In this article we will discuss about:- 1. Origin of Rhizosphere 2. Reasons of Increased Microbial Activity in Rhizosphere 3. Rhizosphere Microorganisms 4. The Rhizosphere Effect 5. Rhizosphere Engineering.
Origin of Rhizosphere:
In 1904, L. Hiltner for the first time coined the term ‘rhizosphere’ to denote the area of intense microbiological activity that extends several millimeters from the root system of the growing plants. Microorganisms growing under the influence of roots are often qualitatively and quantitatively different from those inhabiting remote from this influence in the soil environment.
Therefore, the rhizosphere is a unique subterranean habitat for microorganisms. The rhizosphere microflora of one plant differ from the rhizosphere microflora of the other plant. Thus, rhizosphere microorganisms differ plant to plant both qualitatively and quantitatively.
The rhizosphere region can be divided into two zones:
i. The inner rhizosphere which is in a close vicinity of root surface, and
ii.The outer rhizosphere embracing the immediate adjacent soil (Fig. 30.4).
In 1949, F.E. Clark has suggested to use the term “rhizoplane” for root surface itself in studying the rhizosphere phenomenon. Balandreau and Knowles (1978) have termed the epidermis/cortex region, the endorhizosphere and the zone in the immediate vicinity to epidermis, the exorhizosphere to denote the intimacy of microbial associations.
Between the rhizosphere and soil there is an area of transition in which the root influence diminishes with distance. Therefore, it is generally accepted that the term rhizosphere soil refers to the thin layer adhering to a root after the loose soil and clumps have been removed by shaking. The soil coating varies in thickness according to root types, presence of moisture and condition of soil. This certainly influences the ‘rhizosphere effect’.
Reasons of Increased Microbial Activity in Rhizosphere:
The outer epidermal walls of living root hairs and all plant roots are covered with mucilage and cuticle (see Fig. 30.4). Organic and inorganic compounds accumulated in cytoplasm of root cells are diffused out. This loss occurs probably due to unfavourable conditions external to root. The phenomenon of loss of organic and inorganic compounds from root surface is known as root exudation.
In addition, the root-hairs are sloughed off during secondary thickening. All these root tissues and organic and inorganic compounds constitute a food base (for microorganisms) which are generally lacking in non-rhizosphere soil. The microorganisms colonize the rhizosphere to utilize them as food, and in turn release exudates from their own cells. Thus, they are regarded as selective sieves.
The rhizosphere region is a highly favourable habitat for the proliferation and metabolisms of numerous types of microorganisms. The microbial community of this zone can be examined by means of cultural, microscopic and manometric techniques. Dubey and Dwivedi (1988) screened the rhizosphere and non-rhizosphere microfungi of soybean both qualitatively and quantitatively.
Always, the number of rhizosphere microfungi was higher than the number of non-rhizosphere fungi (Table 30.2) The dominant fungi of rhizosphere were Aspergillus flavus, A. fumigatus, A. luchuensis, A. niger, A. terreus, Cladosporium cladosporioides, Curvularia lunata and Fusarium oxysporum, whereas the dominant fungi of rhizoplane were A. niger, Cladosporium herbarum, F. oxysporum, F. solani, Macrophomina phaseolina, Neocosmospora vasinfecta and Rhizoctonia solani. In addition, mycorrhizal fungi are also known to be present in rhizosphere soil and rhizoplane of roots.
Protozoa are relatively conspicuous particularly the small flagellates, large ciliates and amoeboidal forms. They are situated in the water films on the root hairs and on the epidermal tissue. Cysts of nematodes have also been reported in the rhizosphere region, for example Heterodera, Pectus, Tylenchus, Acrobeles, Helicotylenchus, Meloidogyne, etc.
Less is known about the algae except the blue-green algae present in the rhizosphere soil. This may be because of establishing symbiotic associations in certain plants such as coralloid roots of Cycas.
Bacteria reported from the rhizosphere and rhizoplane regions irrespective of their dominance are: Arthrobacter, Pseudomonas, Bacillus brevis, B. circulans, B. polymyxa, B. megaterium, Agrobacterium radiobacter, A. tumifaciens, Azotobacter, Flavobacterium, Rhizobium spp., Cellulomonas, Micrococcus, Mycobacterium, etc.
Actinomycetes are also important constituents of rhizosphere and rhizoplane microflora of different biosynthetic capabilities, antagonistic potentiality and taxonomic groups. Examples are Actinomyces chromogenes, Frankia (inside root tissues), Nocardia spp., Micromonospora spp., Streptomyces antibioticus, S. scabies, S. griseus etc.
The Rhizosphere Effect:
The rhizosphere is a zone of increased microbial community as well as microbial activities influenced by the root itself. However, this influence can be measured simply by plating technique and expressed as a rhizosphere effect (i.e. a stimulation that can be measured on quantitative basis by the use of rhizosphere: soil (R:S) ratio, obtained by dividing the number of microorganism in the rhizosphere soil by the number of microorganisms in the non-rhizosphere soil).
R:S ratio of soybean changing with different growth stages has been shown in Table 30.2. It is apparent that different types of microbes dominate at a particular growth stage.
A single microbial species will have to compete all the time to become a permanent inhabitant of the rhizosphere region, which is rather impractical. This microbial selection during different growth stages and changing scenario of microbial community in rhizosphere/rhizoplane are ultimately governed by root exudates of host as well as the result of microbial interactions.
As a rule, actinomycetes, protozoa and algae are not significantly benefited by roots, and the R:S ratio rarely exceeds 2:1 or 3:1.
The bacterial count in rhizosphere soil is the maximum (R:S values 10 to 20 or more), and varies with plant species, plant age and fertilization. However, there is no selective stimulation or inhibition.
Generally, bacteria of several distinctly different physiological, taxonomic and morphological groups are found to grow in rhizosphere region. However, on generic basis Pseudomonas, Flavobacterium, Alcaligens and Agrobacterium are especially common.
In contrast, total count of fungi is not altered by the root. Not total, but specific fungal genera are stimulated with plant species, plant age and soil types and environmental factors.
The study of rhizosphere is very important because several pathogenic microorganisms have to pass through the rhizosphere and infect root system. Moreover, manipulation of rhizosphere environment is of recent consideration as far as control of soil-borne plant pathogens is concerned.
Effect of Rhizosphere on Host Plants:
The rhizosphere microorganisms have either beneficial or harmful effects on the development of plant. The microorganisms are intimately associated on the rhizoplane, therefore, any toxic or beneficial substance produced by them has direct effect on plant.
Some of the possible effects are briefly described as below:
(a) The microorganisms catalyse the reactions in the rhizosphere and produce CO2 and form organic acids that in turn solubilize the inorganic nutrients of plants.
(b) Aerobic bacteria utilize O2, and produce CO2, therefore, lower O2 and increase CO2 tension that reduces root elongation, and nutrient and water uptake.
(c) Some of the rhizosphere microorganisms produce growth-stimulating substances and release elements in organic forms through the process of mineralization.
(d) Plant growth regulators such as indole acetic acid, gibberellins, cytokinins, etc. are known to be produced by the rhizosphere microflora.
(e) They influence phosphorus availability to plant through the process of mineralization and immobilization. However, when plant suffers from nutrient scarcity during summer in tropical areas the microorganism release the immobilized nutrients. Therefore, they act as sink between soil and plant roots in nutrient poor systems.
(f) Microorganisms in the rhizosphere zone change the availability or toxicity of sulphur to plants.
(g) The products of microbial metabolisms sometimes have toxic effect on plants; therefore, these are termed as phytotoxins.
Rhizosphere effect is established in the root region due to secretion of root exudates as compared to non-rhizospheric soil. Gene expression is altered through genetic engineering in plants; therefore, the root exudates and secretions are also changed which result in aggressive root colonisation by desirable microorganisms.
Tissue-specific gene expression has great potential for the genetic engineering of the rhizosphere. The use of border cells (i.e. cells released out from root) into the surrounding soil due to their metabolically active nature supports rhizosphere process.
It is now possible to manipulate the rhizosphere through changes in root exudation to alter the microbial flora to influence the microbial gene expression or to alter the chemistry of root region.
Most of the plant growth promoting rhizobacteria (PGPR) inhibit the deleterious plant pathogens by involving proteins, peptides, etc. Their gene manipulation may help in engineering proteins/peptides which ultimately diffuse out in the rhizosphere.
The role of opine-concept in plant- microbe interactions is now evidenced by the fact that these amino acid derivative or sugar derivative molecules are not catabolised by most of the soil microorganisms. Hence, the use of opine- catabolising bacteria has definitely play a role, thus establishing the opine-concept.
The modified form of this concept is to engineer plants to overproduce some common nutrients that do not allow to grow the undesirable microorganisms in the rhizosphere.
Plants might be engineered to release a small molecule inducer of bacterial gene expression into the rhizosphere. In response, an engineered microbial population would synthesise a desired organic molecule e.g. siderophores which indirectly play a role in biological control. Similarly, transgenic plants, release rhizopine (L-3-O-methyl-syllo-inosamine) into the rhizosphere which allow rhizopine catabolising bacteria to grow in their vicinity.
Plants can potentially remediate the soil contaminants, accumulate metals for recovery purposes and clean the contaminated soil. On the other hand, bacteria can contribute to remediation by catabolising organic molecules, mobilising soil bound metals and excreting siderophores. Proton used in decreasing soil pH and solubilising metal cations is also an advantage of rhizosphere engineering.