The Invisible Herd: Rhizophagy and the New Agricultural Paradigm

The essential role soil microorganisms play in agriculture has been documented since at least the 1800s.

We’ve made relatively little progress in the last 200 years in our understanding and practical application of soil life to farming systems.

For the past century, agriculture has followed a chemical paradigm that sees plant nutrition as plant roots' interception and uptake of mineral ions.

This linear and simplistic approach to farming leads to the overapplication of soluble fertilizers at the expense of the linchpin of soil and plant health—the invisible herd of soil microorganisms.

Through photosynthesis, plants produce energy from sugar-rich exudates released into their root zone to attract and feed beneficial microorganisms.

In turn, bacteria and fungi retain nutrients from the soil, which are made plant available when consumed by nematodes and protozoa. Aside from some specialized symbionts like mycorrhizal fungi and Rhizobium, this is the primary mode of nutrient cycling.

In 2013, a group of Australian scientists discovered a plant/microbe interaction that broke all of these rules and completely blurred the line between plants and their bacteria and fungi counterparts. This association between plants and microbes was named rhizophagy.

A few years later, Dr James White of Rutgers University discovered that rhizophagy is more than a one-off interaction. It’s a recurring cycle with far-reaching repercussions for plant health and the future of agriculture.

Rhizophagy literally means “root eating.” Like other plant-microbe interactions in the soil, plants initiate the rhizophagy cycle by attracting microbes into the root zone by exuding sugars, amino acids, organic acids, and fats, the microbial equivalent of a 5-star buffet.

The microbes then colonize the root meristem, which is “internalized” into the root cells. In other words, the plant roots eat the microbes — specifically bacteria and some fungi.

Once inside the root, the microbes, known as endophytes, are shuttled throughout the plant and exposed to reactive oxygen called superoxide. This superoxide ruptures the microbe’s cell wall, allowing the plant to harvest nutrients from the remaining protoplast.

Some subsumed microbes survive the nutrient extraction process and begin to move throughout the plant. Some microbes are deposited into the leaves, seeds, and fruits. Others are cloned within the root, where they spur the rapid growth and elongation of root hairs.

As the root hairs elongate, microbes are ejected back into the soil, along with a trail of exudates to feed them. While in the soil, microbes acquire more minerals and nutrients and are then triggered to follow the exudate trail back to the plant root tips, and the cycle continues.

What makes this so groundbreaking?

Plants are estimated to acquire up to 30% of their nitrogen needs through the rhizophagy cycle. Given most nitrogen inputs' steep economic and ecological costs, this free nitrogen source is a boon to farmers.

Plants receive all other essential plant nutrients, including N. With the rhizophagy cycle, plants have a vast menu of nutrients to choose from as needed, which is far superior to the conventional practice of force-feeding plants with soluble fertilizers.

Through a unique mechanism, the endophytic bacteria in the rhizophagy cycle provide their host plant with heightened resistance to disease and pathogens. Essentially, it gives plants natural immunity for free and without the adverse side effects of chemical interventions.

The big caveat is that we've taken away the ability of plants and microbes to form rhizophagy symbiosis. Due to sterile plant breeding practices, overuse of chemical inputs, and other soil disturbances like frequent tillage, the link between plants and microbe symbionts is broken.

Caveats aside, this new knowledge of the rhizophagy cycle will catalyze more research and insight into the still unknown mysteries of plants and their microbe partners. After all, we’ve only named and described ~2% of soil microbes. Progress and innovation await.