are predominant ( Podile and Kishore, 2006). Bacteria of diverse genera have been identified as PGPR, of which Bacillus and Pseudomonas spp. It is well established that only 1 to 2% of bacteria promote plant growth in the rhizosphere ( Antoun and Kloepper, 2001). Independent of the mechanisms of vegetal growth promotion, PGPRs colonize the rhizosphere, the rhizoplane (root surface), or the root itself (within radicular tissues) ( Gray and Smith, 2005). Beneficial free-living soil bacteria are usually referred to as plant growth-promoting rhizobacteria (PGPR, Kloepper et al., 1989). Plant-associated bacteria can be classified into beneficial, deleterious and neutral groups on the basis of their effects on plant growth ( Dobbelaere et al., 2003). The rhizosphere is populated by a diverse range of microorganisms and the bacteria colonizing this habitat are called rhizobacteria ( Schroth and Hancock, 1982). This situation is reflected by the number of bacteria that are found around the roots of plants, generally 10 to 100 times higher than that in the bulk soil ( Weller and Thomashow, 1994). This zone is rich in nutrients when compared with the bulk soil due to the accumulation of a variety of plant exudates, such as amino acids and sugars, providing a rich source of energy and nutrients for bacteria ( Gray and Smith, 2005). The rhizosphere is the narrow zone of soil specifically influenced by the root system ( Dobbelaere et al., 2003). Plant-Associated Bacteria - A General Introduction Resistance-inducing and antagonistic rhizobacteria might be useful in formulating new inoculants with combinations of different mechanisms of action, leading to a more efficient use for biocontrol strategies to improve cropping systems. Rhizobacteria belonging to the genera Pseudomonas and Bacillus are well known for their antagonistic effects and their ability to trigger ISR. Rhizobacteria induce resistance through the salicylic acid-dependent SAR pathway, or require jasmonic acid and ethylene perception from the plant for ISR. Both types of induced resistance render uninfected plant parts more resistant to pathogens in several plant species. Induced systemic resistance (ISR) in plants resembles pathogen-induced systemic acquired resistance (SAR) under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. Several substances produced by antagonistic rhizobacteria have been related to pathogen control and indirect promotion of growth in many plants, such as siderophores and antibiotics. Their effects can occur via local antagonism to soil-borne pathogens or by induction of systemic resistance against pathogens throughout the entire plant. PGPR are highly diverse and in this review we focus on rhizobacteria as biocontrol agents. Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR).
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