Symbiosis with an endobacterium increases the fitness of a mycorrhizal fungus, raising its bioenergetic potential

Abstract

Arbuscular mycorrhizal fungi (AMF) occur in the rhizosphere and in plant tissues as obligate symbionts, having key roles in plant evolution and nutrition. AMF possess endobacteria, and genome sequencing of the endobacterium Candidatus Glomeribacter gigasporarum revealed a reduced genome and a dependence on the fungal host. To understand the effect of bacteria on fungal fitness, we used next-generation sequencing to analyse the transcriptional profile of Gigaspora margarita in the presence and in the absence of its endobacterium. Genomic data on AMF are limited; therefore, we first generated a gene catalogue for G. margarita. Transcriptome analysis revealed that the endobacterium has a stronger effect on the pre-symbiotic phase of the fungus. Coupling transcriptomics with cell biology and physiological approaches, we demonstrate that the bacterium increases the fungal sporulation success, raises the fungal bioenergetic capacity, increasing ATP production, and eliciting mechanisms to detoxify reactive oxygen species. By using TAT peptide to translocate the bioluminescent calcium reporter aequorin, we demonstrated that the line with endobacteria had a lower basal intracellular calcium concentration than the cured line. Lastly, the bacteria seem to enhance the fungal responsiveness to strigolactones, the plant molecules that AMF perceive as branching factors. Although the endobacterium exacts a nutritional cost on the AMF, endobacterial symbiosis improves the fungal ecological fitness by priming mitochondrial metabolic pathways and giving the AMF more tools to face environmental stresses. Thus, we hypothesise that, as described for the human microbiota, endobacteria may increase AMF innate immunity.

Arbuscular Mycorrhizal Symbiosis Requires a Phosphate Transceptor in the Gigaspora margarita Fungal Symbiont

“…This attempt to understand the phosphate sensing and signaling mechanisms in G. margarita has revealed the presence of common and novel components of the canonical PHO and PKA pathways in this AM fungus. In this study, the comparison of PHO and PKA pathways in R. irregularis and G. margarita ( Tisserant et al, 2012, Lin et al, 2014and Salvioli et al, 2016 further demonstrate that most components are shared between the two species. Comparative transcriptomic analyses of G. margarita and R. irregularis suggest that the core components of the PHO and PKA pathways are highly conserved across glomeraceous AM fungi.…”

“…It has been demonstrated that transcriptomic sequence is predictive of the PHO and PKA signaling pathways in G. margarita ( Salvioli et al, 2016). In brief, the PHO pathway consists of the upstream signaling components (VIP1, IP7, Pho81, Pho80, Pho85, NUC-1, and Pho2) and the downstream transcriptional outputs (Pho5, Pho12, ALP-2, GigmPT1-5), and the PKA pathway is composed of the core signaling cascade (TPK1-3, Bcy1, Rim15, Msn4, and Rap1) and PKA targets (NTH1, TPS2, TSL1, STRE-driven genes, and ribosomal gene) ( Table 1).…”

“…Homology searches in the first draft of the transcriptome of the G. margarita BEG34 (Salvioli et al, 2016) permitted us to characterize the expression profiles of the potential genes in G. margarita regarding the phosphate sensing and signaling networks ( Supplemental Figure 13). We have identified orthologous proteins for each component of PHO and PKA pathways using TBLASTN searches and the R. irregularis transcriptome ( Tisserant et al, 2012) to determine the conservation of PHO, PKA, and other related pathways in G. margarita.…”

“…Table 1 describes 63 unique genes possibly involved in phosphate sensing and signaling networks, all of them being distributed into four pathways with distinct genes. These genes were functionally annotated according to Gene Ontology and G. margarita Transcriptome Database ( Salvioli et al, 2016). To create an expression profile of the effect of phosphate on the transcriptional changes of selected genes in G. margarita, we employed qRT-PCR analysis to evaluate the transcript levels of phosphate signaling-related genes or isoforms identified in the RNA-sequencing (RNA-seq) experiment ( Salvioli et al, 2016;Supplemental Figure 13).…”

“…These genes were functionally annotated according to Gene Ontology and G. margarita Transcriptome Database ( Salvioli et al, 2016). To create an expression profile of the effect of phosphate on the transcriptional changes of selected genes in G. margarita, we employed qRT-PCR analysis to evaluate the transcript levels of phosphate signaling-related genes or isoforms identified in the RNA-sequencing (RNA-seq) experiment ( Salvioli et al, 2016;Supplemental Figure 13). A list of differentially expressed genes related to PHO, PKA, MAPK, and TOR signaling are shown in Table 1 and Figure 7.…”