Agriculture Reference
In-Depth Information
P. aeruginosa
on the basis of
16S rRNA
gene sequencing and subsequent molecular
phylogeny analysis. Phenotypic analyses as well as 16S rRNA and BOX-PCR-
based genotypic analyses revealed a high degree of diversity among PSB as
reported by Naik et al. (
2008
).
7.2.1 Rhizospheric Competence
Rhizospheric competence is a necessary prerequisite for plant growth-promoting
rhizobacteria (PGPR). It involves effective root colonization combined with the
ability of PGPR to survive and proliferate along the roots of growing plant in the
presence of indigenous microbiota over a period of time. Understanding the plant-
microbe communication, which is influenced by genetic and environmental factors,
can contribute significantly toward revealing the mechanistic basis of PGPR action
(Bais et al.
2004
). Among soil bacteria,
Bacillus
species are believed to be less
rhizosphere competent than
Pseudomonas
species. Eventually, most research even
today is focused at the development of biofertilizer and biocontrol agents based on
Pseudomonas
species (Parikh and Jha
2012
; Jha et al.
2012
). However, studies on
the genetic diversity of
Bacillus
inhabiting soil and wheat rhizosphere implied that
rhizosphere competence is a characteristic of the strain (genotype) not exclusive to
the genus or species. Experiments with different wheat varieties conducted by
Milus and Rothrock (
1993
) have revealed that seeds pelleted with selected strains
of
Bacillus
could successfully establish in the rhizosphere. But whether the colo-
nization attained by introduced strains was on the entire root or only on the top few
centimeters of root below the seed could not be confirmed.
7.2.2 Rhizospheric Effect and Host Specificity
Though previous studies have proved that plants opt for taxonomic functional
groups in the rhizosphere (Mittal and Johri
2007
), it is not certain whether plants
dynamically select beneficial soil microbial communities in their rhizosphere
through rhizodeposition. Although some field studies with mixed plant communi-
ties did not find such selections in the rhizosphere, there are reports that suggest a
strong correlation between plant and soil microbial communities (Duineveld
et al.
2001
). The root exudates are believed to be plant specific, and this specificity
may reflect the evolution or specific physiological adaptation to conditions of a
particular soil habitat. Composition of root exudates has been shown to vary with
plant species and stage of plant growth (Mittal and Johri
2007
). Concomitantly, the
plant is supposed to influence the composition of both indigenous and introduced
rhizobacteria. The exudates, for instance, sugar, amino acids, or organic acids, act
as chemoattractants and hence affect structure and functions of soil bacteria
(Somers et al.
2004
). Being a major driving force for microbial root colonization,
