Rhizosphere

Overview

[9]

The rhizosphere is the portion of soil surrounding plant roots, and it is a hot spot for life. It is influenced by chemicals secreted by plants through their roots, called root exudation. Different plants secrete different chemicals and compounds, so the environment is very unique to the local vegetation. Exudates can even alter the pH of the rhizosphere. The uniqueness of the rhizosphere from place to place and plant to plant explain the different types of [microorganisms] that inhabit it.

[5]

Depending on the type of plant, the rhizosphere can extend 2-80 mm away from the roots. In the vicinity of roots, the soil is significantly wetter, and the high moisture levels protect plants from drying out and contribute to the dense population of microorganisms. Rhizodeposition is considered all the material lost from plant roots into the rhizosphere. This includes water soluble exudates, dead roots and root hairs, and gases, like carbon dioxide.

Root Exudation

Root Exudation [2]

Root exudation is the process of chemical excretion from the roots of plants as a means of interaction with the other organisms in soil. Amino acids, carbohydrates, sugars, and vitamins are all examples of exudates. In order for a plant to survive and thrive, it must have the ability to detect and perceive changes in the local environment. It is also one of the most important factors affecting microbial life and growth. Root to root and root to microbe commmunication are two types of interactions that occur in the rhizosphere due to root exudation.

• Root to Root interaction includes the growth and development of other plants nearby. The chemical messages sent out in root exudation are signals to prevent invading roots.
• Root to microbe interaction can be used in both positive and negative situations. Positive communication is used in order to attract Arbuscular Mycorrhizal Fungi colonization on the root as well as nitrogen fixing bacteria. To create nodulation of fungi on their roots, the plants secrete flavonoids that attract the organisms. Negative communication is used when plants need to defend themselves from parasitic microorganisms and pathogenic bacteria. In these cases, defense proteins are secreted and continuously attack pathogens.

Habitat

Mycorrhizal Fungi [3]

The rhizosphere supports a diverse and densely populated microbial community. There are different types of interactions that occur between the plants and microbes in the rhizosphere.

• Pathogenic microbes invade and kill the plant.
• Symbiotic interactions are beneficial to the plant and microbe.
• Harmful microbes reduce plant growth, but not intentionally like pathogenic ones.
• Saprophytic microbes live off of dead roots and plants.

Bacteria, protozoa, mites, earthworms, and many other organisms live within the rhizosphere. Arbuscular Mycorrhizal Fungi are one of the most important microorganisms within the rhizosphere. Even though plants produce their own food through photosynthesis, they have trouble obtaining and absorbing essential nutrients, like nitrogen and phosphorus. Arbuscular Mycorrhizal Fungi can easily obtain these nutrients, and since they live on plant roots, the plants can absorb them as well. The fungi get carbohydrates from the plants that they use for energy, so the relationship between them is symbiotic.

Methods

The rhizosphere is below the ground, in order to examine it special methods have been developed for use. One such method is the use of rhizotrons. A rhizotron is a laboratory constructed underground to study soil and its interactions with plants and animals. There is a central corridor with viewing windows into the soil profile which are usually shaded or covered to prevent light from entering the below surface soil which could influence the results of some studies. On the outside different experiments are conducted in the soil. Experiments of different soil types, and assemblages of plants and animals within the soil. Rhizotrons do not have to be housed in big facilities, in fact minirhizotrons are commonly used for small scale experiments.

Another common way of studying the rhizosphere is through sampling and the use of probes. Probes can be used with minimal disturbance to analyze the chemicals and other properties in the soil. Sampling can also be done, by either extracting the whole plant and do analysis or by removing part of the root system and soil around it for analysis. Sampling is more invasive than probing is, but can be more accurate.

References

[1] Bishnoi, Usha. “Plant Microbe Interactions.” Advances in Botanical Research, 2015, [[1]].

[2] Baetz, Ulrike, and Enrico Martinoia. “Root Exudates: The Hidden Part of Plant Defense.” Science Direct, Feb. 2014, [[2]].

[3] Chadwick, Douglas H. “Mycorrhizal Fungi: The Amazing Underground Secret to a Better Garden.” Mother Earth News, 2014, [[3]].

[4] Cheng, Weixin, and Alexander Gershenson. “Carbon Fluxes in the Rhizosphere.” The Rhizosphere, 2007, [[4]].

[5] “Crop Gene Discovery Gets to the Root of Food Security.” Phys.org, 4 Dec. 2017, [[5]].

[6] Lines-Kelly, Rebecca. “The Rhizosphere.” Soil Biology Basics, [[6]].

[7] Koo, B-J, and CD Barton. “Root Exudates and Microorganisms.” Encyclopedia of Soils in the Environment, 2005, [[7]].

[8] Pace, Matthew. “Hidden Partners: Mycorrhizal Fungi and Plants.” The New York Botanical Garden, [[8]].

[9] Schley, Lacy. “That Word You Heard: Rhizosphere.” Discover, 11 Feb. 2019, [[9]].

[10] Walker, Travis S., et al. “Root Exudation and Rhizosphere Biology.” Plant Physiology, [[10]].