62 J. Jani et al.
Introduction
Biotic threats for plants, such as insects, soil-
borne pathogens, attack plants and cause impor-
tant damage to crop health. Ecological health
concerns confine the use of chemicals in soil
to control root diseases. The treatment of soil
or planting material with certain strains of
plant-beneficial, root-colonizing Pseudomonas
spp. is a promising alternative to control biotic
threats. Strains of Pseudomonas spp. are also
recognized for plant growth promotion activi-
ties (Kumar 1998 ; Kaur et al. 2007 ; Nashwa
et al. 2008 , Mahesh et al. 2010 ; Sharma et al.
2011 ).
Pseudomonas genus comprises more than
hundred species with different lineages, groups
and subgroups based on multilocus sequence
analysis. Many of the plant associated strains
belong to Pseudomonas fluorescens group,
which currently includes more than 50 named
species (Yamamoto et al. 2000 ; Mulet et al.
2010 ). Pseudomonas spp. can utilize a variety
of organic compounds (Frias et al. 1994 ; Ola-
lemi and Arotupin 2012 ) as energy sources, and
produce an array of secondary metabolites fore-
most as 2, 4-diacetylphloroglucinol (DAPG), li-
popeptides, phenazines, pyrrolnitrine, pyochelin
and hydrogen cyanide (Keel et al. 1992 ; Haas
and Defago 2005 ). Certain strains live in a com-
mensal relationship with plants, protecting them
from infection by pathogens that would other-
wise cause disease. Control of root diseases
by beneficial bacteria involves a blend of pos-
sible mechanisms that may complement each
other. Direct antagonism against the pathogen
by production of diffusible or volatile antibiotic
compounds or by inactivation of virulence traits
of the pathogen is considered to be a primary
mechanism of bio control (Diby et al. 2005 ;
Dikin et al. 2007 ). Another important mecha-
nism is the indirect inhibition of the pathogen
by bacterial stimulation of defence responses in
the plant host. As such, Pseudomonas spp. func-
tions as key components of ecological processes
that suppress plant diseases in agricultural and
natural environments.
Metabolites in Pseudomonas:
Biosynthetic Pathway and Genetic
Organization
The genes responsible for the synthesis of antibi-
otics in Pseudomonas spp. are highly conserved.
Phenazines are unusual nitrogen-containing het-
erocyclic molecules, of which over 60 different
derivatives have been identified in nature. Phen-
azines may play important roles in both symbiotic
and pathogenic microbe-microbe and microbe-
host interactions. Phenazines have been classified
as broad spectrum antibiotics and have been
shown to inhibit a wide variety of plant pathogen-
ic organisms (Poritsanos et al. 2006 ). Pyrrolnitrin
or 3-chloro-4- (2′-nitro-chloro-phenyl)-pyrrole
(Prn) is produced by a diverse number of Pseudo-
monas spp. such as Pseudomonas aureofaciens.
It inhibits growth of plant parasitic fungi and bac-
teria (Angayarkanni et al. 2005 ).
Pyrrolnitrin is synthesized from tryptophan
through a biochemical pathway determined pri-
marily by radio labeling studies using tryptophan
analogs. In recent years, it has been identified
that biosynthetic and regulatory loci are required
for the production of these antibiotics. Besides
pathway-specific regulators, a number of global
regulatory elements are involved in the control
of the biosyntheses of these compounds, among
them the sigma factors RpoD, RpoS and RpoN
and a two-component system composed of the
sensor kinase GacS and the response regulator
GacA (Bjornlund et al. 2009 ; Neidig et al. 2011 ;
Lalaouna et al. 2012 ). In addition, numerous bi-
otic and abiotic signals may influence production
of these antipathogenic compounds, including
different mineral and carbon sources and me-
tabolites released by microorganisms and plants.
Recently, in P. fluorescens, CHA0, which keeps
the antibiotics DAPG and PLT at a fine-tuned
balance can be affected by microbial and plant
phenolics. By the help of reporter system based
on autofluorescent green (GFP) and red (DsRed)
proteins to monitor changes in the balance of
DAPG and PLT expression in the rhizosphere of
healthy and pathogen-attacked plants.
Gene activation (GacS/GacA) signal-trans-
duction pathway is one of the proposed mod-