ISOLATION AND SCREENING OF FREE-LIVING NITROGENFIXING BACTERIA, FROM DIFERENT SOIL SAMPLES, AS PLANT GROWTH PROMOTERS Iliyana VALCHEVA*, Donka DRAGANOVA, Jordanka KUZMANOVA, Mladen NAYDENOV Agricultural University - Plovdiv *E-mail: valchevailiana1@gmail.com Abstract Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonize plant roots and enhance plant growth by a wide variety of mechanisms. The use of PGPR is steadily increasing in agriculture and offers an attractive way to replace chemical fertilizers, pesticides, and supplements. Plant growth promoting bacteria can enhance and promote plant growth and development in different ways. These include production of phytochormones, solubilization of phosphorus, nitrogen fixation and biocontrolling effects on phytopathogenic microorganisms. In this study thirty rhizobacterial strains, isolated from different soil samples, were screened for L-tryptophan (L-TRP) – depending indole acetic acid (IAA) production, phosphate-solubilizing activity, production of biocontrol compounds, such as chitinolytic, cellulolytic and gelatinolytic enzymes. The rhizobacteria isolates producing IAA were tested for seed germination and root elongation ability. Results of the laboratory study showed that twenty-two rhizobacterial strains were able to produce indol acetic acid (IAA). Four isolates were able to solubilize insoluble phosphate as evident by production of clear zone on Pikovskaya’s agar. They were analyzed for inorganic P concentration colorimetrically by the molybdenum- blue method. Five isolates showed chitinolytic activity on solid medium by producing halo. Six strains demonstrated cellulolytic activity in the Congo Rod plate test. Only one of the tested rhizobacterial strains was able to produce gelatinolytic enzymes when using the Nutrient Gelatin stab method. All isolates were able to grow on N-free medium. Key words: PGPR, IAA, PSB, LYTIC ENZYMS Intoduction Increasing use of chemical fertilizers and highly productive cropping systems leads to health hazards and environmental pollution. In recent years, biofertilizers have emerged as an important component of the integrated nutrient supply system and hold a great promise to improve crop yields. Beneficial rhizobacteria that stimulate plant growth are usually referred as Plant Growth Promoting Rhizobacteria or PGPR. These bacteria can enhance and promote plant growth and development in different ways. These mechanisms include solubilization of phosphorus, nitrogen fixation, phytohormons production and biocontrolling effects on phytopathogenic microorganisms. Plant growth-promoting rhizobacteria are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Inoculation of crop plants with certain strains of PGPR at an early stage of development improves biomass production through direct effects on root and shoots growth. Microorganisms that colonize the rhizosphere can be classified according to their effects on plants and the way they interact with roots, some being pathogens whereas other trigger beneficial effects. In the last few years, the number of PGPR that have been identified has seen a great increase. This is mainly because the role of the rhizosphere as an ecosystem has gained importance in the functioning of the biosphere. Various species of bacteria like Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes,