Free-living nitrogen fixing bacteria were isolated from rhizosphere of seven different plant namely sesame, maize, wheat, soybean, lettuce, pepper and rice grown in Chungbuk Province, Korea. Five isolates with nitrogenase activity above 150nmol(-1) mg(-1) protein were identified based on, phenotypic and 16S rDNA sequences analysis. The strains were identified as Stenotrophomonas maltophilia (PM-1, PM-26), Bacillus fusiformis (PM-5, PM-24) and Pseudomonas fluorescens (PM-13), respectively. All the isolates produced indole-3-acetic acid (IAA), in the presence of tryptophan, ranging from 100.4 microg ml(-1) (PM-13) to 255 microg ml(-1) (PM-24). The isolate PM-24 (Bacillus fusiformis) exhibiting highest nitrogenase activity (3677.81 nmol h(-1) mg(-1) protein) and IAA production (255microg ml(-1)) has a promising potential for developing as a plant growth promoting rhizobacteria.

Though there is an abundant supply of nitrogen in the atmosphere, it cannot be used directly by the biological systems since it has to be combined with the element hydrogen before their incorporation. This process of nitrogen fixation (N 2 -fixation) may be accomplished either chemically or biologically. Between the two elements, biological nitrogen fixation (BNF) is a microbiological process that converts atmospheric di-nitrogen (N 2 ) into plant-usable form. In this review, the genetics and mechanism of nitrogen fixation including genes responsible for it, their types and role in BNF are discussed in detail. Nitrogen fixation in the different agricultural systems using different methods is discussed to understand the actual rather than the potential N 2 -fixation procedure. The mechanism by which the diazotrophic bacteria improve plant growth apart from nitrogen fixation such as inhibition of plant ethylene synthesis, improvement of nutrient uptake, stress tolerance enhancement, solubilization of inorganic phosphate and mineralization of organic phosphate is also discussed. Role of diazotrophic bacteria in the enhancement of nitrogen fixation is also dealt with suitable examples. This mini review attempts to address the importance of diazotrophic bacteria in nitrogen fixation and plant growth improvement.Key words: Biological nitrogen fixation (BNF), Diazotrophic bacteria, Plant growth promotion, N 2 fixing prokaryotes, nif genes Biological nitrogen fixation (BNF) can convert atmospheric di-nitrogen (N2) into plant-usable form, which improves plant growth and yield.

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Soil salinization refers to the buildup of salts in soil to a level toxic to plants. The major factors that contribute to soil salinity are the quality, the amount and the type of irrigation water used. The presented review discusses the different sources and causes of soil salinity. The effect of soil salinity on biological processes of plants is also discussed in detail. This is followed by a debate on the influence of salt on the nutrient uptake and growth of plants. Salinity decreases the soil osmotic potential and hinders water uptake by the plants. Soil salinity affects the plants K uptake, which plays a critical role in plant metabolism due to the high concentration of soluble sodium (Na + ) ions. Visual symptoms that appear in the plants as a result of salinity include stunted plant growth, marginal leaf necrosis and fruit distortions. Different strategies to ameliorate salt stress globally include breeding of salt tolerant cultivars, irrigation to leach excessive salt to improve soil physical and chemical properties. As part of an ecofriendly means to alleviate salt stress and an increasing considerable attention on this area, the review then focuses on the different plant growth promoting bacteria (PGPB) mediated mechanisms with a special emphasis on ACC deaminase producing bacteria. The various strategies adopted by PGPB to alleviate various stresses in plants include the production of different osmolytes, stress related phytohormones and production of molecules related to stress signaling such as bacterial 1-aminocyclopropane-1-carboxylate (ACC) derivatives. The use of PGPB with ACC deaminase producing trait could be effective in promoting plant growth in agricultural areas affected by different stresses including salt stress. Finally, the review ends with a discussion on the various PGPB activities and the potentiality of facultative halophilic/halotolerant PGPB in alleviating salt stress.

Penicillium oxalicum was isolated from acidic paddy rhizosphere soil, Milyang, Korea using modified media with AlPO 4 and bromo cresol green (BCG). Ability of fungus to solubilize mineral phosphates and its cause for solubilization were studied in vitro. The fungus was grown in liquid medium cultures containing AlPO 4 / FePO 4 /Ca 3 (PO 4 ) 2 (1000 mg P L )1 ). Concentration of organic acids, soluble phosphate and pH were determined periodically during 7 to 78 h incubation. Penicillium oxalicum demonstrated higher levels of Ca 3 (PO 4 ) 2 (129.10 mg L )1 ) and AlPO 4 (119.80 mg L )1 ) solubilization than that of FePO 4 (54.70 mg L )1 ) solubilization. Soluble phosphate concentrations in the culture medium were directly proportional to the organic acids and inversely related to pH. Malic acid production may have contributed the higher Al-P solubilization in culture media. Evidence from abiotic solubilization using sodium malate and HCl to solubilize P also indicated that malic acid may have been a main organic acid involved in the solubilization of AlPO 4 and Ca 3 (PO 4 ) 2 .