Plant Growth-Promoting Actions of Rhizobacteria

The rhizosphere as compared to bulk soil is rich in nutrients because of root exudation and deposits. As a consequence, the number of bacteria surrounding plant roots is 10-100 times higher than in bulk soil. These rhizobacteria, based on their effects on plants, can be largely divided into beneficial, deleterious, or neutral bacteria. The beneficial bacteria, also called plant growth-promoting rhizobacteria (PGPR), exert their beneficial effect through either direct or indirect mechanisms or both. In this chapter, the different mechanisms of plant-growth promotion and their impact are discussed. The mechanisms comprise the production of plant growth-promoting substances, nitrogen transformations, increasing bioavailability of phosphate and micronutrients, and biological control, as the best documented cases. In addition, the bacterial production of some molecules with recently described plant growth-promoting effects is discussed. To indicate the impact of PGPR, the applications and relevance of these bacteria in agricultural practices are highlighted. The importance of the plant genotype, inoculum density and technology, and co-inoculation practices, in terms of plant responsiveness are discussed. It is concluded that basic research should remain a priority in order to be able to develop performing and reliable bacterial inocula as a means to support sustainable agriculture.