic benefits of multiple mutualists.

Nutrient demand and fungal access to resources control the carbon allocation to the symbiotic partners in tripartite interactions of Medicago truncatula

“…Synergistic responses in tripartite interactions have also been described by other authors especially under low P and N supply conditions (Bournaud et al, ; Larimer et al, ). The dual inoculation with rhizobia and AM fungi can lead to higher photosynthetic rates and improves the harvest index (proportion of seed yields in relation to the total plant biomass) of legumes (Afkhami & Stinchcombe, ; Kaschuk, Kuyper, Leffelaar, Hungria, & Giller, ). In our study, the positive impact of tripartite interactions on plant growth was mainly the result of a higher BNF activity of the nodules and the improved plant N nutrition (Figures c and S2b,c).…”

“…The majority of legumes form tripartite interactions and are simultaneously colonized with N‐fixing bacteria and arbuscular mycorrhizal (AM) fungi. It is well known that these interactions can substantially contribute to the nutrient acquisition of legumes and increase the fitness of both the host and the different root symbionts (Afkhami & Stinchcombe, ; Mortimer, Pérez‐Fernández, & Valentine, ; Ossler, Zielinski, & Heath, ).…”

Transcriptomic analysis reveals the possible roles of sugar metabolism and export for positive mycorrhizal growth responses in soybean

“…Therefore, the transcriptome of mycorrhizal plants must be considerably reprogrammed during colonization by AM fungi, which results in distinct physiological responses for specific combinations of AM fungus and host plant (Hohnjec et al , Hogekamp et al , Gutjahr et al ). Transcriptomic studies allow for genome‐wide identification of hundreds of AM‐related genes (Hogekamp et al , Gutjahr et al , Afkhami and Stinchcombe ). However, expression profiling in mycorrhizal roots has mainly focused to date on interactions involving one or two cooperative AM fungi.…”

Cooperation and coexpression: How coexpression networks shift in response to multiple mutualists

“…While natural frequencies of interactions between M. truncatula and each of its microbial partners in nature are needed, experimental manipulations such as ours (that simulate the extremes of variable bacterial and fungal densities) in M. truncatula make an excellent system for understanding the molecular basis of multiple mutualist effects. Previous work in this system has documented genes that are differentially expressed in response to each of these mutualists individually as well as together (Afkhami & Stinchcombe, ). That differential expression study found that while expression of most genes was affected by one symbiont or the other, the expression of 623 genes was significantly altered by multiple mutualists.…”

“…Organisms are enmeshed not only in a matrix of negative associations, like competition and predation, but also positive interactions with diverse and ubiquitous mutualistic communities that provide crucial benefits. A growing number of empirical studies have shown that the effects of multiple mutualists on host fitness and traits can be nonadditive (Afkhami & Stinchcombe, ; Brittain, Williams, Kremen, & Klein, ; Gustafson & Casper, ; Lau & Galloway, ; Ossler, Zielinski, & Heath, ; Palmer et al., ). For example, Stachowicz and Whitlatch () detected synergistic effects of two gastropod species on the growth of their algal host; because each gastropod partner defended the alga against a distinct group of herbivores, both gastropods were required for algal growth.…”