Rhizosphere is the region of intense microbial activity, extending several millimeters from the root system of vascular plants. It is where the soil and root of plant make contact. Rhizosphere soil is the thin layer of soil adhering to the root system after shaking and removing the loose soil.
In 1904 the German agronomist and plant physiologist Lorenz Hiltner first coined the term "rhizosphere” [Greek word "rhiza", meaning root].
Hiltner described the rhizosphere as; “The area around a plant root that is inhabited by a unique population of microorganisms, influenced by the chemicals released from plant roots”.
Rhizosphere microflora is quantitatively and qualitatively different from the non-rhizosphere microflora. Also, rhizosphere microflora of one plant differs from that of another. Thus, rhizosphere is a unique subterranean habitat for microorganisms.
Structure of Rhizosphere
It has three zones which are defined based on their relative proximity to, and thus influence from, the root.
1. Endorhizosphere; in close proximity with the plant cortex and endodermis in which microbes can occupy the "free space" between cells
2. Rhizoplane; is the root surface
3. Ectorhizosphere; the outermost zone which extends from the rhizoplane into the adjacent soil.
The direct influence of plant roots on microbes and microbes on plant root within the rhizosphere is known as Rhizosphere effect. The growth of a soil microorganism is enhanced by the excretions and organic debris of roots within a rhizosphere. These bring about physical and chemical alteration of the soil.
Rhizosphere effect is expressed by R:S ratio, which is the ratio between number of microorganisms in the rhizosphere soil to the number of microorganisms in the non-rhizosphere soil. R: S ratio is different for different plants and changes with the stage of growth of a plant. The values are high for bacteria in rhizosphere region.
During seed germination and seedling growth, the developing plant interacts with microorganisms present in the surrounding soil. As seeds germinate and roots grow through the soil, the release of organic material lead to the development of active microbial populations in rhizosphere region (that includes plant root and surrounding soil in a few mm of thickness).
Root exudation is the release of organic compounds from living plant roots into the surrounding soil. Rates of exudation vary widely among plant species and environmental conditions. It has been estimated that 12-40% of the total amount of carbohydrates produced by photosynthesis is released into the soil surrounding roots.
Root exudates are mainly composed of water-soluble sugars, organic acids, and amino acids, but also contain hormones, vitamins, amino compounds, phenolics and sugar phosphate esters. The qualitative and quantitative compositions of root exudates are affected by various environmental factors including; pH, soil type, oxygen status, light intensity, soil temperature, nutrient availability and the presence of microorganisms. The rhizosphere, is a hot spot for numerous organisms and is considered as one of the most complex ecosystems on Earth. Microorganisms found in the soil include bacteria, fungi, nematodes, protozoa, algae, viruses etc.
Rhizosphere microflora
Rhizosphere organisms that have been well studied for their beneficial effects on plant growth and health are; the nitrogen-fixing bacteria, mycorrhizal fungi, plant growth-promoting rhizobacteria (PGPR), biocontrol microorganisms, myco parasitic fungi, and protozoa. Rhizosphere microorganisms that are deleterious to plant growth and health include the pathogenic fungi, bacteria, and nematodes. A third group of microorganisms that can be found in the rhizosphere are the human pathogens.
Plant-Microbe interactions in the rhizosphere
Microorganisms present in the rhizosphere play important roles in ecological fitness of their plant host.
Important microbial processes that are expected to occur in the rhizosphere include;
✓ pathogenesis
✓ plant protection/growth promotion
✓ production of antibiotics
✓ geochemical cycling of minerals
✓ plant colonization
Plant-microbe interactions may thus be considered beneficial, neutral, or harmful to the plant, depending on the specific microorganisms and plants involved and on the prevailing environmental conditions.
Pathogenic interactions
Roots exudates can attract beneficial organisms, but they can also be equally attractive to pathogenic populations. Many pathogenic organisms, bacteria as well as fungi, have coevolved with plants and show a high degree of host specificity. But even though plants are in permanent contact with potential pathogens such as fungi, bacteria or viruses, successful infection is rarely established. Such a general resistance against most pathogens has been named “horizontal resistance”. These resistance mechanisms include structural barriers and toxic compounds that are present in the unaffected, healthy plant. Phytoanticipins; is a toxin which resist the entry and colonization of pathogenic fungi in plants. In some instances, pathogens can overcome the pre-formed barriers and develop virulent infection processes leading to plant disease.
Beneficial microorganisms and modes of action
Plant-beneficial microbial interactions can be roughly divided into three categories.
First, those microorganisms that, in association with plants, are responsible for its nutrition (i.e., microorganisms that can increase the supply of mineral nutrients to the plant). In this case, most organisms may not directly interact with the plant, but their effects on soil biotic and abiotic parameters have an impact on plant growth. e.g. Microbes involved in biogeochemical cycles.
Second, there is a group of microorganisms that stimulate plant growth indirectly by preventing the growth or activity of pathogens. Such microorganisms are referred to as biocontrol agents, and they have been well documented.
A third group involves those microorganisms responsible for direct growth promotion. For example, by production of phytohormones.
1. Plant growth promoting Rhizobacteria- PGPR improve plant growth. PGPR strains have been used successfully for crop inoculations such as Bacillus, Pseudomonas, Rhizobium etc. PGPR promotes plant growth, development and protection by
- Biofertilization -directly, by helping to provide nutrient to the host plant, or helps in root growth and morphology or other beneficial symbiotic relationships- Rhizobium, Azospirillum etc,
- Phyto stimulation (plant growth promoting, usually by the production of phytohormones [auxins, cytokinins, and gibberellins)
- Biocontrol (controlling diseases, mainly by the production of antibiotics and antifungal metabolites, lytic enzymes etc). Eg., Bacillus thuringiensis (BT)
- Pathogen inhibition can also be by
- Antagonism, including; Antibiosis i.e. the inhibition of microbial growth by diffusible antibiotics, toxins, and biosurfactants,
- Competition for resources such as nutrients and oxygen occur generally in soil between soil-inhabiting organisms.
- Induced resistance -Plant-associated bacteria can reduce the activity of pathogenic microorganisms by helping the plant to better defend itself, a phenomenon termed “induced systemic resistance”, “ISR”.
Neutral interactions
Saprophytic microorganisms are responsible for decomposition of organic residues in soil and associated soil nutrient mineralization or turnover processes. Whereas these organisms do not appear to benefit or harm the plant directly (hence the term neutral), their presence is vital for soil dynamics, and their absence would influence plant health and productivity.
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