Eating the Hand that Feeds You: Rhizophagy and Plant Nutrition

Those crazy scientists have done it again, throwing generally accepted theories of life science out the window. A group of Australian researchers have shown that plants are able to consume whole bacteria and yeast cells.

Prior to this, our understanding of the root-microbe relationship revolved around the idea that microbes provided nutrition to plants. Certain Bacteria can make nitrogen available, as well as solubilize phosphorus, potassium and micronutrients into forms that are plant-friendly.

Fungi perform a similar role, directly transporting nutrients and water into plants via the mycorrhizal networks. These mechanisms are pretty well understood and accepted as common.

What’s not so commonly known is that plants can eat whole microbes. Yes, plant roots are able to devour bacteria and yeasts. The term proposed for this newly discovered mode of nutrition is rhizophagy (rhye-zo-fay-jee).

During the process of rhizophagy, microbes are corralled into protrusions of cell walls arising from root tissue. The cell walls close off and trap the victims inside. The microbial cells are digested inside the plant cell, and the digested nutrients are transported through the plant and used for growth.

This mind-blowing process was discovered when scientists drenched tomato and mustard roots in a liquid solution that contained E. coli (a non-disease causing strain, of course), and the common yeast S. cerevisiae. The microbes were labeled with a fluorescent dye enabling scientists to easily see them as bright glowing dots. After the roots had been soaked for several hours, they were washed to remove any clinging microbes and then examined under a black light and microscope.

The radical scientists found that the microbes were incorporated into the roots cells. E.coli and S. cerevisiae glowed brightly inside root hairs, in the outer layer of epidermal cells and in the apoplastic space between the cells.

The microbes were not found past the border-controlling Casparian strip, so they were unable to flow freely into the plant’s vascular system. Another group of plants was soaked in a solution of fluorescent beads similar in size to E.coli. None of these beads were incorporated into the roots, indicating that the plants were not just taking up anything that resembled a microbe, but specifically going after the nutritious bacteria and yeast.

So how exactly does a plant eat a microbe? Looking into it with a high-powered electron microscope, scientists saw a structure begin to grow around the glowing E. coli cells. The microbes were being enveloped by what looked to be a cell wall growing out of the root.

By adding a cellulase activity indicator (tagged in detectable gold), it was seen that indeed the newly forming wall was chemically similar to the cellulose of a cell wall. To determine what triggered the growth of the cell wall, the researchers grew the plants in a fluorescent broth that glowed when cellulase (an enzyme that would allow a microbe to break into the cell) was used to degrade cellulose.

When E.coli and the fluorescent broth were applied to roots, the glowing markers indicated that the cellulase was breaking apart the cell walls, allowing the microbes to enter the cell. They were not able to determine if the cellulase was coming from the plants or from the bacteria, so keep your eye out for future studies.

To make sure intact microbes were not transported through the plant, scientists examined leaf tissue for the fluorescing microbes by growing out microbes found in and on the leaf. No glowing microbes were found. They also soaked plant roots with disease-causing salmonella and found no evidence of its presence in plant tissue either.

So, what were the plants doing with those yummy microbes? Well, they saw that microbes took anywhere from 10-14 days to be fully engulfed by the plant’s root cells. Then, by growing the plants in the presence of another type of identifiable E.coli (labeled with a radioactive isotope of nitrogen), it was shown that the nitrogen from the E. coli had been moved up the stem and incorporated into the leaf tissue. This proved that the plants were using the microbes for nutrition.

This study raises a lot of questions.

• Do plants use this mechanism for nutrition constantly, or just under certain conditions?
• Are plants selective of which microbes they consume?
• Can we feed them certain microbes to induce a certain response?
• Can we bulk up our microbes with micronutrients so they get passed along to the plant?
• Does the genetically modified DNA of those glowing microbes affect the plant?

Our understanding of plant-microbe dynamics is always evolving. Studies like these pave the way for future bio-fertilization systems. What will you be feeding your garden this year?

Maybe you will first be feeding your microbes, then letting the garden feed itself.