Deleterious Rhizobacteria

Deleterious rhizobacteria (DRB) are predominantly saprophytic bacteria that aggressively colonize plant seeds, roots and rhizospheres and readily metabolize organic substances released by plant tissues. Unlike typical phytopathogens, DRB do not invade and parasitize vascular tissues; DRB that inhabit plants endophytically are found intercellularly beneath epidermal cells and in intracellular spaces of root cortical cells without inducing disease symptoms. Because plant growth suppression by DRB is subtle, the origin, distribution and ecophysiology of DRB are reviewed. The development of growth suppression in plants by DRB depends on interaction of several factors including plant host, soil environmental factors, time, and, in agroecosystems, management. The interacting factors may determine the expression and severity of several mechanisms for growth suppression mediated by DRB. Plant growth suppressive mechanisms include production of phytotoxic substances, plant-growth regulators, or interactions with other microorganisms. As new techniques in biotechnology are applied in investigations of DRB and their metabolites, the new information will aid in management of DRB in various ecosystems and in development of DRB as potential beneficial bacteria.