Description
CHAPTER ONE
1.0 Background of study
Introduction
The rhizosphere can be defined as the soil region where processes mediated by
microorganisms are specifically influenced by the root system . This area includes
the soil connected to the plant roots and often extends a few millimeters off the
root surface, being an important environment for the plant and microorganism
interactions ( Lynch, 1990 ; Gray and Smith, 2005 ), because plant roots release a
wide range of compounds involved in attracting organisms which may be
beneficial, neutral or detrimental to plants (Lynch, 1990 ; Badri and Vivanco,
2009) The plant growth-promoting bacteria (or PGPB) belong to a beneficial and
heterogeneous group of microorganisms that can be found in the rhizosphere, on
the root surface or associated to it, and are capable of enhancing the growth of
plants and protecting them from disease and abiotic stresses ( Dimkpa et al .,
2009a ; Grover et al ., 2011 ; Glick, 2012).
The mechanisms by which PGPB stimulate plant growth involve the availability of nutrients originating from genetic processes, such as biological nitrogen fixation and phosphate solubilization, stress
alleviation through the modulation of ACC deaminase expression, and production
of phytohormones and siderophores, among severalInteractions between plants and
bacteria occur through symbiotic, endophytic or associative processes with distinct
degrees of proximity with the roots and surrounding soil (Figure 1A ).