Agriculture Reference
In-Depth Information
Paracoccus, Xanthobacter, Alcaligenes
and
Pseudomonas
may also exhibit
chemolithoautotrophic growth on inorganic sulfur compounds (Vidyalakshmi et al.
,
2009).
Production of Volatile Compounds
Volatiles constitute a group of compounds that evaporate rapidly at ambient temperature
and pressure. Some strains of rhizobacteria belonging to
Bacillus subtilis
,
Bacillus
amyloliquefaciens
and
Enterobacter cloacae
promote plant growth releasing volatile
compounds (Ryu et al.
,
2003). Growth promoting action by volatile compounds is one of the
mechanisms more recently studied in PGPR. Acetoin (3-hydroxy-2-butanone) and 2,3
butanediol are volatile compounds produced by
B. subtilis
and
B. amyloliquefaciens,
which
promote growth of
Arabidopsis thaliana
in vitro
. Some of these compounds act by regulating
the synthesis of auxin and cell expansion (Zhang et al.
,
2007), but a role has also been
proposed in induction of systemic resistance (Farag et al.
,
2006) and antibiosis (Mitchell et
al.
,
2010). Certainly, this knowledge area requires further research to identify new compounds
and elucidate new plant signaling pathways involved in the interactions with plant growth
promoting microorganisms. For example, two new volatile compounds, the 2-methyl
propanoic acid and 3-methyl butanol, which are synthesized
de novo
during the interaction of
Arabidopsis thaliana
with the bacteria
Bacillus megaterium
and
Stenotrophomonas
maltophilia
has been recently identified (Kai et al., 2007; García-Juárez et al.
,
2010).
Furthermore, in biocontrol fungi (
e.g.
Trichoderma
spp.) certain volatile compounds,
such as acetone, 2-methyl-1-butanol, heptanal and octanal, increase the antagonistic activity
of the beneficial fungi by inhibiting the protein synthesis of pathogen fungi (Humphris et al.
,
2001). Additionally,
Arthrobacter agilis
UMCV2 is capable of producing volatile organic
compounds (VOCs), primarily dimethylhexadecylamine, an amino lipid with antifungal
activity, which exerts a strong inhibition on
Botrytis cinerea
and
Phythopthora cinnamomi.
Conveniently, the effect of this VOC on
Trichoderma virens
is very subtle, suggesting the
possibility of developing comprehensive strategies for disease control considering this
differential action (Velázquez-Becerra et al.
,
2010).
Synthesis of Pyrroloquinoline Quinone
Pyrroloquinoline quinone (PQQ) is a novel, water soluble and heat stable growth factor in
plants that has the ability to carry our redox cycles (Stites et al., 2000). It has been reported
that PQQ acts as a reactive oxygen species (ROS) scavenger by directly neutralizing reactive
species in
Escherichia coli
(Misra et al., 2004). PQQ acts as a non-covalently bound redox
cofactor of several bacterial dehydrogenases. Several gram-negative bacteria are capable of
producing organic acids by direct oxidation of aldehydes, which then diffuse in surroundings
and help in the acidification of poorly soluble mineral phosphates such as calcium phosphate
(Goldstein, 1986; Sashidhar & Podile, 2010). Glucose dehydrogenase (GDH) requires PQQ
as a redox cofactor for direct oxidation of glucose to gluconic acid, which then helps in acidic
solubilization of insoluble phosphates in soil. There are plant growth-promoting bacteria that
use GDH-PQQ holoenzyme for solubilization of both inorganic and /or organic phosphates in
soil (Han et al., 2008).
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