have reported their comparative relevance in identifying beneficial endophytes
(Carvalho et al. 2016).
Plant hormone-signalling pathways are critical in plant defense, and they are well
understood to be also involved in interactions with endophytic bacteria (Lebeis et al.
2015). Studies have reported the functions of ET (ethylene), SA (Salicylic acid), and
JA (Jasmonic acid) not only in regulation of endophytic bacterial colonization but
also the diversity of endophytic bacterial populations. It has been found that a typical
endophytic colonization of Medicago truncatula by Klebsiella pneumoniae 342 led
to activation of the ET signalling pathway, although in contrast, an ET-insensitive
mutant of M. truncatula was hyper-colonized by that endophytic strain (Iniguez et al.
2005). These results were in agreement with a study that found sugarcane colonized
by both the diazotrophic endophyte of Gluconacetobacter diazotrophicus PAL5 and
Herbaspirillum rubrisubalbicans HCC103 showed an increased expression of a
putative ET receptor (SCER1) at 24 h as well as seven days after the inoculation.
The Small RNAs (sRNAs) also affect plant growth and development as posttranscriptional regulators of gene expression (Pinski et al. 2019). They additionally
influence plant responses to abiotic stresses and phytopathogens (Carvalho et al.
2016). Recent studies have also indicated the involvement of sRNAs during interactions of plants with endophytic bacteria. For example, when T. aestivum was
inoculated with endophytic Rhizobium and Azorhizobium caulinodans ORS571, an
altered miRNA (microRNA) expression was observed. The peak response appeared
at 12–24 h after inoculation and the responses were different in the roots and shoots.
The roots seemed more sensitive to the inoculation than the shoots, possibly because
this strain colonizes roots (Qiu et al. 2017). Thiebaut et al. (2014) reported the
response of maize sRNAs to inoculation with the diazotrophic H. seropedicae and
A. brasilense which indicated that there to have been upregulation of the coppermiRNAs (Cu-miRNAs) coupled with an inhibition of their targets. The names for
these copper miRNAs originated from the fact that they target the genes for proteins
with a Cu cofactor such as laccases, superoxide dismutases, and cupredoxins. These
enzymes are known to be involved in rapidly generating an oxidative burst and
response signaling during pathogen attack (Thiebaut et al. 2014; Pilon 2017). In the
study of Thiebaut et al. (2014), Cu-miRNAs’ target genes were downregulated,
suggesting that both diazotropic strains suppress the early plant defense response
(Thiebaut et al. 2014).
The endophytic colonization process is also influenced by the up or
downregulation of genes related to cell wall modifications (Pinski et al. 2019).
These genes encode for hydroxyproline-rich glycoproteins (HRGPs), expansins,
and pectinesterases. The HRGPs implicated in biological functions are usually
grouped into three complex multigene families, i.e., (i) arabinogalactan proteins
(AGPs), (ii) extensins, and (iii) proline-rich proteins (Johnson et al. 2017).
The extensins of plants are well-studied in relation to their interactions with
pathogenic and beneficial bacteria. Extensins play an important role in plant defense
by strengthening the cell wall following a phytopathogen inoculation (Pinski et al.
2019). Interestingly, there have been studies which found that extensins also increase
in the nodules colonized by symbiotic R. leguminosarum. Extensins form part of the
406
S.-C. Chun
(Carvalho et al. 2016).
Plant hormone-signalling pathways are critical in plant defense, and they are well
understood to be also involved in interactions with endophytic bacteria (Lebeis et al.
2015). Studies have reported the functions of ET (ethylene), SA (Salicylic acid), and
JA (Jasmonic acid) not only in regulation of endophytic bacterial colonization but
also the diversity of endophytic bacterial populations. It has been found that a typical
endophytic colonization of Medicago truncatula by Klebsiella pneumoniae 342 led
to activation of the ET signalling pathway, although in contrast, an ET-insensitive
mutant of M. truncatula was hyper-colonized by that endophytic strain (Iniguez et al.
2005). These results were in agreement with a study that found sugarcane colonized
by both the diazotrophic endophyte of Gluconacetobacter diazotrophicus PAL5 and
Herbaspirillum rubrisubalbicans HCC103 showed an increased expression of a
putative ET receptor (SCER1) at 24 h as well as seven days after the inoculation.
The Small RNAs (sRNAs) also affect plant growth and development as posttranscriptional regulators of gene expression (Pinski et al. 2019). They additionally
influence plant responses to abiotic stresses and phytopathogens (Carvalho et al.
2016). Recent studies have also indicated the involvement of sRNAs during interactions of plants with endophytic bacteria. For example, when T. aestivum was
inoculated with endophytic Rhizobium and Azorhizobium caulinodans ORS571, an
altered miRNA (microRNA) expression was observed. The peak response appeared
at 12–24 h after inoculation and the responses were different in the roots and shoots.
The roots seemed more sensitive to the inoculation than the shoots, possibly because
this strain colonizes roots (Qiu et al. 2017). Thiebaut et al. (2014) reported the
response of maize sRNAs to inoculation with the diazotrophic H. seropedicae and
A. brasilense which indicated that there to have been upregulation of the coppermiRNAs (Cu-miRNAs) coupled with an inhibition of their targets. The names for
these copper miRNAs originated from the fact that they target the genes for proteins
with a Cu cofactor such as laccases, superoxide dismutases, and cupredoxins. These
enzymes are known to be involved in rapidly generating an oxidative burst and
response signaling during pathogen attack (Thiebaut et al. 2014; Pilon 2017). In the
study of Thiebaut et al. (2014), Cu-miRNAs’ target genes were downregulated,
suggesting that both diazotropic strains suppress the early plant defense response
(Thiebaut et al. 2014).
The endophytic colonization process is also influenced by the up or
downregulation of genes related to cell wall modifications (Pinski et al. 2019).
These genes encode for hydroxyproline-rich glycoproteins (HRGPs), expansins,
and pectinesterases. The HRGPs implicated in biological functions are usually
grouped into three complex multigene families, i.e., (i) arabinogalactan proteins
(AGPs), (ii) extensins, and (iii) proline-rich proteins (Johnson et al. 2017).
The extensins of plants are well-studied in relation to their interactions with
pathogenic and beneficial bacteria. Extensins play an important role in plant defense
by strengthening the cell wall following a phytopathogen inoculation (Pinski et al.
2019). Interestingly, there have been studies which found that extensins also increase
in the nodules colonized by symbiotic R. leguminosarum. Extensins form part of the
406
S.-C. Chun