Symbiosis between an endobacterium and a mycorrhizal fungus has an impact on the trascriptional profile of the plant partner

Arbuscular mycorrhizal fungi (AMF) are crucial drivers of plant evolution: as obligate biotrophs, they have a key role in plant health. Some AMF possess endobacteria, whose genome sequencing revealed a reduced genome and a dependence on fungal host. We previously combined transcriptomics and cell biology to demonstrate that Candidatus Glomeribacter gigasporarum (CaGg) positively influences the physiology of its fungal host, Gigaspora margarita. We demonstrated how CaGg rises host's bioenergetic potential in terms of ATP production, enhancing its capability in detoxifying endogenous reactive oxygen species (Salvioli et al., 2016). Starting from these results we wondered whether such a positive impact of the bacterial presence on the fungal physiology may be transmitted to the host plant. With this aim, we performed an RNA-seq analysis of Lotus japonicus roots colonized by both the B+ (containing CaGg) and B- (without CaGg) G. margarita lines after 28 days. As expected, only a few genes (112) resulted to be differentially expressed: among them plant genes involved in hormones signaling, flavonoid biosynthesis and mineral exchange resulted to be sensitive to the presence or absence of the endobacterium inside the AMF. Interestingly, some genes involved in defense response to pathogens, such as PR10, were up-regulated in the B- colonized roots. The results suggest that the intracellular fungal microbiota may influence plant responses.