Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

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Physiological and Molecular Mechanisms of Plant Growth
Promoting Rhizobacteria (PGPR)
Beatriz Ramos Solano, Jorge Barriuso, and Francisco J. Gutiérrez Mañero
3.1
Introduction
Plant growth promoting rhizobacteria (PGPR) include bacteria that inhabit the
rhizosphere, improve plant health and may also enhance plant growth. The term
PGPR was coined by Kloepper and coworkers in 1980 [35], although PGPR was first
mentioned in 1978 by the same author in the Proceedings of the Fourth International
Congress of Bacterial Plant Pathogens, conducted in France. Since then, research
on PGPR has increased noticeably, with 11 reports appearing in the USDA (US
Department of Agriculture) between 1980 and 1990, 34 from 1990 to 1995 and 72
from 1995 to 2000.
Currently, the number of works per year on this topic has seen a 10-fold increase,
creating a new discipline that has changed the basic traditional concepts of plant
physiology and microbial ecology.
Bashan and Holguin [8] proposed a revision of the original definition of the term
PGPR, since there are a number of bacteria that may have a beneficial effect on the
plant even though they are outside the rhizosphere environment.
Bacteria identified to be PGPR could be members of several genera such as
Azotobacter, Acetobacter, Azospirillum, Burkholderia, Pseudomonas and Bacillus
[3,7,13,36,37,45,46,52]. The positive effect of PGPR occurs through various mechanisms.
This role involves not only the direct effect of a single bacterial strain but also
that of the molecular dialogue established among soil microorganisms and between
microorganisms and the plant, including quorum-sensing mechanisms.
3.2
PGPR Grouped According to Action Mechanisms
A thorough understanding of the PGPR action mechanisms is fundamental to
manipulating the rhizosphere in order to maximize the processes within the system
Plant-Bacteria Interactions. Strategies and Techniques to Promote Plant Growth
Edited by Iqbal Ahmad, John Pichtel, and Shamsul Hayat
Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31901-5
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