The Importance of the Rhizosphere in Nature - Article Example

Rhizosphere Name: Institution: Rhizosphere The rhizosphere refers to soil layer below the earth surface where plant roots, diverse microbial populations and mycorrhizal fungi interact. The rhizosphere hosts a variety of beneficial microbial communities, which play a critical role in accelerating mineralization of nutrients from the soil organic materials (Gregory, 2008). As result, the rhizosphere has gained significant interest from ecologist and other scientists involved in the study of the interaction between plants and soil microbes. Rhizosphere studies are particularly crucial in attempts to re-establish biodiversity within the soil zone as one of the effective strategies for creating self-sustainable agricultural systems that require limited fertilizer inputs. Rhizosheath is an important component of the rhizosphere, which entails the soil particles attached to the plant root system. This component specifically refers to the soil particles bound together by root exudates and root hairs. As such, this paper explores the meaning of rhizosheath, its importance, factors influencing its formation and difference between the rhizosheath soil and the bulk soil. The paper will also identify the most important processes in the rhizosphere. Definition of Rhizosheath Rhizosheaths are recognized as structures within the rhizosphere comprising of persistent root hairs, mucilage secreted from the plant roots and soil particles forming a cylinder around the actual plant root (Gregory, 2008). Such structures form only around the distal root regions where there is significant higher water content compared to the proximal regions. This can be attributed to the relative immaturity of the xylem in the distal region, which account for the relative higher water content than the proximal regions. Ecological research has shown that some desert grasses and herbaceous perennials produce rhizosheaths of soil particles bound together and to the root surface by mucilages and other complex carbohydrates (Gregory, 2008). Rhizosheaths forming on root surfaces extend up to several times in diameter of the root secreting the mucilage that holds the soil particles together. During the formation of rhizosheaths, root soil particles are embedded on root hairs with soil solutes and microorganisms inundating the apoplast of the epidermal and cortical cells of the roots system. Importance of Rhizosheath Rhizosheath play a critical role in promoting water absorption by roots as well as provision of protection to the roots against desiccation. This is particularly important in some desert grasses, a phenomenon that enhance their survival amidst possible desiccation and limited water (Lambers, Chapin, Chapin (III) & Pons, 2008). The soil particles attached to the hair roots bring moisture close to the root to facilitate the water absorption process. Rhizosheath facilitate water uptake by the roots from, moist soil through elimination of root-soil gaps. The movement of water between root and soil is limited by incomplete root-soil contact (Witzany, 2010). Rhizosheaths confer to plants growing in the deserts enhanced water stress tolerance by ensuring that the little available moisture is held close to the roots for effective absorption. (Shahid, Abdelfattah & Taha, 2013) Plants growing in sandy soils particularly benefit from the formation of rhizosheaths as one of the adaptations plants growing in soils with poor water retention capacity. As such, some of the desert grasses have well developed rhizosheaths that enable them utilize the little available moisture held by the rhizosheaths. Other than enhancing water absorption through the roots system, rhizosheath offer an ideal habitat for a variety of soil microorganisms involved in important nutrient cycles and mutually beneficial interactions with the plants (Witzany, 2010). Notably, some microorganisms inhabiting rhizosheaths of different plants belong to species of free-living nitrogen fixing bacteria, which provide the plant with nitrogen. The micro-ecosystem formed by rhizosheath greatly influence plant nutrition and roots development as well as the quality of soil around plants that form rhizosheath. The formation of rhizoheaths contributes to improved uptake of nutrients from the soil especially in plants growing in areas with limited nutrients Yanai, Majdi & Park, 2003). Plants such as desert grasses are exposed to possible desiccation out to the poor water retention capacity of the sandy soils, a risk that is significantly reduced through the formation of rhizosheath (Witzany, 2010). Formation of rhizosheaths among plants growing in areas characterized by drought stress plays an important role in minimizing water loss from the roots with subsequent improvement in their survival chances (Whalley, Riseley, Leeds-Harrison, Bird, Leech & Adderley, 2005). Therefore, the formation of rhizosheath plays a critical role in enhancing waterholding capacity within the rhizosphere zone making it more water-stable zone favorable for the survival of plants growing on soils with poor water retention capacity. Despite the numerous benefits associated with formation of rhizosheaths, the embedment of soil particles on the root hairs significantly interfere with root development and architecture. The growth and development of root hairs and other parts of the root system is particularly limited as the mucilage continues to shrink leading to stronger embedment of soil particles on the root cells (Shahid, Abdelfattah & Taha, 2013). This limits the development of root cells into new root hairs that later develop into sub-branches of the main root thereby affecting the overall root architecture. Factors That Affect the Formation of the Rhizosheath The formation of rhizosheaths is influenced by several factors including environmental, soil factors and the development of root hairs. The development of root hairs offers contact between root and soil through the formation of rhizosheaths essential in the uptake of water and nutrition. The penetration of root hairs into the soil and the release of mucilage from the roots play a critical role in the formation of rhizosheath. The root architecture and the inundation of mucilage from the plant roots form essential components of the rhizosheath formation process (Haling, Simpson, Delhaize, Hocking & Richardson, 2010). Plants must be able to release adequate mucilage to allow sufficient binding of the soil particles around the root hairs leading to the formation of rhizosheaths. In this case, the type of plant and root architecture significantly influences the formation of rhizosheaths with only a few plant species including some desert grass species and some perennial herbaceous plants showing the formation of rhizosheaths. Moreover, formation of rhizosheaths is influenced by certain properties such as soil acidity, water retention, and salinity. Microorganisms involved in the synthesis of polymers of monosaccharides such as exo-polysaccharides, which play a critical role in the formation of rhizosheath, are affected by the levels of salinity (Delhaize, James, & Ryan, 2012). Studies have shown demonstrated that increased soil acidity interfere with root formation as well as the formation of rhizosheath with plants growing in highly acidic soil environments showing reduced formation of rhizosheaths (Haling, Simpson, Delhaize, Hocking & Richardson, 2010). However, that some plant species such as acid-soil sensitive and resistant genotypes show better development of root hairs and rhizosheaths in highly acidic soil environments. According to Haling et al. (2010), studies have shown that acid-soil sensitive genotypes such as cocksfoot show improved development of root hair and rhizosheaths in terms of root hair length and rhizosheath size per unit root hair length. Several differences exist between rhizosheath and bulk soil among them the distribution of viral abundance in which the rhizosheath comprises of a high population of microorganisms. Roots within the rhizosphere exude a range of organic molecules including organic acids, polysaccharides, and proteins, which plays a critical role in attracting a wide range of microbes as well as repelling other potentially harmful microbes. As a result, bacterial abundance within the rhizosheath and the generally the rhizosphere emerges to be significantly higher than in the bulk soil. Exudations from the root system in the rhizosheath also contribute to a lower virus-to-bacterium ration (VBR) compared to bulk soil in which the ratio is quite higher (Witzany, 2010). Such differences could be attributed to the chemical composition in the rhizosheath in which some microbes may be poorly adapted to such environments. Another significant difference revolves around moisture content with rhizosheath having a higher moisture content compared to the bulk soil (Whalley, Riseley, Leeds-Harrison, Bird, Leech & Adderley, 2005). This is particularly evident in soil types that have low to poor water or moisture retention capacity. In addition, the rhizosheath or generally the rhizosphere has highly depleted nutrient levels compared to the bulk soil (Yanai, Majdi & Byung, 2003). Conclusion The rhizosphere zone is of great significance to scientists interested in understanding ways in which soil composition can be manipulated to increase plant growth and yield. As such significant focus in the study of the rhizosphere should be on the topic of microbes around the rhizosheath owing to the important relationship such as symbiotic relations that influence certain nutrients uptake. Rhizosheaths are recognized as structures within the rhizosphere comprising of persistent root hairs, mucilage secreted from the plant roots and soil particles forming a cylinder around the actual plant root. Rhizosheath play a critical role in promoting water absorption by roots as well as provision of protection to the roots against desiccation. The formation of rhizosheaths is influenced by several factors including environmental, soil factors and the development of root hairs. Several differences exist between rhizosheath and bulk soil among them the distribution of viral abundance in which the rhizosheath comprises of a high population of microorganisms, nutrients, and moisture content. References Delhaize, E., James, A & Ryan, R. (2012). Aluminium tolerance of root hairs underlies genotypic differences in rhizosheath size of wheat (Triticum aestivum) grown on acid soil. New Phytologist, 195 (3), 609-619. Gregory, P. (2008). Plant roots: Growth, activity and interactions with the soil. Hoboken, NJ: John Wiley & Sons. Hailing, E., Richardson, E., Culvenor, A., Lambers, H & Simpson, J. (2010). Root morphology, root-hair development and rhizosheath formation on perennial grass seedlings is influenced by soil acidity. Plant & Soil, 335 (1/2), 457-468. Hailing, E et al. (2010). Effects of lime on root growth, morphology and the rhizosheath of cereal seedlings growing in acid soil. Plant & Soil, 327 (1/2), 199-212. Lambers, H., Chapin, F., Chapin (III), F & Pons, T. (2008). Plant physiological ecology. New York, NY: Springer. Shahid, S., Abdelfattah, M & Taha, F. (2013). Developments in soil salinity assessment and reclamation: Innovative thinking and use of marginal soil and water resources in irrigated agriculture. New York, NY: Springer. Whalley, R., Riseley, B., Leeds-Harrison, B., Bird, A., Leech, K & Adderley , P. (2005). Structural differences between bulk and rhizosphere soil. Journal of soil science, 56 (3), 353-360. Witzany, G. (2010). Biocommunication in soil microorganisms. New York, NY: Springer. Yanai, D., Majdi, H & Park, B. (2003). Measured and modeled differences in nutrient concentrations between rhizosphere and bulk soil in a Norway spruce stand. Plant & Soil, 257(1), 133-142. Young, I. (1995). Variation in moisture contents between bulk soil and the rhizosheath of wheat (Triticum aestivum L. cv. Wembley). New Phytologist, 130 (1), 135-139. Read More

Formation of rhizosheaths among plants growing in areas characterized by drought stress plays an important role in minimizing water loss from the roots with subsequent improvement in their survival chances (Whalley, Riseley, Leeds-Harrison, Bird, Leech & Adderley, 2005). Therefore, the formation of rhizosheath plays a critical role in enhancing waterholding capacity within the rhizosphere zone making it more water-stable zone favorable for the survival of plants growing on soils with poor water retention capacity.

Despite the numerous benefits associated with formation of rhizosheaths, the embedment of soil particles on the root hairs significantly interfere with root development and architecture. The growth and development of root hairs and other parts of the root system is particularly limited as the mucilage continues to shrink leading to stronger embedment of soil particles on the root cells (Shahid, Abdelfattah & Taha, 2013). This limits the development of root cells into new root hairs that later develop into sub-branches of the main root thereby affecting the overall root architecture.

Factors That Affect the Formation of the Rhizosheath The formation of rhizosheaths is influenced by several factors including environmental, soil factors and the development of root hairs. The development of root hairs offers contact between root and soil through the formation of rhizosheaths essential in the uptake of water and nutrition. The penetration of root hairs into the soil and the release of mucilage from the roots play a critical role in the formation of rhizosheath. The root architecture and the inundation of mucilage from the plant roots form essential components of the rhizosheath formation process (Haling, Simpson, Delhaize, Hocking & Richardson, 2010).

Plants must be able to release adequate mucilage to allow sufficient binding of the soil particles around the root hairs leading to the formation of rhizosheaths. In this case, the type of plant and root architecture significantly influences the formation of rhizosheaths with only a few plant species including some desert grass species and some perennial herbaceous plants showing the formation of rhizosheaths. Moreover, formation of rhizosheaths is influenced by certain properties such as soil acidity, water retention, and salinity.

Microorganisms involved in the synthesis of polymers of monosaccharides such as exo-polysaccharides, which play a critical role in the formation of rhizosheath, are affected by the levels of salinity (Delhaize, James, & Ryan, 2012). Studies have shown demonstrated that increased soil acidity interfere with root formation as well as the formation of rhizosheath with plants growing in highly acidic soil environments showing reduced formation of rhizosheaths (Haling, Simpson, Delhaize, Hocking & Richardson, 2010).

However, that some plant species such as acid-soil sensitive and resistant genotypes show better development of root hairs and rhizosheaths in highly acidic soil environments. According to Haling et al. (2010), studies have shown that acid-soil sensitive genotypes such as cocksfoot show improved development of root hair and rhizosheaths in terms of root hair length and rhizosheath size per unit root hair length. Several differences exist between rhizosheath and bulk soil among them the distribution of viral abundance in which the rhizosheath comprises of a high population of microorganisms.

Roots within the rhizosphere exude a range of organic molecules including organic acids, polysaccharides, and proteins, which plays a critical role in attracting a wide range of microbes as well as repelling other potentially harmful microbes. As a result, bacterial abundance within the rhizosheath and the generally the rhizosphere emerges to be significantly higher than in the bulk soil. Exudations from the root system in the rhizosheath also contribute to a lower virus-to-bacterium ration (VBR) compared to bulk soil in which the ratio is quite higher (Witzany, 2010).

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