Bacterial biofertilizers

Abstract

Many bacteria and fungi can enhance plant growth. The present review is limited to plant growth promoting rhizobacteria (PGPR). However, it includes endophytic bacteria that show plant growth enhancing activity as well. Also the best studied bacterial mechanisms of plant growth promotion are discussed, with a special emphasis on biological nitrogen fixation and synthesis of phytohormones, including less understood mechanisms like inhibition of plant ethylene synthesis, degradation of organic-P compounds, phenazine-related mineral solubilization, and synthesis of lumichrome. In addition, examples of PGPR genes that show activation in the interaction with plants, and beneficial events resulting from plant-bacterial interactions like stress relief and enhancement of other ecological associations are presented. Plant growth promoting activity and more precisely, crop yield enhancement are the final effects of the different mechanisms that PGPR possess and are the applicative goal of the agricultural microbiology research. Despite the undoubted economic and ecological benefits of utilizing some PGPR species as biofertilizers, the application of such a species must be very carefully assessed because of their importance as opportunistic pathogens in nosocomial infections and in patients with other diseases. On this basis, PGPR species must be selected for producing safe biofertilizers. Strain selection, as also the number of the bacterial cells, and characteristics of the bacterial cultures used in the production of biofertilizers, as well as, results of inoculation of different crops and cultivars with Azospirillum under field conditions are also included in the discussion. © 2006 Springer.