The rhizosphere is a narrow region in the soil surrounding the plants’ root, where biological and chemical properties of the soil are influenced by the root exudates. This region is rich with many microbes, which are collectively known as the root microbiome. Plant growth is strongly influenced by the rhizosphere. Recently, scientists have designed an optical tool that offers clear three-dimensional images of the rhizosphere. This innovative tool will help scientists to study plant-microbe interactions and biomolecular mechanisms.
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Role of Rhizosphere in Plant Development and Metabolite Production
Previous studies have shown that the activity of the plant root system influences the physicochemical conditions and microbial activity of the surrounding rhizosphere, and vice versa. These processes determine the nutrient availability and solubility of toxic elements for plants and microbes. Scientists have documented that the rhizosphere comprises a unique micro-ecosystem that drastically differs from the bulk soil, as it is not influenced by root exudates. Although root-soil interaction affects the pore structure within the rhizosphere, in a complex manner, the mechanism is poorly understood. Scientists have indicated that the presence of several factors is linked to the plant-microbe interactions, such as pH, acidity, and porosity.
It is important to understand the rhizosphere as it is associated with plant stress management, plant growth, and sustainable production. It is essential to observe root distribution under natural conditions as well as under the influence of various stresses. The real-time monitoring of root activity that influences the physicochemical changes in the rhizosphere and characterization of regulatory mechanisms of biochemical reactions will help develop strategies to improve plant growth and production.
Some of the structural imagining techniques used by scientists to study the rhizosphere are X-ray Computed Tomography, neutron Computed Tomography, and Magnetic Resonance Imaging. These techniques help scientists to study the microscopic heterogeneities of soil close to the root surfaces, soil compactness, soil-root contact, and hydraulic conductivity of the root.
Joel Kostka, professor at the Georgia Institute of Technology, stated that although many studies associated with the identification of microbes present in the soil had been conducted, very little research exists that reflects the contribution of these microbes or reveals their dynamics under real soil conditions.
The lack of information about the free-living bacteria exists owing to the absence of tools that enables scientists to visualize these microbes and image their interaction with plant roots. Kostka stated that the rhizosphere is the hotspot of microbes that influences plant growth and development. He further added that the rhizosphere is an extremely important zone as this is where the plant communicates with the external environment.
Development of a Novel Optical Tool to Study Rhizosphere
Kostka and his colleagues received a federal grant to develop tools that could provide clear images of the rhizosphere so that scientists could elucidate the chemical and biomolecular interactions between plant and soil. The research team envisioned creating a novel optical instrument that will produce 3-dimensional images of the metabolic processes bearing phytochemical or microbial specificity. In other words, this tool will potentially identify carbon sources, e.g., sugar, secreted by the plant root systems as well as identify nitrogen-rich compounds synthesized by the nitrogen-fixing microbes and offered to plant roots. These nitrogen-fixing microbes are also known as diazotrophs.
This tool should be able to produce real-time images of the microbes attached to the roots in the rhizosphere region and visualize the oxygen-carbon-nitrogen chemical exchanges that occur.
Scientists explained that the problems of soil visibility are typically associated with understanding how photons (particles of light) get scattered once they hit the soil surface. For instance, when a red light is focussed on a thumb, it glows even when one moves the thumb around. This means that some light particles come through, whilst most get scattered. Scientists pointed out that the unscattered light contains spatial information but as it is extremely weak it cannot be visualized by naked eyes. This spatial information remains unidentified. A similar phenomenon occurs with soil where spatial information is lost as it is not detected appropriately.
Researchers have planned to design an instrument that will focus light on the soil. This highly sensitive instrument will detect the weakest amount of light scattered when it reaches the target. Analysis of this light will help researchers to elucidate the biochemical processes that occur in the rhizosphere. This optical instrument is based on Raman scattering and optical coherence tomography (OCT).
Typically, Raman scattering and OCT are used in non-invasive imaging of thin biological materials such as the retina of the eye and the tiniest section of plant roots. The amount of light passing through a sample determines the refractive index of the material. The change in the light frequency also describes the chemical composition of a material. This real-time imaging will help scientists to focus on optimizing plant-microbe interactions to obtain sustainable products (e.g., biofuels, fertilizers, etc.) and improve plant growth.
Importance of the Optical Imaging Tool
Scientists believe that understanding metabolic processes that occur in the rhizosphere is extremely important as it could help develop a broad range of sustainable products such as biofuels and biofertilizers. This would also assist in developing improved agricultural practices for better crop management. This tool will enable scientists to use plants and soil as an effective carbon trap, i.e., separate greenhouse gases present in the atmosphere and trap them into the soil. This optical tool will allow scientists to follow the biochemical reactions under the soil in real-time. This would aid them in identifying the precise contribution of each microbe, present in the rhizosphere, on plant growth and production.
References and Further Reading
Getting to the root of plant-soil interactions: Optical instrument to give clearest 3D images yet of rhizosphere. (2021) [Online] Available at: https://www.eurekalert.org/news-releases/936888
Roose, T. et al. (2016) Challenges in imaging and predictive modeling of rhizosphere processes. Plant and Soil. 407, pp. 9–38. https://doi.org/10.1007/s11104-016-2872-7
Downie, H.F. et al. (2015) Challenges and opportunities for quantifying roots and rhizosphere interactions through imaging and image analysis. Plant, Cell and Environment. 38. pp. 1213–1232. https://doi.org/10.1111/pce.12448