An interdomain network: the endobacterium of a mycorrhizal fungus promotes antioxidative responses in both fungal and plant hosts

Abstract

SummaryArbuscular mycorrhizal fungi (AMF) are obligate plant biotrophs that may contain endobacteria in their cytoplasm. Genome sequencing of Candidatus Glomeribacter gigasporarum revealed a reduced genome and dependence on the fungal host.RNA-seq analysis of the AMF Gigaspora margarita in the presence and absence of the endobacterium indicated that endobacteria have an important role in the fungal pre-symbiotic phase by enhancing fungal bioenergetic capacity. To improve the understanding of fungalendobacterial interactions, iTRAQ (isobaric tags for relative and absolute quantification) quantitative proteomics was used to identify differentially expressed proteins in G. margarita germinating spores with endobacteria (B+), without endobacteria in the cured line (BÀ) and after application of the synthetic strigolactone GR24.Proteomic, transcriptomic and biochemical data identified several fungal and bacterial proteins involved in interspecies interactions. Endobacteria influenced fungal growth, calcium signalling and metabolism. The greatest effects were on fungal primary metabolism and respiration, which was 50% higher in B+ than in BÀ. A shift towards pentose phosphate metabolism was detected in BÀ. Quantification of carbonylated proteins indicated that the BÀ line had higher oxidative stress levels, which were also observed in two host plants.This study shows that endobacteria generate a complex interdomain network that affects AMF and fungal-plant interactions.

Gigaspora margarita with and without its endobacterium shows adaptive responses to oxidative stress

“…Taking in account the results discussed above, we argued that CaGg may have a protective effect on G. margarita, making the fungal host more efficient in detoxifying endogenous ROS. Consistent with this, proteomic data confirmed that the B-spores produced less thioredoxin and other ROS-detoxifying proteins, and showed signs of oxidative damage (Vannini et al 2016).…”

“…This result may be explained by considering that the expression of these ROS scavengers is already higher in the B+ control fungus, when compared to the B-control (B+ control vs B-control comparison). A similar expression pattern has already been described for glutathione S-transferase and peroxiredoxin, which were shown to be specifically up-regulated at transcript and protein levels in the B+ line under normal growth conditions Vannini et al 2016). Thus, given that G. margarita B+ efficiently elicits the ROS scavengers, it potentially needs a lower activation of these genes when compared to the less-efficient B-line upon H2O2 treatment.…”

“…In yeast, the pentose phosphate pathway is essential for sulfur assimilation, being the main reservoir of NADPH used by the cell for sulfite to sulfide conversion (Slekar et al 1996). In G. margarita both the absence of CaGg Vannini et al 2016) and the H2O2 treatment seemed to induce the shift towards the pentose phosphate pathway. Taken in the whole, the GO enrichment analysis reveals that the H2O2 treatment pushes G. margarita to deeply change its transcriptome profile, shifting towards alternative metabolic pathways that are directly linked to mitigation of oxidative stress.…”

“…G. margarita catalase was also up-regulated upon treatment, but its expression level was not above the applied log2 fold change threshold. Other players of the ROS-scavenging system, such as thioredoxin reductase, have already been identified as markers of G. margarita endogenous oxidative stress response Vannini et al 2016). Taken as a whole, these observations indicate that G. margarita modulates the expression levels of many redox enzymes in a way comparable to other filamentous fungi, as well as the model AM fungus R. irregularis.…”

“…Several types of ROS react with methionine, forming methionine sulfoxide, which is reduced by methionine sulfoxide reductase (Luo and Levine 2009). We previously demonstrated that under normal growth conditions, the B-line contains a higher quantity of carbonylated proteins (Vannini et al 2016), which are considered a major hallmark of oxidative damage (Fedorova et al 2014) and specifically involve methionine oxidation. The gene coding for methionine sulfoxide reductase showed a significant up-regulation in our data set upon H2O2 treatment (comp18281_c1, log2 fold change:…”