Focus on Environment

<strong>Focus</strong> Environ (2016)
Plant Growth Promoting Bacteria
some identified antibiotics produced by
PGPB. These antibiotics are formed through
metabolic pathways in the bacterial cell
through usage of available nutrients, biotic
and environmental stimuli such as minerals,
carbon source, pH, temperature and trace
elements (Compant et al., 2005). However,
some pathogens will develop resistance to
these antibiotics. Therefore, usage of multiple
bacteria that produce multiple antibiotics
which acts synergistically will show better
effects (Glick, 2012). In addition, formation
of allelochemicals by PGPB has the potential
to suppress pathogens activities (Saraf et
al., 2014). Besides that, the capability of
PGPB in producing chitinase, cellulase, β-
1,3 glucanase, protease and lipases will
breakdown the cell walls of pathogenic bacteria
and fungus (Hamid et al., 2013, El-
Katatny, 2010). In addition, the formation of
siderophores will prevent some pathogenic
bacteria from acquiring iron nutrient directly
from the soil. This somehow will affect
pathogenic bacterial proliferation and
growth (Gupta et al., 2014). On the other
hand, application of PGPB will increase the
content of beneficial bacteria in the soil.
Abundant of beneficial bacteria will rapidly
colonize plant roots before pathogenic bacteria
could actually invade into the plant root
system (Kundan et al., 2015; Glick, 2012).
Furthermore, PGPB are also capable to detoxify
pathogen virulence factor by producing
proteins that reversibly bind to the toxins
(Gaiero et al., 2013). Recently, it was reported
that PGPB suppress the virulence
genes by quenching pathogen quarom sensing
capacity by degrading autoinducer signals
(Compant et al., 2005). Therefore,
PGPB can be used as a biocontrol agent to
defeat the pathogens.
Induced systemic resistance in plants
Munusamy
Biopriming plants with PGPB will trigger
the induced systemic resistance (ISR)
through flagellation, siderophore, lipopolysacharides
and volatile organic compounds
formation (Compant et al., 2005). This type
of resistance is mainly demonstrated by rhizobacteria
and endophytes, and they will not
cause any visible symptoms of disease.
However, defence mechanism which is a
type of resistant mechanism triggered by
PGPB will regulate different sets of genes
such as peroxides, phenylalanine, ammonia
lyase, phytoalexins, polyphenol oxidase and
chalcone synthase (Choudhary and Johri,
2009). Through this mechanism, accumulation
of salicylic acid, jasmonate and ethylene
will increase the strength of plant cell wall
and alters host physiology and metabolic
responses leading to an enhanced synthesis
of plant defence against abiotic stress (Compant
et al., 2005). Besides that, since water
is one of the most limiting factors for plant
development in semi arid climates, xerotolerant
microorganisms can be used to increase
growth of plants in such climatic
condition (Petrovic et al., 2000). It is because
microorganisms that can survive under
drought conditions have several mechanisms
such as the production of exopolysacharides,
biofilm formation and osmolytes production
that help to avoid cell water loss and boost
the plant growth (Kavamura et al., 2013). In
addition, PGPB can offer plant protection
against desiccation through the maintenance
of moist environment and by supplying nutrients
and hormones which act as a plant
growth promoter for root development
(Vacheron et al., 2013).
Environmental sustainability
Application of PGPB will naturally enhance
soil fertility (Roychowdhury et al., 2014).
Increase of PGPB concentration in the soil
will enhance the degradation of resources
ISBN: 978-967-14475-0-5; eISBN: 978-967-14475-1-2 100