H. seropedicae SmR1was inactivated. Even if there was no structural change in LPS
as in the wild-type strain, LPS quantity reduced leading into a ten-fold reduction in
the endophytic colonization of maize (Tadra-Sfeir et al. 2011).
A proteomic analysis of G. diazotrophicus PAL5 in response to plantlets indicated a higher amount of the outer membrane lipoprotein (Omp16) (Dos Santos et al.
2010). The OprF membrane protein is a trait for pseudomonads and was also
reported to play a role in the attachment to the root surface. This protein is a major
porin and facilitates movement of polar solutes across the outer envelope and is a
part of membrane integrity as it maintains stability of membrane as integral structural
protein.
The inactivation of the oprF gene of Pseudomonas fluorescens CHA0 resulted in
a significant decrease in its attachment to cucumber and tomato roots (Crespo and
Valverde 2009). Plant cell walls are composed of cellulose and pectin, and could be
degraded by some bacterial endophytes (Reinhold-Hurek et al. 2006). For example,
rice root colonization by an Azoarcus sp. BH72 mutant significantly decreased and
was unable to spread to the aboveground parts of the plant when the microbes
endoglucanase activity that degrades cell wall was lost. Furthermore, the
endoglucanase was greatly expressed in the time when the bacterial cell contacted
with the rice roots (Reinhold-Hurek et al. 2006). Also, in Bacillus mycoides EC18,
the genes encoding hydrolases, pullulanase, and a chitin-binding protein are
upregulated when exposed to root exudates.
Plants are well known for producing expansin compounds that play a role in cell
enlargement and other developmental events requiring cell wall loosening
(Sampedro and Cosgrove 2005). The expansins seem limited to bacteria from
Bacillus, Xanthomonas, Xylella, Ralstonia, and Erwinia genera (Kerff et al. 2008).
Expansins can facilitate the loosening of the cell wall components during division
which could also be features of endophytic interactions (Irizarry and White 2018). In
their study, upregulation of expansin gene expression was observed in the cotton
seedling roots following inoculation with Bacillus amyloliquefaciens (Irizarry and
White 2018). However, in rare cases, endophytic bacterial genomes, containing
genes for expansins, could facilitate cell wall extension by the plants (creep) without
any actual breakdown or covalent modification of the cell wall polymers. Thus, it
could be well understood that an expansin deficient mutant of Bacillus subtilis
168 did not colonize roots of maize efficiently, although the extension activity
expressed by the parental wild-type strain was weak but existent when assessed
in vitro (Kerff et al. 2008).
Despite all these results that demonstrate that cell wall-degrading enzymes and
bacterial expansins are involved in attachment, cell wall-degrading enzymes and
expansins are not prerequisite for most successful colonizations. This lack of enzyme
requirement may relate to the fact that many endophytes penetrate through wounds
and natural openings such as the stomata, particularly on the leaves and young stems
(Wallace and May 2018). In addition, genes for plant cell wall-degrading enzymes
have not been discovered in most of the genomes of endophytes (Ali et al. 2014;
Martin-Moldes et al. 2015).
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
403
as in the wild-type strain, LPS quantity reduced leading into a ten-fold reduction in
the endophytic colonization of maize (Tadra-Sfeir et al. 2011).
A proteomic analysis of G. diazotrophicus PAL5 in response to plantlets indicated a higher amount of the outer membrane lipoprotein (Omp16) (Dos Santos et al.
2010). The OprF membrane protein is a trait for pseudomonads and was also
reported to play a role in the attachment to the root surface. This protein is a major
porin and facilitates movement of polar solutes across the outer envelope and is a
part of membrane integrity as it maintains stability of membrane as integral structural
protein.
The inactivation of the oprF gene of Pseudomonas fluorescens CHA0 resulted in
a significant decrease in its attachment to cucumber and tomato roots (Crespo and
Valverde 2009). Plant cell walls are composed of cellulose and pectin, and could be
degraded by some bacterial endophytes (Reinhold-Hurek et al. 2006). For example,
rice root colonization by an Azoarcus sp. BH72 mutant significantly decreased and
was unable to spread to the aboveground parts of the plant when the microbes
endoglucanase activity that degrades cell wall was lost. Furthermore, the
endoglucanase was greatly expressed in the time when the bacterial cell contacted
with the rice roots (Reinhold-Hurek et al. 2006). Also, in Bacillus mycoides EC18,
the genes encoding hydrolases, pullulanase, and a chitin-binding protein are
upregulated when exposed to root exudates.
Plants are well known for producing expansin compounds that play a role in cell
enlargement and other developmental events requiring cell wall loosening
(Sampedro and Cosgrove 2005). The expansins seem limited to bacteria from
Bacillus, Xanthomonas, Xylella, Ralstonia, and Erwinia genera (Kerff et al. 2008).
Expansins can facilitate the loosening of the cell wall components during division
which could also be features of endophytic interactions (Irizarry and White 2018). In
their study, upregulation of expansin gene expression was observed in the cotton
seedling roots following inoculation with Bacillus amyloliquefaciens (Irizarry and
White 2018). However, in rare cases, endophytic bacterial genomes, containing
genes for expansins, could facilitate cell wall extension by the plants (creep) without
any actual breakdown or covalent modification of the cell wall polymers. Thus, it
could be well understood that an expansin deficient mutant of Bacillus subtilis
168 did not colonize roots of maize efficiently, although the extension activity
expressed by the parental wild-type strain was weak but existent when assessed
in vitro (Kerff et al. 2008).
Despite all these results that demonstrate that cell wall-degrading enzymes and
bacterial expansins are involved in attachment, cell wall-degrading enzymes and
expansins are not prerequisite for most successful colonizations. This lack of enzyme
requirement may relate to the fact that many endophytes penetrate through wounds
and natural openings such as the stomata, particularly on the leaves and young stems
(Wallace and May 2018). In addition, genes for plant cell wall-degrading enzymes
have not been discovered in most of the genomes of endophytes (Ali et al. 2014;
Martin-Moldes et al. 2015).
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
403