Microbial bioinoculants and their role in plant growth and development

The increase in the world's population, coupled with the limitations in the world's supply of natural resources and widespread degeneration of the environment presents a major challenge to agriculturalists. Chemical fertilizer is used to give the plant nutrient requirements within a short period to get faster results. Newly improved varieties of crops need high proportions of fertilizer. But chemical fertilizer has certain limitations and entails a lot of disadvantages. No doubt, the application of chemical fertilizer provides nutrition in high concentration in the soil and plants. When chemical fertilizer is applied, the entire contents would not be absorbed by the plants and the remaining parts would react in the soil. Part of it would be washed away and would contaminate water and some part of it would evaporate to the atmosphere; thereby the environment is polluted. Intense activity is involved in efforts to create plants that by themselves are able to fix nitrogen from the atmosphere – that is, convert nitrogen gas into nitrates that can be used by plant’s metabolic machinery. At present, only certain plants called legumes are naturally able to do so and even legumes require the aid of symbiotic bacteria. Such a development would drastically curtail the amount of fertilizer required by agricultural crops (Tilak, 2001). The main factor is the price, biotechnology could provide an alternative to technologies that have harmful effects on the environment and it would have the potential of enhancing production on a sustainable basis (Al- Garni, 2006). Microorganisms are useful for biomineralization of bound soil and make nutrients available to their host and/or its surroundings. Nitrogen and phosphorus are major plant nutrients which occupy a key place in balanced use of fertilizer. Phosphorus is an important requirement of legumes for their nitrogen fixation process (Huda et al., 2007). All tropical legumes fix the atmospheric nitrogen by Rhizobium which requires optimum level of phosphorus in plant tissue. Their seedlings establish better in presence of mineral solubilizers because more of the tropical soils are phosphate fixing and make it unavailable to the plants (Dabas and Kaushik, 1998; Sahgal et al., 2004; Tilak et al., 2005; Hameeda et al., 2008; Gupta et al., 2007). It is due to the phosphate solubilizing organisms those solubilize the bound form of phosphorus and AM fungi acts as up-taker of phosphorus and make it available to the host plants. Microorganisms facilitate plant mineral nutrition by changing the amounts, concentrations and properties of minerals available to plants. These changes lead to change in growth, development and chemical composition of plant that are common and substantial enough to encourage the exploitation of plant microbe interaction for improvement of crop productivity. Possible approaches include both the introduction of foreign microorganisms and capitalization on the indigenous microflora. There are various groups of organisms that can be solubilize and/or leaching of phosphate, iron and other mineral metals. Since the chemical fertilizers are becoming important ingredient of the agricultural farming and production, need based technology should be given priority. As the production and manufacture cost of the chemical fertilizer are very high, its availability and uses are also becoming imperative. Biomineralizing phenomenon of the microbes is very important in this regard. Plant microbe interaction is an important phenomenon and also useful in the development of most suitable bioinoculant which may be able to improve the plant productivity under adverse condition. A large number of literatures are available regarding the microbial interaction and beneficial uses in plants of agriculture, horticulture and forestry. Keeping this in view, some important information regarding the biofertilizing potential of some important group of microbes and their application for the development of sustainable technology has been reviewed here.