Root-associated archaea: investigating the niche occupied by ammonia oxidising archaea within the wheat root microbiome

Root associated microbiomes (RAMs) are complex communities which provide benefits to host plants via disease suppression, abiotic stress relief and increased nutrient bioavailability. Most RAM studies have focussed on bacteria and fungi, archaea have largely been overlooked as many studies fail to utilize archaea-specific 16S primers. However, there are reports of archaea being detected and isolated from the rhizosphere and endosphere of crop species, and one report of a plant-growth promoting (PGP) ammonia oxidising archaeon (AOA). Here, we aimed to assess the role of AOA within the wheat (Triticum aestivum) RAM. We applied archaea-specific primers and 16S amplicon sequencing to profile the archaeal community associated with wheat roots grown in agricultural soil. To assess PGP capacity we treated wheat seeds with a concentrated inoculum of model AOA Candidatus Nitrosocosmicus franklandus C13. In contrast to prior reports this had no impact on plant biomass, indicating N. franklandus may be a passive member of the wheat RAM. Stable isotope probing (SIP) experiments have confirmed that bacterial species metabolise Arabidopsis thaliana root exudates. Fractions are being examined to assess whether archaeal species can do the same, and a similar SIP experiment will be performed in wheat. An enrichment culture experiment using root exudates will also be applied to identify and isolate archaea capable of metabolising wheat root exudates. Here we show that AOA are present within the wheat RAM; to understand the niche occupied by these microbes we must further probe how they interact with host metabolites, and whether they contribute to host fitness.