Focus on Environment

<strong>Focus</strong> Environ (2016)
Plant Growth Promoting Bacteria
absorption in plant root cells (Vacheron et
al., 2013). They can also produce phytohormones
such as indole acetic acid (IAA), gibberellic
acid (GA) and cytokinins (Gupta et
al., 2015). Different PGPB produce different
phytohormones (Pontes et al., 2015). In addition,
it is also capable of inducing modifications
in plant gene expression, increasing
drought resistance associated genes like
ERD15 (early response to dehydration) or
DREB (dehydration responsive element protein)
(Gagne-Bourque et al., 2015). Inoculation
of PGPB will increase the uptake of
NH4 + , HPO4 2- /H2PO4 - by the roots, mineralize
organic soil and induce tolerance or resistance
to the biotic stress (Nkebiwe et al.,
2016). Most PGPB can also facilitate the
uptake of environmental nutrients such as
sulphur, magnesium and calcium. It has
shown to solubilise and mineralize organic
soil (Calvo et al., 2014) these will induce
biochemical changes in the plant which will
lead to beneficial effects on the plant health,
growth and also in decreasing plant disease
(Tak et al., 2013).
Phosphate solubilisation
Phosphorus is a major macronutrient needed
by plants; however, it is present in unavailable
form in the soil (Yuan, 2015). In addition,
the rainfall and leaching will continuously
reduce the phosphorus level in the soil
(Brar et al., 2012). The presences of PGPB
will enable the conversion of phosphorus
into more available forms, such as orthophosphates
which plant roots can absorb
easily (Rodriguez et al., 2006).
Nitrogen fixation
According to the Fertilizers Institute of
United States (2016), there is 78% of nitrogen
in the air and 98% presence in the soil.
Therefore, there is no limitation in nitrogen
Munusamy
content for all living things especially for
plant. Fixing atmosphere nitrogen (N2) and
stimulation of nitrate transport system by
PGPB increases the nitrogen availability for
the plants (Mantelin and Touraine, 2004).
Besides plant growth, nitrogen is required
for synthesis of enzyme, proteins, chlorophyll,
DNA and RNA (Saeed et al., 2015).
Sequestration of Iron
Iron mainly affects the variety of bacterial
communities in the soil as they compete
among themselves to absorb the available
iron (Woitke and Schnitzler, 2005). Therefore,
PGPB synthesize low molecular mass
known as siderophore under iron limiting
conditions. This molecule will competitively
bind to ferric ion Fe +3 to form Fesiderophore
complex that facilitate better
iron uptake (Gupta et al., 2014). Various
bacterial strains will synthesize different
types of siderophores that function differently
(Ahmed and Holmstrom, 2014). They also
generally remove any kind of siderophores
with lower affinity and draw irons
from heterologous siderophores that are
coproduced by other microorganisms. All
these will increase the uptake of iron in
plants (Compant et al., 2005).
ALTERATION OF PHYTOHORMONE
LEVELS
Modulation of ethylene
Methionine is the initial substrate involve in
the ethylene production. This substrate is
converted into S-adenosyl-L-methionine
(SAM) by SAM synthase. It is then hydrolyzed
to 1-aminocyclopropane-1-carboxylic
acid (ACC) and 5- methyl thioadenosine by
ACC synthase (Gamalero and Glick, 2015).
Finally, ACC molecule is metabolized to
ethylene, carbon dioxide and cyanide by
ISBN: 978-967-14475-0-5; eISBN: 978-967-14475-1-2 98