I was amazed as I learned about the soil food web and mycorrhizae fungi and all the benefits they provide plants and soil. We know that bacteria play a role providing those benefits and we are just learning that some of those benefits are provided while bacteria reside within the plant root system.
Endophytic (Endo-within, phytic-plant) bacteria are bacteria that live for a time within the plant without causing harm or any symptoms of plant response. Every plant has one or more microorganisms that live within the plant as endophytic organisms. There is a remarkable technology called confocal laser scanning microscopy and when combined with very specialized bacterial staining processes provides clear, detailed images of endophytes. Those images also reveal details about the chemical processes that allow them to invade and live within a plant, either between cells or within cells. Some of those endophytic bacteria that are part-time inhabitants of plant roots are called rhizophagy bacteria (rhizo-root, phagy-to eat) and make up the Rhizophagy Cycle.
Plant roots purposely leak exudates composed of carbohydrates (carbon sugar), amino acids, vitamins, and organic acids to attract and feed bacteria and fungi in the rhizosphere. At the expanding root tip, special exudates invite select bacteria to enter the plant root. Not necessarily a nice invite because the plant root reacts to the invading bacteria by producing a reactive oxidizing agent, a superoxide, designed to kill a bacterium. The rhizophagy bacteria respond by producing ethylene, an antioxidant that reduces the superoxide to an oxide that is not so lethal. This reduced oxide (hydrogen peroxide) dissolves the bacteria cell wall without killing the bacteria. It also reduces the possibility of the superoxide harming the root cell wall. Root cells of the plant use the nutrients from the bacteria cell wall, especially the nitrogen and iron. Bacteria cell walls contain siderophores (chelating agents) releasing iron and other metal ions in the presence of oxidizing agents.
Why would a bacterium possibly want to enter a root to have its cell wall stripped? The plant provides nutritious exudates to the bacteria while providing protection from grazing protozoa and nematodes. Loss of a cell wall makes reproduction much easier and the bacteria multiply in ideal growing conditions with plenty of food. And nothing makes a bacteria happier than producing large colonies of like microorganisms.
As bacteria populations rapidly expand, more ethylene gas is produced. This causes a root cell to elongate and form a root hair. As bacteria multiply, they grow into the root hair, more ethylene accumulates, and the root hair expels the wall-less bacteria out into the soil within the rhizosphere. In that exudate rich zone, the bacteria quickly rebuild their cell walls. Many are again attracted by the special exudate invitation, entering the root, and continuing the rhizophagy cycle. The trip takes a couple of days to complete.
The rhizophagy bacteria also produce argine, another antioxidant, which is converted to nitric oxide. The nitric oxide reacts with carbon dioxide. This forms nitrate providing the plant with usable nitrogen.
The rhizophagy bacteria also secrete other phytohormones (it is a wonder what you can do without a cell wall) that are involved with root system development (auxins, cytokinins, gibberellins). And they cause an induced resistance reaction in root cells. Root cells develop thicker cell walls (partly to protect against their own superoxide) making them more resistant to heat, cold, drought, and bad guys.
Urban soil is not native. It is an unnatural compacted, homogenized mix. As we push to grow native plants, they often struggle in an urban soil where they did not evolve to tolerate the poor growing conditions. We need to remember to include their necessary microorganisms with them as they try to grow in this foreign environment. The seeds from trees are inoculated with beneficial microorganisms. Natural born human infants are inoculated with beneficial bacteria from their mothers. We should pay attention.
While we were discovering mycorrhizae and the soil food web, rhizophagy bacteria were at work in a world too small for us to see clearly until we developed confocal laser scanning microscopy and incredible staining processes. We are just learning about rhizophagy bacteria; we are at the beginning of discovery.
Trees are amazing organisms.
David M Vaughan
Certified Arborist TX 0118
References
Lowenfels, J. 2022. Teaming with Bacteria: the organic gardener’s guide to endophytic bacteria and the rhizophagy cycle. Timber Press, Portland, OR. 177 pages.
White, J. F., Kingsley, K.L., Verma, S.K., and K. P. Kowalski. 2018. Rhizophagy cycle: an oxidative process in plants for nutrient extraction from symbiotic microbes. Microorganisms 6(3):95, doi: 10.3390/microorganisms6030095.
ASCA Webinar 10-25-2023, A Tree is Never Just a Tree, presentation by Lynne Boddy, Cardiff University, UK, 2023.
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