The soil is a unique environment in which complex interactions take place between plants and the soil organisms.It plays host to thousands of different microbial species.One gram of soil may contain 106-1010 microorganisms (Rossello-Mora and Amann,2001).These complex soil microbial populations play important roles in plant health and productivity (Berg,2009; Berendsen et al.,2012;Chaparro et al.,2012),with the interactions occurring mainly in the rhizosphere.The rhizosphere is the thin layer of soil directly influenced by plant roots and root exudates.It is regarded as an important and active zone for microbial colonization and activity (Hartmann et al.,2008).