Most Plants Depend On A Fungi-Bacteria Relationship We've Been Overlooking

The hyphae, which are smaller than a human hair, can be seen here among the roots of a grass plant. Image credit: Maria Harrison

Agricultural scientists may have overlooked a symbiotic relationship essential to the success of food production. Its discovery may bring potential to supercharge it to enhance crop growth, replacing artificial fertilizers that come with a host of undesirable consequences.

Botanists have been aware for well over a century of the role bacteria play in fixing nitrogen in the roots of some plant families, an essential step in making soils rich enough to support the abundance we see. The role of symbiotic fungi, which similarly occupy plant root systems, has been known for almost as long but is much less studied, despite actually being more widespread. Now, it has been revealed that when plants form a partnership with these fungi, they’re actually entering into a triad, with bacteria playing a hidden role.

Arbuscular mycorrhizal fungi (AMF) are found in the roots of 72 percent of land plants, with most others associated with other sorts of fungi. The fungi are better able to access nitrogen and phosphorus in the soil than plants, and so extract these vital nutrients and pass them to their hosts. As well as a place to live, the plants provide fatty acids in return.

Professor Maria Harrison of the Boyce Thompson Institute wondered where the AM fungi were getting these nutrients from, since they lack the enzymes to release them from complex organic molecules that represent their main soil reservoir.

Harrison and colleagues studied bacteria on the root-like filaments, known as hyphae, that AMF project into the soil to extend their reach. In The ISME Journal, they report the presence of bacteria on the hyphae of two fungi species differing from those found in the surrounding soil.

“Just like the human gut or plant roots, the hyphae of AM fungi have their own unique microbiomes," Harrison said in a statement. Harrison is seeking further confirmation but considers it likely the bacteria are helping the hyphae extract nutrients, particularly vital phosphates, from the soil.

Her theory is the bacteria break down complex organic molecules left in the soil by previous generations of plants and animals. This makes phosphorus – and possibly other nutrients – available to the fungi, using some themselves and passing the rest on to the plants.

These arbuscular mycorrhizal fungi form a crucial substrate for bacterial colonies. Image Credit: Maria Harrison

"If we're right, then enriching the soil for some of these bacteria could increase crop yields and, ultimately, reduce the need for conventional fertilizers along with their associated costs and environmental impacts," Harrison said

Many species of AMF exist, but Harrison and co-authors got similar results with two species, each planted in three types of soil – suggesting the microbiomes are widespread.

Harrison also thinks fungi recruit these useful bacteria from the soil by releasing molecules that act as attractants. It took just 14 days after hyphae formation for bacteria to establish communities on them, which continued to grow until the experiment stopped at 65 days.

Harrison also found evidence that families of predator bacteria that consume other microbes are particularly common around the hyphae, which the paper says “may serve as linear feeding lanes”. Although these predators pose a threat to their prey, they – like animal apex predators – apparently enhance the health of the system overall, releasing nutrients stored by bacteria further down the food chain. This makes the phosphates available to the fungi, and ultimately plants and humans.

Besides increasing the growth of the plants with which they collaborate, AMF increase carbon flow into the soil, sequestering carbon dioxide, at least temporarily.

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