In the study of Arsene et al. (1994), the root surface of wheat was predominantly
colonized with Azospirillum brasilense Sp7 whereas a spontaneous mutant Sp7-S
generated from Sp7 was hardly found to be attached to the root surface and did not
bind Congo red (Katupitiya et al. 1995a, b). In enriched media, there was little
difference between a spontaneous mutant Sp7-S and its parental strain Sp7 (PeregGerk et al. 1997). The only difference between two strains was that the Sp7-S did not
show flocculation and swarming in specialized growth media. The spontaneous
mutation occurred in a regulatory gene for flocculation (flcA) (Levy et al. 2018).
As a result, production of exopolysaccharide significantly reduced in the mutant
Sp7-S (flcA-) and surface colonization reduced. However, it could colonize between
cortical cells and in crevices where lateral roots emerge and para-nodules, formed
responding to the amendment of 2,4-D (Deaker and Kennedy 2001). The colonization of the mutant Sp7-S (flcA-) in this way shows higher rates of nitrogenase
activity than is appeared by Sp7. In the study explained here, a laboratory model
of para-nodulation in wheat was applied as several researchers had conducted
(Tchan and Kennedy 1989; Katupitiya et al. 1995a, b; Sriskandarajah et al. 1993).
The application of synthetic auxin has been shown to arrest normal lateral root
development (Sriskandarajah et al. 1993) and provides numerous modified lateral
root structures from a greater number of primordia, in turn enabling a more extensive
colonization by the Sp7-S strain of A. brasilense (Deaker and Kennedy 2001).
Deaker and Kennedy (2001) confirmed that by using nifH-lacZ fusions,
A. brasilense Sp7-S, a mutant capable of more endophytic colonization of wheat
roots than the wild type Sp7, fixed more N 2 than the wild type Sp7 strain. In their
study, the nifH nitrogenase gene was strongly expressed in the wheat rhizosphere of
para-nodulated wheat inoculated with A. brasilense and grown in different hydroponic systems, and apparently this effect was attributed to the bacteria having
improved access to carbon compounds and a more favorable microaerobic
concentration.
20.3.2 Genetics of Plants on Interactions with Endophytic
Bacteria
Not only the genetic elements of endophytes, but also those of plants, are important
and interact for successful achievement of colonization and endosymbiosis. Plants
have various receptors and symbiosis can involve changes in plant hormone signalling pathways. In addition, small RNAs (sRNAs) could also be involved in symbiosis (Carvalho et al. 2016; Thiebaut et al. 2014).
Plant receptors play a role in recognizing bacterial pathogen signals, and that is
mainly mediated by the family of receptor-like kinases (RLK), which include a
leucine-rich repeat LRR-RLKs), wall-associated kinases (WAK), lectin receptor-like
kinases (LecRLKs), and Lys-motif receptors (LysM). However, only a few studies
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
405
colonized with Azospirillum brasilense Sp7 whereas a spontaneous mutant Sp7-S
generated from Sp7 was hardly found to be attached to the root surface and did not
bind Congo red (Katupitiya et al. 1995a, b). In enriched media, there was little
difference between a spontaneous mutant Sp7-S and its parental strain Sp7 (PeregGerk et al. 1997). The only difference between two strains was that the Sp7-S did not
show flocculation and swarming in specialized growth media. The spontaneous
mutation occurred in a regulatory gene for flocculation (flcA) (Levy et al. 2018).
As a result, production of exopolysaccharide significantly reduced in the mutant
Sp7-S (flcA-) and surface colonization reduced. However, it could colonize between
cortical cells and in crevices where lateral roots emerge and para-nodules, formed
responding to the amendment of 2,4-D (Deaker and Kennedy 2001). The colonization of the mutant Sp7-S (flcA-) in this way shows higher rates of nitrogenase
activity than is appeared by Sp7. In the study explained here, a laboratory model
of para-nodulation in wheat was applied as several researchers had conducted
(Tchan and Kennedy 1989; Katupitiya et al. 1995a, b; Sriskandarajah et al. 1993).
The application of synthetic auxin has been shown to arrest normal lateral root
development (Sriskandarajah et al. 1993) and provides numerous modified lateral
root structures from a greater number of primordia, in turn enabling a more extensive
colonization by the Sp7-S strain of A. brasilense (Deaker and Kennedy 2001).
Deaker and Kennedy (2001) confirmed that by using nifH-lacZ fusions,
A. brasilense Sp7-S, a mutant capable of more endophytic colonization of wheat
roots than the wild type Sp7, fixed more N 2 than the wild type Sp7 strain. In their
study, the nifH nitrogenase gene was strongly expressed in the wheat rhizosphere of
para-nodulated wheat inoculated with A. brasilense and grown in different hydroponic systems, and apparently this effect was attributed to the bacteria having
improved access to carbon compounds and a more favorable microaerobic
concentration.
20.3.2 Genetics of Plants on Interactions with Endophytic
Bacteria
Not only the genetic elements of endophytes, but also those of plants, are important
and interact for successful achievement of colonization and endosymbiosis. Plants
have various receptors and symbiosis can involve changes in plant hormone signalling pathways. In addition, small RNAs (sRNAs) could also be involved in symbiosis (Carvalho et al. 2016; Thiebaut et al. 2014).
Plant receptors play a role in recognizing bacterial pathogen signals, and that is
mainly mediated by the family of receptor-like kinases (RLK), which include a
leucine-rich repeat LRR-RLKs), wall-associated kinases (WAK), lectin receptor-like
kinases (LecRLKs), and Lys-motif receptors (LysM). However, only a few studies
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
405