3 Microbial Endophytes: Sustainable Approach …
59
bacterium abolished bacterial twitching and motility as well as the endophytic colonization of the roots of rice plants (Bohm et al. 2007), where pilT and pilA genes
encode the pilus retraction protein and the pilin structural protein, respectively. On
the other hand, the colonization of host plants by bacterial endophytes, showed that
the gumD gene from the nitrogen-fixing endophyte G. diazotrophicus, involved in
exopolysaccharide biosynthesis, is required for biofilm formation and subsequent
root colonization (Meneses et al. 2011). It was later demonstrated the significance of
endophyte colonization in rice plants of the gor and sod genes, a glutathione reductase, and a superoxide dismutase, analyzed in the N 2 -fixing strain G. diazotrophicus
(Alquéres et al. 2013). A series of DNA cytosine methylation changes were observed
as a consequence of plant inoculation with the endophytic PGPB B. phytofirmans
PsJN (Da et al. 2012). In this study, 30 plant proteins (thought to be involved in growth
and signaling) and their methylation status was significantly altered (increased or
decreased) and identified following interaction with the bacterium. When the effect
of the endophytic PGPB G. diazotrophicus on sugarcane plants was assessed, using
the proteomics approach of the more than 400 proteins that were analyzed, 78 were
differentially expressed in the presence of the bacterium (Lery et al. 2011).
To improve phosphate-dissolving capacity by PGPB strains, genetic transfer of
any isolated gene involved in mineral phosphate solubilization (MPS) is an interesting
approach. An attempt was made to improve MPS in PGPR strains, using a PQQ
synthase gene from E. herbicola (Rodriguez et al. 2001). This gene was subcloned
in a broad-host-range vector pKT230 and the recombinant plasmid was expressed in
E. coli and subsequently transferred to PGPR strains of B. cepacia and P. aeruginosa.
Several of the ex-conjugants that were recovered in the selection medium showed
a larger clearing halo in medium with tricalcium phosphate as the sole P source.
This indicated the heterologous expression of this gene in the recombinant strains
and gave rise to improved MPS ability in these PGPRs. However, expression of the
mineral phosphate solubilizing (mps) genes in a different host may also be influenced
by the genetic background of the recipient strain, the copy number of the indigenous
plasmids, and metabolic interactions.
Mineral P solubilization involves the synthesis of gluconic acid, which is produced
from glucose involving glucose dehydrogenase (GDH) enzyme (Goldstein and Liu
1987). The cofactor pyrroloquinoline quinone (PQQ) is required for GDH activity
(Goldstein 1995). In pqq operon, pqqA, pqqB, pqqC, pqqD, and pqqE genes are
conserved and arranged in an orderly manner, whereas, pqqF and pqqG are located
either proximal or distal to the pqq operon (Shen et al. 2012). Moreover, a twocomponent regulatory system, consisting of a DNA-binding transcriptional regulator (PhoB) and transmembrane histidine kinase (PhoR) involved in regulation
and secretion of various enzymes such as alkaline phosphatases, acid phosphatases,
phytases, and phosphodiesterase in response to inorganic phosphate (Pi) scarcity
(Santos-Beneit 2015). The alkaline phosphatase enzyme (encoded by gene phoA)
of E. coli was fully induced when the Pi concentration was reduced from 100 mM
to 0.16 mM (Lopez-Bucio et al. 2003), suggesting that a regulatory element called
as Pi transport operon and the sensor-activator operon, both are involved in this
mechanism. The genes controlled by Pi and activated by PhoB constituted the PHO
59
bacterium abolished bacterial twitching and motility as well as the endophytic colonization of the roots of rice plants (Bohm et al. 2007), where pilT and pilA genes
encode the pilus retraction protein and the pilin structural protein, respectively. On
the other hand, the colonization of host plants by bacterial endophytes, showed that
the gumD gene from the nitrogen-fixing endophyte G. diazotrophicus, involved in
exopolysaccharide biosynthesis, is required for biofilm formation and subsequent
root colonization (Meneses et al. 2011). It was later demonstrated the significance of
endophyte colonization in rice plants of the gor and sod genes, a glutathione reductase, and a superoxide dismutase, analyzed in the N 2 -fixing strain G. diazotrophicus
(Alquéres et al. 2013). A series of DNA cytosine methylation changes were observed
as a consequence of plant inoculation with the endophytic PGPB B. phytofirmans
PsJN (Da et al. 2012). In this study, 30 plant proteins (thought to be involved in growth
and signaling) and their methylation status was significantly altered (increased or
decreased) and identified following interaction with the bacterium. When the effect
of the endophytic PGPB G. diazotrophicus on sugarcane plants was assessed, using
the proteomics approach of the more than 400 proteins that were analyzed, 78 were
differentially expressed in the presence of the bacterium (Lery et al. 2011).
To improve phosphate-dissolving capacity by PGPB strains, genetic transfer of
any isolated gene involved in mineral phosphate solubilization (MPS) is an interesting
approach. An attempt was made to improve MPS in PGPR strains, using a PQQ
synthase gene from E. herbicola (Rodriguez et al. 2001). This gene was subcloned
in a broad-host-range vector pKT230 and the recombinant plasmid was expressed in
E. coli and subsequently transferred to PGPR strains of B. cepacia and P. aeruginosa.
Several of the ex-conjugants that were recovered in the selection medium showed
a larger clearing halo in medium with tricalcium phosphate as the sole P source.
This indicated the heterologous expression of this gene in the recombinant strains
and gave rise to improved MPS ability in these PGPRs. However, expression of the
mineral phosphate solubilizing (mps) genes in a different host may also be influenced
by the genetic background of the recipient strain, the copy number of the indigenous
plasmids, and metabolic interactions.
Mineral P solubilization involves the synthesis of gluconic acid, which is produced
from glucose involving glucose dehydrogenase (GDH) enzyme (Goldstein and Liu
1987). The cofactor pyrroloquinoline quinone (PQQ) is required for GDH activity
(Goldstein 1995). In pqq operon, pqqA, pqqB, pqqC, pqqD, and pqqE genes are
conserved and arranged in an orderly manner, whereas, pqqF and pqqG are located
either proximal or distal to the pqq operon (Shen et al. 2012). Moreover, a twocomponent regulatory system, consisting of a DNA-binding transcriptional regulator (PhoB) and transmembrane histidine kinase (PhoR) involved in regulation
and secretion of various enzymes such as alkaline phosphatases, acid phosphatases,
phytases, and phosphodiesterase in response to inorganic phosphate (Pi) scarcity
(Santos-Beneit 2015). The alkaline phosphatase enzyme (encoded by gene phoA)
of E. coli was fully induced when the Pi concentration was reduced from 100 mM
to 0.16 mM (Lopez-Bucio et al. 2003), suggesting that a regulatory element called
as Pi transport operon and the sensor-activator operon, both are involved in this
mechanism. The genes controlled by Pi and activated by PhoB constituted the PHO