Long-Term Persistence of Arbuscular Mycorrhizal Fungi in the Rhizosphere and Bulk Soils of Non-host Brassica napus and Their Networks of Co-occurring Microbes

Abstract

Arbuscular mycorrhizal fungi (AMF) are obligate plant symbionts that improve the nutrition and health of their host. Most, but not all the crops form a symbiosis with AMF. It is the case for canola (Brassica napus), an important crop in the Canadian Prairies that is known to not form this association. From 2008 to 2018, an experiment was replicated at three locations of the Canadian Prairies and it was used to assess the impact of canola on the community of AMF naturally occurring in three cropping systems, canola monoculture, or canola in two different rotation systems (2-years, canola-wheat and 3-years, barley-pea-canola). We sampled canola rhizosphere and bulk soils to: (i) determine diversity and community structure of AMF, we expected that canola will negatively impact AMF communities in function of its frequency in crop rotations and (ii) wanted to assess how these AMF communities interact with other fungi and bacteria. We detected 49 AMF amplicon sequence variants (ASVs) in canola rhizosphere and bulk soils, confirming the persistence of a diversified AMF community in canola-planted soil, even after 10 years of canola monoculture, which was unexpected considering that canola is among non-mycorrhizal plants. Network analysis revealed a broad range of potential interactions between canola-associated AMF and some fungal and bacterial taxa. We report for the first time that two AMF, Funneliformis mosseae and Rhizophagus iranicus, shared their bacterial cohort almost entirely in bulk soil. Our results suggest the existence of non-species-specific AMF-bacteria or AMF-fungi relationships that could benefit AMF in absence of host plants. The persistence of an AMF community in canola rhizosphere and bulk soils brings a new light on AMF ecology and leads to new perspectives for further studies about AMF and soil microbes interactions and AMF subsistence without mycotrophic host plants.

Crop rotation significantly influences the composition of soil, rhizosphere, and root microbiota in canola (Brassica napus L.)

“…In the case of canola, the stronger impact of crop rotation on the fungal community may be related to the fact that canola is a non-mycorrhizal crop [ 43 ] and thus with increasing canola frequency there will be a selective pressure on these important symbionts. Indeed, the relative abundance of Glomerales and Paraglomerales was lower in C-C soil, a finding consistent with previous research quantifying the effects of canola intensity on AMF diversity in soil [ 18 ]. The fungi that were consistently associated with monocropping are known pathogens of canola including Alternaria , Leptosphaeria and Phaeomycocentrospora .…”

“…Non-host crops decrease the AMF inoculum potential in the soil and mycorrhizal colonization of subsequent host crops (e.g. cereals) and may decrease translocation of fresh plant C to soil microbes [ 18 – 20 ]. Because canola is not an AMF host crop, it may have other means of acquiring P such as direct excretion of P solubilizing organic acids as reflected in the root exudates.…”

The mysterious non‐arbuscular mycorrhizal status of Brassicaceae species

“…We also conclude that microbiome structure is an important aspect of Brassicaceae for establishing new mutualisms that compensate for the nutritional benefits the family is bereft of due to its non‐AM status. An excellently structured study published recently further shows how important it is to study the Brassicaceae‐related microbiome in the context of the possibility of sustenance of AMF in the absence of their host plants (Floc'h et al, 2022). The study for the first time reported that certain bacteria and fungi in the bulk soils of the AM non‐host plants help in the sustenance of AMF in the absence of their hosts.…”

Does intercropping of legumes with oilseeds modify mycorrhizal colonisation?

“…Gimsing & Kirkegaard, 2009;Couëdel et al, 2019), that are toxic to fungi and are thought to contribute to the plant's non-mycorrhizal status(Floc'h et al, 2022). Indeed, mycorrhizal plants, such as maize and linseed, had reduced mycorrhizal colonisation and yield in years where they follow canola in crop rotation, compared with another mycorrhizal crop(McGonigle et al, 2011;Higo et al, 2017).…”