encoding for MCPs in H. seropedicae SmR1, nine were differentially expressed in
the cells associated with roots. Inactivation of one of these, the gene (accession
number Hsero_3720), resulted in a two-fold reduction in the ability of the mutant
strains to attach compared to the wild-type strain (Balsanelli et al. 2016; Pinski et al.
2019) (Table 20.2). This MCP is a key transducer required to sense rhizosphere
compounds and to direct bacteria towards the host-secreted compounds. However,
there was no difference in epiphytic and endophytic colonization by the SmR1
mutant strain compared with the wild-type strain (Balsanelli et al. 2016).
The inactivation of another MCP chemotaxis-like protein encoded by tlp1 gene
has resulted in the impairment of chemotaxis to several terminal electron acceptors
(oxygen and nitrate) and redox active chemicals, indicated by studying a mutant of
the rhizospheric strain Azospirillum brasilense Sp7 that displayed impaired colonization of the plant roots (Greer-Phillips et al. 2004; Michiels et al. 1991; Pinski et al.
2019) (Table 20.2).
Defining the precise function of flagella is complicated because their protein
constituents are microorganism-associated molecular patterns (MAMPs) that elicit
response by the plant against phytopathogens. A lack of flagellum synthesis in
Azoarcus sp. BH72 has yielded mutants that were not motile, although the mutants’
attachments to the root surface remained the same (Buschart et al. 2012). The
impaired Azoarcus sp. BH72 flagella could not be recognized by the plant, resulting
in failure to induce a plant defensive response (Buschart et al. 2012). However, in
A. brasilense Sp7 the flagellum was found critical to achieve adherence to wheat
roots (Croes et al. 1993) (Table 20.2).
The pili produced by Azoarcus sp. BH72 are also involved in colonizing the
surface and interior of roots. The inactivation of the pilus-associated pilX gene has
resulted in a greatly reduced root colonization that could be due to the impaired
twitching motility of the mutant (Shidore et al. 2012). In another study, pili mutants
with no twitching motility also showed a significant decrease in colonization of the
surface and interior of roots (Böhm et al. 2007). A microscopy study on the type IV
pili of H. seropedicae SmR1 during the colonization of wheat roots offered further
suggestion for the involvement of this structure in attachment to host cells
(Pankievicz et al. 2016) (Table 20.2). Some contradictory information on the
involvement of type IV pili was presented by Cole et al. (2017). In their study,
mutations in the pilus locus increased the colonization fitness of the rhizospheric
strain Pseudomonas simiae WCS417r. However, this study may imply that the
mutation promotes a planktonic lifestyle, possibly resulting in a reduction in cellto-cell communication and cell-to-surface interactions, leading to increased motility
and colonization efficiency (Cole et al. 2017).
Biofilm formation could be a possible factor for attachment of bacteria to the root
surface (Pinski et al. 2019). The biofilm typically contains water, proteins, polysaccharides, extracellular DNA (eDNA), RNA, and ions (Żur et al. 2016). A series of
mutants representing a range of species has suggested that additional polymers
contribute to bacterial attachment and colonization. In Gluconacetobacter
diazotrophicus PAL5, exopolysaccharide (EPS) is also involved in forming the
biofilm. The inactivation of the gumD gene responsible for the first step of EPS
biosynthesis resulted in a decrease in the rhizospheric and endophytic colonization
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
401
the cells associated with roots. Inactivation of one of these, the gene (accession
number Hsero_3720), resulted in a two-fold reduction in the ability of the mutant
strains to attach compared to the wild-type strain (Balsanelli et al. 2016; Pinski et al.
2019) (Table 20.2). This MCP is a key transducer required to sense rhizosphere
compounds and to direct bacteria towards the host-secreted compounds. However,
there was no difference in epiphytic and endophytic colonization by the SmR1
mutant strain compared with the wild-type strain (Balsanelli et al. 2016).
The inactivation of another MCP chemotaxis-like protein encoded by tlp1 gene
has resulted in the impairment of chemotaxis to several terminal electron acceptors
(oxygen and nitrate) and redox active chemicals, indicated by studying a mutant of
the rhizospheric strain Azospirillum brasilense Sp7 that displayed impaired colonization of the plant roots (Greer-Phillips et al. 2004; Michiels et al. 1991; Pinski et al.
2019) (Table 20.2).
Defining the precise function of flagella is complicated because their protein
constituents are microorganism-associated molecular patterns (MAMPs) that elicit
response by the plant against phytopathogens. A lack of flagellum synthesis in
Azoarcus sp. BH72 has yielded mutants that were not motile, although the mutants’
attachments to the root surface remained the same (Buschart et al. 2012). The
impaired Azoarcus sp. BH72 flagella could not be recognized by the plant, resulting
in failure to induce a plant defensive response (Buschart et al. 2012). However, in
A. brasilense Sp7 the flagellum was found critical to achieve adherence to wheat
roots (Croes et al. 1993) (Table 20.2).
The pili produced by Azoarcus sp. BH72 are also involved in colonizing the
surface and interior of roots. The inactivation of the pilus-associated pilX gene has
resulted in a greatly reduced root colonization that could be due to the impaired
twitching motility of the mutant (Shidore et al. 2012). In another study, pili mutants
with no twitching motility also showed a significant decrease in colonization of the
surface and interior of roots (Böhm et al. 2007). A microscopy study on the type IV
pili of H. seropedicae SmR1 during the colonization of wheat roots offered further
suggestion for the involvement of this structure in attachment to host cells
(Pankievicz et al. 2016) (Table 20.2). Some contradictory information on the
involvement of type IV pili was presented by Cole et al. (2017). In their study,
mutations in the pilus locus increased the colonization fitness of the rhizospheric
strain Pseudomonas simiae WCS417r. However, this study may imply that the
mutation promotes a planktonic lifestyle, possibly resulting in a reduction in cellto-cell communication and cell-to-surface interactions, leading to increased motility
and colonization efficiency (Cole et al. 2017).
Biofilm formation could be a possible factor for attachment of bacteria to the root
surface (Pinski et al. 2019). The biofilm typically contains water, proteins, polysaccharides, extracellular DNA (eDNA), RNA, and ions (Żur et al. 2016). A series of
mutants representing a range of species has suggested that additional polymers
contribute to bacterial attachment and colonization. In Gluconacetobacter
diazotrophicus PAL5, exopolysaccharide (EPS) is also involved in forming the
biofilm. The inactivation of the gumD gene responsible for the first step of EPS
biosynthesis resulted in a decrease in the rhizospheric and endophytic colonization
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
401