The Rhizophagy Cycle

:: Tuesday, September 1, 2020
Third in the series Unlocking Plant Root Potential: A Conversation with Dr. James White
This month, our series continues with the third installment, as we focus on the core component of Dr. White’s work: The Rhizophagy Cycle

In 2010, a team of Australian microbiologists published research findings on the interactions between plants and microbes in the soil. They dubbed the process the Rhizophagy. Dr. James White and his team at Rutgers University have spent the last several years investigating this process. Research findings have led the team to conclude that the Rhizophagy process is cyclical and can impose effects on a plant’s ability to obtain nitrogen and other essential elements.

In essence, the Rhizophagy Cycle refers to a plant’s ability to farm the beneficial microbes they need. Plants exact this “farming” process through the attraction of beneficial microbes to their rhizosphere by producing the exudates they feed on. White’s team has found that the process of microbe establishment is methodical and perpetual when outside interference does not interrupt the cycle.

A 12-step cycle

The cyclical process follows a 12-step sequence:
  1. Microbes colonize the root tip’s meristem where plants secrete exudates such as carbohydrates and organic acids
  2. Microbes establish intracellularly within the plant root cells (between cell wall and the plasma membrane of root cells)
  3. Cell walls are oxidized off microbes using superoxide produced by root cells
  4. Constant movement of microbe protoplasts around the root cells (a process called cyclosis) and exposure to superoxide produced on root cell plasma membranes extracts nutrients from microbe protoplasts
  5. Microbe protoplasts are replicated within root cells so that many clones of internalized microbes are made
  6. Internalized microbe protoplasts stimulate root hair formation in the root epidermis
  7. Microbe protoplasts are ejected back into the soil from pores in tips of elongating root hairs
  8. Root hairs secrete exudates onto recently ejected microbes from tips of root hairs
  9. Microbe protoplasts reform cell walls once ejected back into the soil
  10. Microbes colonize the soil rhizosphere
  11. Microbes acquire additional nutrients they need to grow in the rhizosphere
  12. Microbes re-colonize root tip meristems (restart of cycle)
This cyclical sequence provides the highway by which plants obtain nutrients from the soil using soil microbes.

“Plants attract microbes to the meristem and internalizes them into the root cells. In this process, the plant exposes the microbes to reactive oxygen, called superoxide, to extract nutrients from them. Superoxide oxidizes cell walls off of microbes and causes the microbe protoplasts to leak nutrients that are absorbed into the root cells. The circular rotation of the microbes around the periphery of root hair cells facilitates flow of nutrients from microbes to root cells by breaking down gradients that would reduce nutrient flow from microbes to root cells.” Dr. White says, “Eventually, the microbes that survive the process and are cloned within root hairs will trigger root hairs to elongate, and through the elongation process, the microbes are ejected back into the rhizosphere along with a trail of exudates that rebuild microbe cell walls. The microbes then acquire needed nutrients in the soil and are eventually triggered to follow the exudate ‘food’ trail back to the tip of the root and the process begins again.”

Why do plants need the Rhizophagy Cycle?

Dr. White shares that the key to the efficacy of the entire process lies with the superoxide and its ability to extract nutrients from the microbes’ cells.

“Superoxide oxidizes the cells walls off of the bacteria so that they become naked protoplasts. They become spherical structures that are many different sizes,” he says. “They bud very quickly, and the plant moves them around, through a process called cyclosis, and replicates them so that by the time the root hair is developed, there are many, many clones to be ejected back into the soil.”

The exudate zone lies at the intersection of the root tip and the soil microbiome. The root tips secrete carbohydrates, amino acids, organic acids, and a host of other microbe food sources for, what White and his team believe to be, the sole purpose of cultivating and growing microbes within that zone for the plant to internalize.

To put the cycle into perspective, White says that a simple way to think of the activity frequency is: The more root tips a plant has, the more cycling of nutrients is occurring.

“If you think of a corn plant and what its roots look like—lots of little branch roots—those little root tips are where the microbes are cultivated and internalized,” he says, “Naturally, grasses are going through the Rhizophagy Cycle a lot.”

A second, but certainly not secondary, role of the Rhizophagy Cycle is its impact on a plant’s available nitrogen, with research concluding that plants with rhizosphere microbes receive and uptake, on average, 30% more nitrogen than those plants without microbes present in their rhizosphere. And while it is still not fully understood, White and his team believe that plants are forcing microbes to fix nitrogen. While it is clear that nitrogen is obtained in the Rhizophagy Cycle, other macro- and micro- nutrients are also absorbed by plant tissues due to the Rhizophagy Cycle.

To maintain the Rhizophagy Cycle, White says that nature must be allowed to work in perpetuity with very limited interruption, noting that a lack of microbes in the soil microbiome changes the entire plant-soil relationship.

“The Rhizophagy Cycle makes a difference in terms of plant health and soil relationship,” he says, “For example, without the presence of microbes, plant roots form very few root hairs, which we know is detrimental to the health of the plant.”

Dr. White recommends that to preserve a fully functional soil microbiome, non-disruptive agricultural practices such as reduced levels of synthetic fertilizers and no-till practices should be considered and implemented.
As we continue to explore the soil microbiome and its impact on plant performance, look for my fourth and final post of the series where I’ll share insights on the soil’s potential to cycle nutrients and boron’s critical role in plant health.

Diagrammatic representation of the rhizophagy cycle

Diagram of the Rhizophagy Cycle
A. Diagram of the rhizophagy cycle showing microbes entering root cells at the root tip meristem and exiting root cells at the tips of elongating root hairs. Rhizophagy cycle microbes alternate between an intracellular endophytic phase and a free-living soils phase; soil nutrients are acquired in the free-living soil phase and extracted oxidatively in the intracellular endophytic phase.
B. Shows bacteria (arrow) in the periplasmic space of parenchyma cell near root tip meristem of an Agave sp. seedling (bar = 20 µm; stained with diaminobenzidine tetrahydrochloride followed by aniline blue).
C. Bacteria (arrow) emerging from root hair tip of grass seedling (bar = 20 µm; stained with fluorescent nucleic stain SYTO 9).

As we continue to explore the soil microbiome and its impact on plant performance, look for my fourth and final post of the series where I’ll share insights on the soil’s potential to cycle nutrients and boron’s critical role in plant health.
Continue to part four of the series: Boron and Nutrient Cycling

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