root mucilage along with arabinogalactan-proteins, pectic polysaccharides, secondary metabolites, antimicrobial compounds, and extracellular DNA, and play a key
role on root defense through the formation of a root extracellular trap. The expansins
are also involved in the development of the plant, loosening the cell wall constituents
during division.
Pectinesterases catalyse the de-esterification of polygalacturonans, which might
also be characteristics of endophytic interactions (Irizarry and White 2018). This
enzyme was associated with the lateral root development in cucumber seedlings
(Zhang et al. 2014). In the study of Xie et al. (2012), the gene of predicted enzyme
pectate lyase was upregulated. Pectate lyase degrades pectin and has been previously
reported to contribute to the nodule-formation of rhizobacteria to colonize roots
during early stages of symbiosis (Xie et al. 2012). Xie et al. (2012) described that a
legume pectate lyase (LjNPL) was essential for normal initiation of infection of
Mesorhizobium loti in the plant L. japonicus.
The increase of lignin in cell walls is a key reaction to plant pathogens and is a
barrier against the spread of bacterial cells (Liu et al. 2018; Pinski et al. 2019). Also,
greater lignification decreases the infiltration of fungal enzymes and toxins through
plant cell walls (Liu et al. 2018). Considering these protective responses associated
with lignification, it is perhaps unexpected that endophytic colonization would
trigger an accumulation of lignin, i.e., a higher lignin content was observed in the
roots of cotton following a B. amyloliquefaciens pb1 challenge (Irizarry and White
2018). An increase in the expression of cell wall bound peroxidase that is coinvolved
in lignification of cell walls was found in A. thaliana in response to an endophytic
colonization by P. putida BP25 (Sheoran et al. 2016). An increased expression of
cinnamyl alcohol dehydrogenase (CAD), which is known to be involved in lignin
biosynthesis, was reported in M. sinensis followed by an endophytic colonization by
H. frisingense GSF30T (Straub et al. 2013). However, the expression of CAD was
downregulated in wheat inoculated with A. brasilense FP2 (Camilios-Neto et al.
2014). The patterns and types of lignin produced with inoculation of endophytes
seem distinctive (Pinski et al. 2019). An analysis of the profile for miRNAs expression of maize challenged with diazotrophic bacteria indicates decrease in lignin
biosynthesis, because miR408 was induced followed by the downregulation of its
targets, which are laccases (Thiebaut et al. 2014). Therefore, miRNA expression
seems likely that these endophytes affect lignification in the host (Pinski et al. 2019).
20.3.3 Diversity of Diazotrophs
20.3.3.1 Azotobacter
Azotobacter species (perhaps most notably Azotobacter vinelandii and
A. chroococcum) are free-living bacteria appearing round and oval with flagella
(Yamagata and Itano 1923). Azotobacter are aerobic heterotrophic organisms that
act as diazotrophs under either aerobic or microaerobic conditions, depending on an
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
407
role on root defense through the formation of a root extracellular trap. The expansins
are also involved in the development of the plant, loosening the cell wall constituents
during division.
Pectinesterases catalyse the de-esterification of polygalacturonans, which might
also be characteristics of endophytic interactions (Irizarry and White 2018). This
enzyme was associated with the lateral root development in cucumber seedlings
(Zhang et al. 2014). In the study of Xie et al. (2012), the gene of predicted enzyme
pectate lyase was upregulated. Pectate lyase degrades pectin and has been previously
reported to contribute to the nodule-formation of rhizobacteria to colonize roots
during early stages of symbiosis (Xie et al. 2012). Xie et al. (2012) described that a
legume pectate lyase (LjNPL) was essential for normal initiation of infection of
Mesorhizobium loti in the plant L. japonicus.
The increase of lignin in cell walls is a key reaction to plant pathogens and is a
barrier against the spread of bacterial cells (Liu et al. 2018; Pinski et al. 2019). Also,
greater lignification decreases the infiltration of fungal enzymes and toxins through
plant cell walls (Liu et al. 2018). Considering these protective responses associated
with lignification, it is perhaps unexpected that endophytic colonization would
trigger an accumulation of lignin, i.e., a higher lignin content was observed in the
roots of cotton following a B. amyloliquefaciens pb1 challenge (Irizarry and White
2018). An increase in the expression of cell wall bound peroxidase that is coinvolved
in lignification of cell walls was found in A. thaliana in response to an endophytic
colonization by P. putida BP25 (Sheoran et al. 2016). An increased expression of
cinnamyl alcohol dehydrogenase (CAD), which is known to be involved in lignin
biosynthesis, was reported in M. sinensis followed by an endophytic colonization by
H. frisingense GSF30T (Straub et al. 2013). However, the expression of CAD was
downregulated in wheat inoculated with A. brasilense FP2 (Camilios-Neto et al.
2014). The patterns and types of lignin produced with inoculation of endophytes
seem distinctive (Pinski et al. 2019). An analysis of the profile for miRNAs expression of maize challenged with diazotrophic bacteria indicates decrease in lignin
biosynthesis, because miR408 was induced followed by the downregulation of its
targets, which are laccases (Thiebaut et al. 2014). Therefore, miRNA expression
seems likely that these endophytes affect lignification in the host (Pinski et al. 2019).
20.3.3 Diversity of Diazotrophs
20.3.3.1 Azotobacter
Azotobacter species (perhaps most notably Azotobacter vinelandii and
A. chroococcum) are free-living bacteria appearing round and oval with flagella
(Yamagata and Itano 1923). Azotobacter are aerobic heterotrophic organisms that
act as diazotrophs under either aerobic or microaerobic conditions, depending on an
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
407