Associative and Antagonistic activities in the Rhizosphere
In natural environments (eg. Soil, Air, Water etc.) a number of relationships exist between individual microbes, microbial species and between individual cells. The composition of microflora of any habitat (soil / rhizosphere) is governed by the biological equilibrium created by the associations and interactions of all individuals found in the community. In soil and rhizosphere region, many microorganisms live in close proximity and their interactions with each other may be associative or antagonistic.
A. Associative interactions / activities in rhizosphere: The dependence of one microorganism upon another for extra-cellular products (eg. amino acids & growth promoting substances) can be regarded as an associative activity / effect in rhizosphere. There is an increase in the exudation of amino acids, organic acids and monosaccharide by plant roots in the presence of microorganisms. Gibberellins and gibberellin- like substances are known to be produced by bacterial genera viz Azotobacter, Arthrobacter, Pseudomonas, and Agrobacterium which are commonly found in the rhizosphere. Microorganisms also influence root hair development, mucilage secretion and lateral root development. Fungi inhabiting the root surface facilitate the absorption of nutrient by the roots.
Mycorrhiza is one of the best known associative / symbiotic interactions which exist between the roots of higher plants and fungi. This mycorrhizal association has been found to improve plant growth through better uptake of phosphorus and zinc from soil, suppression of root pathogenic fungi and nematodes. Another example is association between the bacterium Rhizobium and roots of legumes and Azospirillum with cereal crops (wheat, rye, bajara, maize etc).
B. Antagonistic interactions / activities in rhizosphere: The biochemical qualities of root exudates and the presence of antagonistic microorganisms, plays important role in encouraging or inhibiting the soil borne plant pathogens in the rhizosphere region. Several mutualistic, communalistic, competitive and antagonistic interactions exist in the rhizosphere. The number and qualities of antagonistic microorganisms in the rhizosphere could be increased through artificial means such as fertilizer application, organic amendments, foliar spraying of chemicals etc.
Antagonistic microorganisms in the rhizosphere play an important role in controlling some of the soil borne plant pathogens. Stanier et al (1966) discovered the bacterial strain Pseudomonas fluorescens and the fluorescent pigments of this species in biological control of root pathogens. Strains of P. fluorescence are collectively called as "Fluorescent Pseudomonads". They produce variety of biologically active compounds such as plant growth substances, cyanides, antibiotics and iron chelating substances called "Siderophores" Rovira and Campbell (1975) , showed that bacterial strains of P fluorescens could lyse the hyphae of Gaumannomyces graminis var. Tritici, the causative agent of take-all disease of wheat. Fluorescent pseudomonads (P. fluorescens, P. putida) are known to produce iron chelating substances called Siderophores. These are low molecular weight, extra cellular, iron-binding agents produced by pseudomonads in response to low iron stress or when Fe3 is in short supply. Thus, iron stress triggers the formation of iron-binding ligands called siderophores. Siderophores contains the pigments Pyovirdin (Fluorescent) and Pyocyanin (non-Fluorescent) having iron chelating properties. Another pigment "Pseudobactin" is a fluorescent chelator of iron which is known to promote plant growth and inhibition of pathogenic bacteria in the rhizosphere. An antibiotic called "Pyrrolnitrin" reduces damping-off disease in cotton caused by Rhizoctonia solani. Several species of Bacillus are known to cause mycolysis in the rhizosphere. eg. Fusarium oxysporum hyphae are known to undergo lysis in soil due to these bacterial metabolites.
The successful antagonists among fungi are Trichoderma sp (T. viride and T. harzianum, T. hamatum) and Gliocladium virens which parasitize, lyse or kill the phytopathogenic fungi in the soil. Antifungal and antibacterial actinomycetes in the rhizosphere play an important role in controlling pathogenic fungi and bacteria, for example Micromonospora globosa is a potent antagonist of Fusarium udum causing wilt of pigeon pea. Amoebae are also known to play an antagonistic role in controlling soil fungi, eg. control of take-all disease of wheat caused by Gaumannomyces graminis through the use of Myxamoebae. There can also occur antagonisms between two fungi producing metabolite and interfering the growth of the other fungus as in case of Peniophora antagonizing Heterobasidium.