Introduction
Actinomycetes are widely distributed in soil, compost, fodder, leaf litter, water and other environments. They are often considered as the prokaryotic equivalent of fungi. They are gram positive bacteria with DNA rich in guanine plus cytosine. They have an important role in recycling of complex organic material. They effectively compete with other microbes and survive under unfavorable environmental conditions.The emphasis attributed to the actinomycetes in biotechnological application is a result of the vast metabolic diversity of these organisms and their long association with environment and needs of humans. Biodegradation by these organisms plays an extremely useful role in waste removal. (Piret and Demain, 1988)Actinomycetes are well known as potential producers of biologically active secondary metabolites of economic significance to the pharmaceutical, chemical and agricultural industries. The need to bioprospect alternative sources of natural products from actinomycetes is stressed.This group encompasses genera covering a wide range of morphology extending from the coccus and rod-coccus cycle through fragmenting hyphal forms to genera with permanent and highly differentiated, branched mycelium (Barakate et al., 2002).Since soil is the best source of actinomycetes, most of the studies were concentrated on soil ecosystem. They are found abundantly in all soils, cultivated and uncultivated, fertile and infertile, in various regions throughout the world. (Barakate et al., 2002). Recently, the studies were conducted in aquatic habitats such as freshwater and marine situations.Several investigators demonstrated the occurrence and significance of actinomycetes in freshwater ecosystems. The genera of actinomycetes in these habitats are, Actinoplanes, Micromonospora, Nocardia, Rhodococcus, Streptomyces and Thermoactinomyces. As the frequency of novel bioactive compounds obtained from terrestrial actinomycetes decreases with time, actinomycetes from diverse environments have been increasingly screened for their ability to produce new secondary metabolites. (Cui et al., 2006)Actinomycetes were found to produce many bioactive compounds of high commercial value. So, emphasis is given in many research and industrial laboratories for the isolation of rare genera / species found in unusual environments. Many Actinomycetes viz., Actinoplanes, Coryneforms, Micromonospora, Micropolyspora and marine species have been isolated from sea water and marine habitats. Many investigators considered actinomycetes as part of indigenous micro flora in marine habitats, whereas others considered them as wash-in components and then survive in marine and littoral sediments as spores. The species from marine habitats showed salt tolerance. Many species like Streptomyces thermogriseus, from hot spring, Yunan, Streptomyces sp. 173, from sea sediments of east coast of China, Streptacidiphilus jiangxiensis sp nov. from acidic rhizosphere soil in China, are few examples to cite. (Barkate et al., 2002; Huang et al., 2004; Xiong and Kong 2004; Xu et al., 1998).It has been reported that many soil bacteria like Streptomyces produce siderophores (Neilands, 1981). Siderophores are defined as “Ferric iron specific binding compounds produced by bacteria and fungi growing under low iron stress.” (Lankford, 1973). The single term “Siderophores” (Sidero-iron, phores - bearer) has replaced all earlier designations like siderochromes, sideramines and sideromycins. They are low molecular weight compounds of 500-1000 Daltons. They are significant for solubilization, transport and storage of iron in micro-organisms. This term is strictly used for metal free ligands (Neilands, 1981). During the number of stages of microbial growth, iron is involved as inorganic micronutrient. Microorganisms allow iron into the cell in dissolved form. This high affinity system has two parts, the production of an iron binding molecule, which is excreted from the cell and the elaboration of a membrane receptor molecule, which binds to the iron complex and transport the metal to the cell’s interior. The membrane receptors and enzymes that synthesize iron binding molecules are coded by five genes in one operon, which is turned off when sufficient iron has been taken into the cell. (Lewin, 1984.)The importance of these siderophores extends to their immediate role in microbial physiology (Messenger and Rattledge, 1985). Siderophores and their substituted derivatives have a lot of applications. Siderophores have great potential in medicinal field and also in the field of agricultural biotechnology. Desferrioxamine–B, which is obtained from Streptomyces pilosus, is used to remove excess of iron in diseases like Hematocromatosis and anemia like Thalassaemia. (Winkelman, 1986). The Scandium and Indium siderophore complexes have antibacterial activity against some bacteria (Roger et al., 1982); Siderophore can act as vaccines when coupled with larger molecules (Chaudhary and Chincholkar, 1998). In agriculture, siderophores of Pseudomonas putida, Pseudobactin was found to increase growth and yield of various plants (Kloepper et al., 1980). Siderophoregenic Bradyrhizobium japonicum showed a marked increase in the percentage of germination, nodulation, chlorophyll, oil, protein content and number of pods (Khandelwal et al., 2002). A marine isolate of fluorescent Pseudomonas sp produced pyoverdine type of siderophore and exhibited potential biocontrol activity by showing good antifungal activity against plant deleterious fungi like A. niger, A. falvus, Fusarium oxysporum and Sclerotium rolfsii (Manwar et al., 2004). So, modern agricultural practices which require eco-friendly and sustainable technologies find siderophores as a potential agent in biocontrol area. (Chincholkar et al., 2000).Extensively studied siderophore of actinomycetes is Desferrioxamine (DFX) which is a low molecular weight compound and produced by species of Streptomyces, Nocardia and Micromonospora. (Gunter et al., 1993). Structurally different siderophores also are reported recently to be co- produced in some species like Coelichelin and Coelibactin in Streptomyces coelicolor or Enterobactin in Streptomyces tendae. A pathogenic actinomycete Actinomadura madurae produces Madurostatin (Harda et al., 2004) as well as some strains of Nocardia asteroids produces siderochelin whereas some also produces 2-3, dihydroxybenzoic acid (Fiestener and Beaman, 1987). Strains of Frankia, a symbiotic nitrogen fixer of Casuarinaceae when screened for siderophore production was found to produce catechol type of siderophores in iron deficient media. Enterobactin, another characteristic catechol type siderophore was produced by Streptomyces sp. (Arahou et al., 1998). Sideromycins are iron chelating antibiotics produced by Streptomyces (Mohandas, 2005). 1.1 Actinomycetes in Extreme Environment:The studies on occurrence, survival and activity of actinomycetes in certain extreme habitats are available. The acidophilic and aciduric streptomycetes are widespread in acidic soils. Mikami et al., (1982) isolated actinomycetes from soil with optimum growth at pH of 9.0-9.5. These isolates also could grow at pH 7.0 and hence they were designated as ‘alkali resistant actinomycetes’ or ‘alkali tolerant actinomycetes’. Actinomycetes have been reported from various extreme environments like thermal spring, marine sediments and crater lakes (soda lakes). Lonar Lake, a crater formed by meteoritic impact, offers a unique ecological niche. The lake water is salty and alkaline (pH 9.5 to 10.0) due to high content of sodium carbonate. The microbial ecosystem is reported by researchers was found to have Methanosarcina (Thakkar & Ranade, 2002), alkalliphilic bacteria of genera Bacillus, Staphylococcus, Micrococcus, Pseudomonas and Arthrobacter. Extremophilic actinomycetes are reported to produce novel secondary metabolites. Thus, interest in extremophilic actinomycetes may pave the new avenue for various purposes.1.2 Sustainable Agriculture:Harwood (1988) defined “Sustainable Agriculture” as an “agriculture that can evolve indefinitely toward greater human utility, greater efficiency of resource use and a balance with the environment that is favorable to humans and to most other species.” The definition of Sustainable Agriculture (SA) ranges from subsistence farming to dynamic systems capable of meeting the demands of the growing population. The objective of SA should be to maintain production at levels necessary to meet the increasing demand of an expanding world population without degrading the environment. (Vyas et al., 1998)Biotechnogical applications will certainly contribute to sustainable agriculture. Isolation and screening of microorganisms beneficial to crop agriculture is the basic need for use of microorganisms for SA. These microorganisms should be developed for production of metabolites which would be useful as plant growth promoters and/or useful as a biocontrol for phytopathogenic fungi. Integrated nutrient management and integrated pest management are two important aspects related with plant growth promotion and biocontrol to be considered for sustainable agriculture.Sustainable agriculture also encompasses biofertilizers and biopesticides and traditional biological inputs and their integration into modern agricultural technology (Sinha, 1998).The explosion of population in the twentieth century compelled for excessive agricultural outputs. To achieve this, in 1960, India launched a massive green revolution programme, which on one side increased the agriculture produces and on the other side tampered with soil fertility. The agricultural ecosystem was destroyed by the ambitious use of agrochemicals. Scientific community now wants a safer alternative for the chemically dominated agricultural practices. Shifting from environmentally destructive chemical farming practices to environment friendly natural and ecological farming was one of the chapters on sustainable agriculture in Agenda 21 at the Earth summit at Rio, Brazil in 1992.Soil health enhancement is of utmost importance in farming system. Healthy soil support several living organisms like Bacteria, Actinomycetes, Fungi and Algae, most of which are involved in various biogeochemical activities (Sinha, 1998). Soil in general and plant roots in particular, the immediate environment of roots and the rhizosphere is colonized by a variety of microorganisms. The plant provides the major source of carbon for maintenance of the microbial community in the rhizosphere and the quantity and nature of the root exudates determines the composition of the rhizosphere microflora. The plant is also dependent on the microbial community for nutrients like Nitrogen Phosphorous and Iron, the availability of which in turn is dependent on the ability of microorganisms for biotransformation. Different microbial communities interact with each other affecting plant growth. Microbial population and its activity differ from soils from nutrient sufficient soils to the nutrient insufficient soils. Production of siderophores is one of the mechanisms, along with other mechanisms like competition for nutrients, production of antibiotics or bacteriocins, secretion of lytic enzymes and production of secondary metabolites which permit some microbial population to be sustained, eliminated or favored.1.3 Siderophores for plant growth:Iron plays an important role in the crop development. Hydroxamate types of siderophores to the tune of 10-7 – 10-8 were found to be present in soils of United States. A siderophore, schizokinen was also detected in paddy crop. These observations clearly point out that the siderophores act as a plant growth promoter. It is further demonstrated that the plants have the ability to incorporate Fe3+ of siderophores into their biomass. So, it is confirmed that plants accept iron available through iron-siderophore chelates. Chincholkar et al., (2000) have reported that an iron inefficient variety of peanut plant when grown hydroponically with catechol type of siderophore, showed increased growth and chlorophyll content. It is indicated that siderophore solubilizes and supplies iron to the plant.There are several lines of evidence to suggest that beneficial Pseudomonas enhance plant growth by producing extra cellular siderophores which bind the iron available in the rhizosphere and making it less available for other microorganisms (Loper and Schroth, 1988). 1.4 Siderophores in Biological Control:It was suggested that siderophore producing microbes can effectively be used as biocontrol agents. The mechanism by which it is achieved is that, they apparently limit the amount of iron available to plant pathogens. There are reports that Colletotrichum gossypii is inhibited by rhizobacteria. Similarly, siderophore induced resistance was imparted by Pseudomonas aeruginosa to infection of bean leaf caused by Botrytis cinerea (Frietas and Pizzinatto, 1997; De-Meyer and Hofte, 1997). The role of fluorescent pseudomonad isolated from rhizoplane was reported in antagonism against several bacterial and fungal pathogens and also increase in crop production. This is mainly due the production of siderophores. (Chincholkar et al., 2000). Symbiotic nitrogen fixer, Rhizobium sp. produces catechol type of siderophore, it promotes the acquisition of iron and inhibited several fungal pathogens including Fasarium oxysporum, F. udum, F. solani and Ustulina zonata (Fekete et al., 1989)Actinomycetes are well known saprophytic bacteria. Other than effective decomposers of various polymers such as lignocelluloses, starch and chitin, they also have other roles also in the soil. It is evident that actinomycetes are qualitatively and quantitatively important in the rhizosphere, where they may influence plant growth and protect plant roots against invasion by root pathogenic fungi. Intensive research on actinomycetes in soil show that actinomycetes are a promising group of fungus antagonistic, root colonizing microbes. Streptomyces and other actinomycetes have been shown to protect different plants from soil borne fungal pathogens in field experiments. Yet, undefined actinomycetes may also play roles in the colonization and formation of mycorrhiza and certain actinomycetes appear to be hyper parasites of fungi, fungal oospores, or fungal sclerotia .Actinomycetes were also found to antagonize Pythium ultimum, which is a cause for damping off disease of lettuce seeds.
