Molecular Ecotoxicology of Plants

Volume 170 of the series Ecological Studies pp 177-205

Increased Plant Fitness by Rhizobacteria

  • Leendert C. van Loon
  • , Bernard R. Glick

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About half of the recently fixed carbon in plant leaves is transported below ground where a substantial fraction is released by growing plant roots as exudates and lysates. These nutrients attract bacteria and fungi, which multiply in the rhizosphere to densities up to and exceeding 100 times those in the bulk soil (Lynch and Whipps 1991). Some of these microorganisms can reduce plant growth by acting as pathogens. However, other microorganisms can promote growth by alleviating growth-restricting conditions (Schippers et al. 1987; Glick et al. 1999). Plant growth-promoting rhizobacteria (PGPR) can affect plant growth and development in two different ways: indirectly or directly (Glick 1995; Glick et al. 1999). Indirect promotion of plant growth occurs when these bacteria decrease or prevent some of the deleterious effects of a pathogenic organism by any one or more of several different mechanisms. For example, production of antibiotics can interfere directly with growth and activity of deleterious soil microorganisms (Glick and Bashan 1997), whereas induction of resistance in the plant increases the plant’s defensive capacity (Van Loon et al. 1998). In addition, bacteria may reduce stresses resulting from the presence of toxic wastes by sequestering heavy metals or degrading organic pollutants.