It has long been believed that plants get nutrients from the soil microbes but how the transfer of nutrients from microbes to the plant occurs has not been understood. That changed in recent years when a team of researchers from Rutgers University discovered one of the ways that plants harness microbes in the soil to obtain nutrients.
In a process the researchers have named the “rhizophagy cyle” with rhizophagy meaning root eating, soil microbes such as bacteria and fungi cycle between living in the soil and a plant-dependent phase within the cells of plant roots. While in the soil, the microbes obtain nutrients and these nutrients are extracted from the microbes when they are internalized by the plants.
This is an oversimplification but the rhizophagy cycle works like this: Plants cultivate or basically farm microbes around their root tips by secreting sugars and proteins. These microbes either enter or are drawn into the root cells at the tips. Once in the plant, the microbes are stripped of their cell walls which contain nitrogen and then become trapped in the plant cells. The plant then hits the microbes with a superoxide to break down moreof the microbe cell to further extract nutrients from them. The microbes that have been internalized by the plants spur the development of hairs on the roots and they are then expelled back into the soil through the growing hair tip. Once back in the soil, the microbes rebuild their cell walls and start acquiring nutrients from the soil, until they are attracted to the root tips by the exudates from the plant roots when the cycle starts over again.
Microbes that plants associate with in this cycle are referred to as endophytes. An endophyte is basically any microbe that gets into a plant that doesn’t cause disease and generally has positive effects.
The research at Rutgers on these microbial endophytes has found that in addition to providing a source of nutrients and improving nutrient absorption in plants, the rhizophagy symbiosis also provides other benefits to the plant.
One is that the association with microbial endophytes helps improve plant stress tolerance. Another is that rhizophagy cycle microbes help to modulate root development by triggering root hair development as was mentioned previously and also by increasing root and shoot elongation and root branching.
Interestingly, they also found that the microbial endophytes can help suppress pathogenic fungi in the soil. Once ejected from the root the rhizophagy microbes need to rebuild and acquire nutrients. To do so they often colonize soil fungal pathogens, draining them of nutrients and reducing their virulence.
So far over 50 plant species have been examined and show evidence that they engage in rhizophagy. As such, it appears that the rhizophagy cycle occurs in all plants and may be an important way plants acquire some nutrients along with other symbiotic relations that plants form such as with mycorrhizal fungi or rhizobium bacteria in the case of legumes.
Researchers have also discovered that endophytes found in the microbial colony of a particular plant are often carried with them on the seed to serve as an inoculant so to speak when the seed is planted. In trials where plants were denied access to the microbial endophytes by sterilizing seed and soil, plant growth and development was significantly reduced compared to plants that were allowed to retain the endophytes carried on the seed.
In a trial with perennial grass, researchers compared two seed lots, one which was produced under moisture stressed conditions and the other produced with normal to above average growing conditions. When the seed of these two lots was planted, they found more problems with stand establishment from the seed lot produced under drought conditions with higher levels of seedling mortality that they attributed at least in part to less endophytes being carried on the seed.
These microbial endophytes may also alter the chemical constituents of plants, which could improve the quality of the crops. For example, carrots produced in the presence of endophytes had more carotene than carrots produced when plants were denied access to the endophytes by sterilizing the seed.
According to James White, a professor in the Department of Plant Biology at Rutgers University-New Brunswick and one of the lead researchers on the rhizophagy cycle, the understanding of how this process works may allow us to grow plants without fertilizers or with minimal fertilizers inputs.
Based on this, tests that provide information on microbial biomass in the soil could provide a good benchmark for tracking soil health. Practices that build soil health will also help to build up microbial communities and should strengthen the rhizophagy cycle.
Keith Brown, Soil Health & Cropping Systems Specialist
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