By James Hoorman Hoorman Soil Health Services
(Source: Jeff Lowenfels, Teaming with Bacteria, 2022)
Do plant roots really eat bacteria? The answer is Yes (sort of!). In the last 5-10 years, our understanding of how plants acquire nutrients has changed dramatically. With new stronger microscopes; Australian scientist and Dr. James White, Rutgers university have discovered that plant roots are taking in endophytic (translation: “within the plant”) bacteria and acquiring nutrients from these microbes. One study estimates that 47% of the atmospheric nitrogen (N) and perhaps as much as 70% of the plants N might be acquired from bacteria absorbed and living between plant cells and within plant cells. This newly discovered processed is called rhizophagy.
We should not be too surprised. Farmers inoculate legumes (soybeans, peas) with Rhizobium bacteria which reside in plant nodules and fix N . Arbuscular Mycorrhizal Fungi (AMF) also enter roots and live between plant cells. AMF are like root extenders, bringing back water and soil nutrients in exchange for plant sugar. Endophytic bacteria do the same thing and all three (Rhizobia, AMF, bacteria) use similar processes to get into the plant. Important note: This is a symbiotic relationship (mutually beneficial to both parties) and the plant is the one that initiates and controls this process.
There are three plants zones for nutrient acquisition. First, the bulk soil away from the plant roots hold many nutrients. AMF can extend 6-18 inches away from a root to access water and nutrients a root can not reach. Second, the rhizosphere right around the plant roots send out root exudates to communicate or signal certain microbes what the plants needs nutritionally. Plants feed the microbes and the microbes make nutrients plant available. The third zone is now endophytic microbes that live within the plant supplying nutrients through rhizophagy.
The rhizophagy cycle is complex and very tightly managed by plant roots to avoid cellular root damage. Bacteria cluster together and live in dense biofilms around the root cap (root tip). Biofilms are full of nutrients (plant root exudates) and provides protection from predators. These bacteria are making nutrients plant available, but the plant wants more because it wants to grow (think about teenagers and how much they need to eat to grow quickly). The plant signals certain bacteria to enter the thin-walled root cell membranes (meristem cells) close to the root cap. The bacteria use arginine (a plant amino acid root exudate) to make ethylene to gain root entry. Once inside the root, these bacteria lose their cell wall (think about tumbling around in a washing machine) but they are still alive. Some stay between cell walls, others enter the plant cell themselves, or move to other parts of the plant (stems, leaves, flowers, seed).
The stripped off cell wall of endophytic bacteria supplies a banquet for the plant. Carbohydrates (sugars), amino acids, plant growth hormones, and macro and micro nutrients all are released to the hungry plant for faster growth and higher yields. The wall-less bacteria thrive in the protected plant (lots of food, no predators, little stress) and they multiply extremely fast (every 20 minutes). As the microbial population expands, it gets crowded, so they start pushing outward forming a new root hair. Roots hairs can only form from endophytic bacteria! These bacteria crowd the root hair in such large numbers, they are expelled in waves back into the soil. Once in the soil, these bacteria fatten up with nutrients, grow a new cell wall, and start looking for the next ride into the plant. The whole process takes about 2 days.
A few bacteria do not survive this wild ride (possibly as many as 10%). Inside a bacteria cell, the cytoplasm is full of nutrients and intact phytohormones that greatly stimulate plant growth, which in the long run, is good for both healthy plants and healthy microbial populations long-term. Since bacteria divide often and produce by division, they really do not have an individual lifespan unless they die or are consumed by another organism.
There is no difference between plant nutrients derived from rhizophagy and those derived directly from the soil. The big difference, rhizophagy supplies the plant with easy-to-get nutrients when it is really hungry and want to grow and produce seeds. The intact plant growth hormones stimulate fast growth and increased crop yields. Also, the endophytic bacteria produce nitric oxide to protect themselves inside the plant. This nitric oxide is converted to nitrates which the plant can use directly and be converted into plant N to make amino acids, proteins, and enzymes. This new information could help farmers and gardeners raise higher yielding, nutrient dense crops with much less commercial fertilizer in the near future.