Rhizophagy
From Triple Performance
The cycle of rhizophagy is a symbiotic process occurring at the level of plant roots, the rhizosphere. Soil microorganisms enter root cells to exchange nutrients.
Rhizophagy allows plants to obtain nutrients directly from microorganisms, such as nitrogen, and other minerals. Moreover, these microorganisms can also suppress plant pathogens and increase their tolerance to oxidative stress.
The rhizophagy cycle
- Colonization: Microorganisms initially develop on the root tip, at the meristem, where carbohydrates and other nutrients are secreted that attract them.
- Penetration: Microorganisms enter root cells at the apical meristem, the growth zone of roots, as protoplasts, meaning without a cell wall.
- Nutrient extraction: Inside root cells, plants produce reactive oxygen species (ROS), such as superoxide, which degrade some of the bacteria, thereby releasing the nutrients they contain into the plant.
- Release of microorganisms: Surviving bacteria stimulate the formation of root hairs and are expelled at the tip of these hairs, thus returning to the rhizosphere.
- Recovery of soil nutrients: Fed by exudates, microorganisms are released into the soil, where they replenish their nutrient reserves using various strategies such as producing siderophores (molecules with a strong affinity for iron), retention on their cell walls, direct absorption... The cycle then restarts.
Benefits
- Better growth and higher yield of crops: Microorganisms involved in this cycle can extract nutrients from complex organic sources that plants alone cannot assimilate.
- Additional supply of nitrogen: Rhizophagy increases the movement of nitrogen within plants. According to research[1], plants with rhizosphere microorganisms receive and absorb on average 30% more nitrogen than plants without microorganisms present in their rhizosphere.
- Better availability of boron: Boron deficiency is one of the most widespread micronutrient deficiencies, yet it is very important for root development. Microbial activity improves soil structure by increasing overall soil stability.
- Better accessibility to micronutrients (trace elements): Microorganisms absorb micronutrients, which are more difficult to obtain, such as boron, cobalt, copper, iron, manganese, magnesium, and zinc, using biogenic ligands called siderophores. Siderophores, due to their strong affinity for metals, notably bind to iron molecules.
- Crop protection: Beneficial microorganisms associated with the rhizophagy cycle can contribute to suppression of soilborne pathogens. Some microorganisms that become intracellular inside plant roots inhibit pathogenic fungi. Others, bacteria of the genus Pseudomonas, can produce antifungal compounds.
- Better soil health: The increase in microbial activity and organic matter in the soil contributes to overall soil health, favorable for good nutrient assimilation.
- More sustainable agriculture: By reducing fertilizer (better nutrient assimilation) and chemical pesticide needs, the rhizophagy cycle promotes more sustainable agricultural practices.
Promoting this process
The rhizophagy cycle is a natural process that occurs spontaneously under healthy soil conditions. To promote this cycle, one can:
- Preserve soil microbial life: Avoid excessive tillage (no-till farming), practice no-till seeding to maintain the integrity of the microbial community in the topsoil, avoid pesticide use that disrupts microbial communities.
- Promote soil organic matter: Adding organic matter to the soil encourages growth and activity of microorganisms, intensifying the rhizophagy cycle.
- Preserve indigenous seed microorganisms: Avoid using antimicrobials on seeds and removal of seed tissues (cleaning, dehulling), as they eliminate beneficial microorganisms. Seeds naturally carry microorganisms to aid their development.
- Reduce dependence on inorganic fertilizers: The application of inorganic fertilizers, such as nitrates, can inhibit natural processes like nitrogen fixation in the soil, thus reducing the benefits of nutrient acquisition via the rhizophagy cycle.
- Use biostimulants: These are substances whose function, applied to plants or the rhizosphere, is to stimulate natural processes by developing a favorable microbial life.
Detecting the presence of the cycle
To detect the presence of the rhizophagy cycle, one should look for signs of interaction between microorganisms and plant roots:
- Presence of microorganisms in and around roots.
- Increased formation of root hairs. Without microorganisms, there are no root hairs.
- Presence of reactive oxygen species (ROS) in root cells. Using diaminobenzidine (DAB) staining, a reaction used to visualize bacterial infections in seedling root cells, an organic compound that reacts with hydrogen peroxide to form a brown/red coloration.
Similarities with allelopathy
Rhizophagy helps plants obtain essential nutrients from microorganisms, while allelopathy helps plants in competition for resources (water, light, nutrients) and defense against pathogens and herbivores. Allelopathy is a phenomenon where a plant releases chemical substances into the environment that have inhibitory or stimulatory effects on other organisms (plants and microorganisms).
Rhizophagy and allelopathy are two distinct phenomena but with common points in the context of plant/microorganism interactions and plant survival and competitiveness strategies. They can also interact complexly in natural ecosystems, contributing to the dynamics of plant and microbial communities:
- Soil microorganisms: Both processes involve interactions with soil microorganisms. Rhizophagy depends on their capture and partial digestion, while allelopathy can influence soil microbial composition by inhibiting or stimulating certain microbial populations.
- Secretion of substances: Plants practicing rhizophagy may secrete enzymes and other chemicals into the soil to attract and manipulate microorganisms. Similarly, allelopathic plants release chemical compounds that can affect neighboring organisms, including microorganisms.
- Competition and survival: Both processes have strategies to improve plant competitiveness and survival. Through rhizophagy, plants obtain additional nutrients, while through allelopathy, they can reduce competition by inhibiting the growth of competing plants and modifying the soil microbial community.
References
Sources
- How Plants Extract Nutrients, James F White, 2019
- How Plants Harness Microbes to Get Nutrients, Rutgers, 2018
- Rhizophagy Cycle: An Oxidative Process in Plants for Nutrient Extraction from Symbiotic Microbes, Multidisciplinary Digital Publishing Institute, 2018
- Unlocking Plant Root Potential: A Conversation with Dr. James White, Borax, 2020
