variety of other phytosymbiont bacteria and their host plants (De Almeida et al.
2009; Thomas and Reddy 2013; White et al. 2014; Thomas and Sekhar 2014).
Facultative intracellular symbionts show their adaptive flexibility by possessing a
relatively greater number of mobile genetic elements as compared with obligate
intracellular symbionts (Toft and Anderson 2010). For example, G. diazotrophicus
has 109 transposases, which is four to five times more mobile elements than
possessed by many other endophytes (Bertalan et al. 2009; Miter et al. 2013),
reflecting a high degree of adaptive flexibility on the part of G. diazotrophicus.
Such flexibility might be needed to overcome host-related constraints such as the
requirement for bacteria to attach to host cells, subsequently enter the cytoplasm,
multiply, exiting, and then being transmitted to new host individuals. All that
activity must occur without the bacteria being recognized by the host immune
system (Toft and Anderson 2010) or at least not targeted for destruction even if
immune recognition does happen.
Genetic diversity and adaptive flexibility can also be achieved though bacteria
possessing plasmids that carry genes responsible for necessary functions such as
nitrogen fixation, sulfur utilization, and hydrocarbon degradation. Nitrogen-fixing
genes could, in particular, be conserved in chromosomal DNA and within plasmids
(Banu and Prasad 2017).
The symbiotic bacteria of genus Rhizobium carry high molecular weight plasmids
(90–350Â10
6 daltons) and it is important to note that the plasmids of R. leguminosarum
have a role in nodule formation (nod genes) symbiosis as well as carrying nitrogen
fixation (nif) genes (Nuti et al. 1979).
Plasmids do occur in G. diazotrophicus but their numbers and sizes vary between
strains, with for example G. diazotrophicus UAP8070 and UAP5665 each having
three plasmids of 93, 22, and 22 kb in size (Caballero-Mellado and Martínez-Romero
1994), PR2 has two plasmids one of which is particularly large at 170 kb and a smaller
one at 24 kb, whereas Pal5 has two plasmids of 38.8 and 16.6 kb, and strain UAP5541
has no plasmids at all (Dent 2018; Fuentes-Ramírez et al. 1993, 1999).
The colonization of plant hosts by endophytic bacteria is described as a complex
process that could be distinguished into five steps as described by Pinski et al.
(2019): (1) recognizing root exudates and motility towards the plant, (2) adhering to
the surface of roots, (3) biofilm formation, (4) root surface penetration, and (5) colonization of the internal parts of a plant (Kandel et al. 2017; Hardoim et al. 2015).
Each of these steps is mediated by various biomolecules which drive dynamic
changes in the expression of the bacterial genes as well as in the colonized host
(Pinski et al. 2019). Colonizing the roots by endophytic bacteria starts with the
chemotaxis of planktonic bacteria towards the roots followed by attachment to the
rhizoplane. It was demonstrated that methyl-accepting chemotaxis proteins (MCPs)
could play a key role in these first stages (Scharf et al. 2016). These are transmembrane sensors that result in the signal compounds surrounding the bacteria being
detected to either direct the bacteria towards attractants or away from repellents
(Scharf et al. 2016).
The involvement of the MCPs in plant colonization was demonstrated by the
inactivation of MCP genes in Herbaspirillum seropedicae SmR1. Of the 66 genes
400
S.-C. Chun
2009; Thomas and Reddy 2013; White et al. 2014; Thomas and Sekhar 2014).
Facultative intracellular symbionts show their adaptive flexibility by possessing a
relatively greater number of mobile genetic elements as compared with obligate
intracellular symbionts (Toft and Anderson 2010). For example, G. diazotrophicus
has 109 transposases, which is four to five times more mobile elements than
possessed by many other endophytes (Bertalan et al. 2009; Miter et al. 2013),
reflecting a high degree of adaptive flexibility on the part of G. diazotrophicus.
Such flexibility might be needed to overcome host-related constraints such as the
requirement for bacteria to attach to host cells, subsequently enter the cytoplasm,
multiply, exiting, and then being transmitted to new host individuals. All that
activity must occur without the bacteria being recognized by the host immune
system (Toft and Anderson 2010) or at least not targeted for destruction even if
immune recognition does happen.
Genetic diversity and adaptive flexibility can also be achieved though bacteria
possessing plasmids that carry genes responsible for necessary functions such as
nitrogen fixation, sulfur utilization, and hydrocarbon degradation. Nitrogen-fixing
genes could, in particular, be conserved in chromosomal DNA and within plasmids
(Banu and Prasad 2017).
The symbiotic bacteria of genus Rhizobium carry high molecular weight plasmids
(90–350Â10
6 daltons) and it is important to note that the plasmids of R. leguminosarum
have a role in nodule formation (nod genes) symbiosis as well as carrying nitrogen
fixation (nif) genes (Nuti et al. 1979).
Plasmids do occur in G. diazotrophicus but their numbers and sizes vary between
strains, with for example G. diazotrophicus UAP8070 and UAP5665 each having
three plasmids of 93, 22, and 22 kb in size (Caballero-Mellado and Martínez-Romero
1994), PR2 has two plasmids one of which is particularly large at 170 kb and a smaller
one at 24 kb, whereas Pal5 has two plasmids of 38.8 and 16.6 kb, and strain UAP5541
has no plasmids at all (Dent 2018; Fuentes-Ramírez et al. 1993, 1999).
The colonization of plant hosts by endophytic bacteria is described as a complex
process that could be distinguished into five steps as described by Pinski et al.
(2019): (1) recognizing root exudates and motility towards the plant, (2) adhering to
the surface of roots, (3) biofilm formation, (4) root surface penetration, and (5) colonization of the internal parts of a plant (Kandel et al. 2017; Hardoim et al. 2015).
Each of these steps is mediated by various biomolecules which drive dynamic
changes in the expression of the bacterial genes as well as in the colonized host
(Pinski et al. 2019). Colonizing the roots by endophytic bacteria starts with the
chemotaxis of planktonic bacteria towards the roots followed by attachment to the
rhizoplane. It was demonstrated that methyl-accepting chemotaxis proteins (MCPs)
could play a key role in these first stages (Scharf et al. 2016). These are transmembrane sensors that result in the signal compounds surrounding the bacteria being
detected to either direct the bacteria towards attractants or away from repellents
(Scharf et al. 2016).
The involvement of the MCPs in plant colonization was demonstrated by the
inactivation of MCP genes in Herbaspirillum seropedicae SmR1. Of the 66 genes
400
S.-C. Chun