Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I recently participated in a workshop on soil health at the New Jersey Vegetable Growers Conference. Dr. James White of Rutgers University gave a fascinating talk on the importance of endophytes in plants.
I spent several years working on endophytes in corn and other crops with a small startup company in the 1980’s and was interested to see how the science had evolved.
Endophytes are microorganisms that live inside of plants but do not cause disease and often provide substantial benefits to the plant. There are both fungal and bacterial endophytes. Some familiar systems are the fungal endophytes of fescue that allow for better stress tolerance in turfgrass, mycorrhizal fungi that help extract nutrients such as phosphorus from soils, and the symbiotic Rhizobium bacterial associations that fix atmospheric nitrogen that plants then can use.
Dr. White and his students have investigated the process of rhizophagy. In this process endophytic microbes that live within the plant supply nutrients and elicit root responses. Studies have shown that as much as 70% of the nitrogen in plants might be acquired from bacterial endophytes in natural systems.
Dr. White noted that “many bacteria and fungi cycle between a free-living phase in the soil and a plant-dependent phase within cells of plant roots. Microbes obtain nutrients (nitrogen and other minerals) in soil, and then nutrients are extracted from microbes in the cells of plant roots”.
In the rhizophagy process, bacteria are absorbed by roots and live between plant cells and within plant cells. The plants then break down the bacteria and then use them as nutrient sources. There are many bacterial that live in the soil that can fix nitrogen freely. When these are then internalized and consumed by plant, this nitrogen is supplied to the plant.
This is a separate process from the symbiosis seen in legumes with Rhizobium bacterial and one that non-legumes can benefit from. The recently commercialized EnvitaTM inoculant uses Gluconacetobacter bacteria to provide nitrogen to non-legumes. Again this is interesting as I had been doing research on this bacteria over 20 years ago. It was commonly found in sugar cane fields that required little nitrogen fertilizer.
The rhizophagy process occurs initially in plant roots. Bacteria are common around root tips where new root cells are formed and feed on the exudates from plants. The plant then signals certain bacteria to enter the newly formed cells close to the root cap. Once inside the root, these bacteria lose their cell wall but they are still alive. The plant uses these stripped bacterial cell walls as food. Some of the bacteria stay between cell walls, others enter the plant cells themselves, or make their way up the plant to other growing parts. These endophytes can even enter fruits and seeds. The internalized bacteria can then be “eaten” as necessary by the plant to provide more nutrients.
Some of the internalized bacteria survive and move to newly forming root hairs where they release plant hormones that cause root hairs to grow. The bacterial then multiply and eventually are released by the root hairs back into the soil, forming clusters around the root, extracting nutrients from the soil, and repeating the process. Not only is nitrogen supplied to plants in this way but other nutrients such as phosphorus and the trace metals (iron, manganese, zinc) are also made more available.
There is no difference between plant mineral nutrients derived from rhizophagy and those derived directly from the soil water. However, this root association with bacteria has other benefits to the plant. Hormones released by the bacteria are growth promoting in the roots and other plant parts. Also, the endophytic bacteria produce nitric oxide to protect themselves inside the plant and this becomes a nitrogen source to the plant.
These root-associated endophytic bacteria also are protective against plant diseases by competition or by altering the microbial composition in the root environment.
According to Dr. White “people have speculated that plants can get nutrients from microbes, but mechanisms for transfer of nutrients from microbes to plants have been elusive – until now”. “Understanding how this process works may allow us to grow plants with reduced fertilizers”.
Some of these bacteria also will limit the growth of certain plants. Again from Dr. White: “we can manipulate the system to increase the growth of desirable plants and decrease the growth of undesirable plants (weeds), potentially using the same microbes.”
As we further understand endophytes there is potential for expanded use in crop production systems.