Vessey 2003_Biofertilizers - Plant and Soil 255 571586 2003...

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Plant and Soil 255: 571–586, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 571 Plant growth promoting rhizobacteria as biofertilizers J. Kevin Vessey Department of Plant Science, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada Received 14 August 2002. Accepted in revised form 27 March 2003 Key words: Diazotrophs, endophytes, growth-promotion, phytohormones, rhizosphere, root morphology Abstract Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may grow in, on, or around plant tissues, stimulate plant growth by a plethora of mechanisms. These bacteria are collectively known as PGPR (plant growth promoting rhizobacteria). The search for PGPR and investigation of their modes of action are increasing at a rapid pace as efforts are made to exploit them commercially as biofertilizers. After an initial clari- fication of the term biofertilizers and the nature of associations between PGPR and plants (i.e., endophytic versus rhizospheric), this review focuses on the known, the putative, and the speculative modes-of-action of PGPR. These modes of action include fixing N 2 , increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant–microbe symbioses. The combination of these modes of actions in PGPR is also addressed, as well as the challenges facing the more widespread utilization of PGPR as biofertilizers. Introduction Plant growth promoting rhizobacteria (PGPR) repres- ent a wide variety of soil bacteria which, when grown in association with a host plant, result in stimulation of growth of their host. Biofertilizer is a recently coined term whose exact definition is still unclear, but which most commonly refers to the use of soil microorgan- isms to increase the availability and uptake of mineral nutrients for plants. The focus of this review is the mode of action of PGPR which act as biofertilizers, either directly by helping to provide nutrient to the host plant, or indirectly by positively influencing root growth and morphology or by aiding other beneficial symbiotic relationships. Not all PGPR are biofertil- izers. Many PGPR stimulate the growth of plants by helping to control pathogenic organism (for reviews of PGPR as biocontrol agents see, Whipps, 2001; Zehnder et al., 2001). Although bacteria were not proven to exist un- til von Leeuwenhoek in 1683 discovered microscopic ‘animals’ under the lens of his microscope, their utiliz- FAX No: +1-204-474-7528. E-mail: [email protected] ation to stimulate plant growth in agriculture has been exploited since ancient times. Theophrastus (372–287 BC) suggested the mixing of different soils as a means of ‘remedying defects and adding heart to the soil’ (Tisdale and Nelson, 1975). Such mixing of soils may have had various positive effects, but undoubtedly the introduction of beneficial microflora, particularly rhizobia for legume production, would have been one of the positive effects. In his poetic work, Georgics ,
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