The hidden symbiosis between blueberries and ericoid mycorrhizal fungus

Contrary to their highbush blueberry relatives, wild blueberries are not planted in fields like a conventional crop. Instead, farmers locate a field where they are pre-existing, usually in the boreal forest, or abandoned farmland. The vegetation is removed to allow the wild blueberry rhizomes to spread. Another particularity of this crop is its 2-year cycle. In the first year, blueberry shoots emerge from the rhizome and by the end of the growing season, they will have formed leaf and flower buds. After spending the winter under a thick layer of snow, the leaf and flower buds burst open during spring to be pollenated by bumblebees, and fruits are harvested at the end of the summer. This 2-year process leads farmers to manage their fields in a way to have both growth stages occurring in the same season, allowing for annual fruit production. Once the fruits are harvested, the shoots are pruned either mechanically or thermally. Pruning allows enhanced fruit production, which would otherwise drop gradually.

One can wonder how a crop derived from such harsh conditions can lead to successful agricultural businesses. Blueberries belong to the Ericaceae plant family alongside cranberries, rhododendrons and heather. This plant family is known to form a unique, specialised symbiosis with ericoid mycorrhizal fungi. Estimated to date back to 117 million years, this type of mycorrhizal symbiosis is the most recent to have evolved, compared with the two more common mycorrhizal symbioses: arbuscular mycorrhizae and ectomycorrhizae. The hyphae of the ericoid fungi involved penetrate the cell wall of the epidermal layer of the thin (50–100 µm) Ericaceae roots. Mycelial coils, which occupy most of the cell’s volume, are formed in order for the mutualistic exchange to take place.

Ericoid mycorrhizal fungi can be beneficial for their plant host in several ways. They contribute to heavy metal tolerance, and they also produce a variety of enzymes able to degrade organic matter. In that sense, a recent study has shown that a few known species of ericoid mycorrhizal fungi shared more genes with saprotrophic fungi than with other kinds of mycorrhizal fungi (ectomycorrhizal or orchid mycorrhizal fungi). This capacity to degrade organic compounds is a blessing for the Ericaceae host plants. Indeed, most of the nutrients present in the soil in which they grow are trapped in organic forms, which are not readily absorbable by plants. By decomposing these compounds, ericoid mycorrhizal fungi provide an essential source of nutrients to their Ericaceae hosts. In return, they receive photosynthates in the form of sugars. It is believed that without this mutualism, Ericaceae would be unable to thrive in such harsh conditions.

This symbiosis has been known for almost 50 years. The first ericoid mycorrhizal fungus was isolated in 1973 by Pearson and Read, who also proved the nature of the symbiosis. Nevertheless, it has been poorly studied compared with other forms of mycorrhizae. Furthermore, research on the wild blueberry root and the microbial community of the rhizosphere is only just beginning - whereas it has been studied for decades for other crops such as wheat, corn and pulses. Therefore, in 2020, we intended to fill this gap by studying the fungal and bacterial communities found in the root and rhizosphere environment of wild blueberries in Quebec. To do so, we have relied on DNA extraction and sequencing of both the 16S rRNA gene and the internal transcribed spacer (ITS) gene allowing us to investigate both the bacterial and fungal communities, respectively. Our first study, focusing on describing these communities in wild blueberry rhizospheric soil, has confirmed the prevalence of the Helotiales fungal order, which contains most of the known ericoid mycorrhizal fungi. As for bacteria, the Rhizobiales order, which contains several nitrogen fixers, was the most abundant. By looking at the correlation between the leaf nitrogen content and the abundance of both bacterial and fungal taxa, we have found a positive correlation between several ericoid mycorrhizal fungi and the presence of nitrogen-fixing bacteria. Though correlation does not imply causation, these taxa are potential candidates for an improved nutrient intake for wild blueberries. Further studies are needed to test this hypothesis but if the results are conclusive, bio-inoculants adapted to wild blueberries containing a mixture of these microorganisms could be engineered in the near future.