microbially provided nitrogen supply, but also from other biologically advantageous
effects due to indoleacetic acid production (IAA) and stimulations of plant responses
that are triggered by the diazotrophs. Some diazotrophs may stimulate the plant in
ways that promote cell growth and induce defensive systemic responses of the plant
against pathogens. Diazotrophs include among their membership many species of
bacteria. Notably, the Rhizobia are well known as symbiotic bacteria which often act
by making nodules on the roots and even on the stems on of leguminous plants,
fixing atmospheric nitrogen and providing it to the plants (Singh et al. 2006).
However, it also has been reported that bacteria belonging to the beta-subclass of
proteobacteria such as Burkholderia and Ralstonia similarly could induce nodules
on legumes and can fix atmospheric nitrogen (Chen et al. 2001; Moulin et al. 2001;
Vandamme et al. 2002; Verma et al. 2004). Many nodule-forming bacteria are
collectively referred to as legume-nodulating bacteria (Zakhia et al. 2004).
Much of the atmospheric nitrogen fixed on earth is attributed to the symbiosis of
Rhizobia with legumes (Chaintreuil et al. 2000). Although these genera of Rhizobia
do not induce nodulation on the plants other than legumes, they can still fix nitrogen
from air and plants can utilize this nitrate. All rhizobial genera including
Azorhizobium, Bradyrhizobium, Ensifer, Mesorhizobium, Methylobacterium, and
Rhizobium belong to the alpha-subclass of proteobacteria (http://www.rhizobia.co.
nz/Rhizobia_Taxonomy.html). It has been much reported that rhizobia may occur as
root endophytes (colonize intercellular spaces of roots) in nonlegumes such as rice,
promoting the plants growth and crop productivity, in addition to those symbiotic
associations which rhizobia have with legumes (Singh et al. 2006; Biswas et al.
2000; Matiru and Dakora 2004; Peng et al. 2002; Perrine et al. 2001; Yanni et al.
1997, 2001).
The rhizobia and other diazotrophs reside inside or outside of the root surface and
fix nitrogen from the atmosphere, providing their product as nitrate that plants can
utilize. It is obvious that many of these endophytes can promote plant growth and
productivity. What has not been much studied is the possibility that these endophytic
rhizobacteria could induce disease and drought resistance in addition to supporting
plant growth promotion, producing IAA and nitrogen fixing (Ji et al. 2014a). The
endophytic green fluorescent protein gene (gfp)-tagged Bacillus subtilis CB-R05
could colonize well on the root tissues of rice (O. sativa Ilpum cv.) for 16 days of
growth after inoculation (Ji et al. 2014b). B. subtilis CB-R05 was originally isolated
from endophytic tissues of rice and demonstrated to promote plant growth and
induce disease resistance. In their works, B. subtilis CB-R05 was transformed with
gfp gene and inoculated into rice seeds through immersion into the suspension of
gfp-tagged B. subtilis CB-R05 for 1 h. The seeds were planted into the soil and
seedlings had grown for 16 days, being observed for their colonization to root
system with confocal fluorescent microscopy (Ji et al. 2014b).
Bowen and Rovira (1999) wrote a valuable review of research and described that
proper management of the rhizosphere could improve plant growth. They reviewed
studies on the increases in growth of tomatoes inoculated with Azotobacter (Brown
et al. 1964). Similar results were also obtained by Rovira (1965) for wheat, following
inoculation with Azotobacter chroococcum, Clostridium pasteurianum, and Bacillus
392
S.-C. Chun
effects due to indoleacetic acid production (IAA) and stimulations of plant responses
that are triggered by the diazotrophs. Some diazotrophs may stimulate the plant in
ways that promote cell growth and induce defensive systemic responses of the plant
against pathogens. Diazotrophs include among their membership many species of
bacteria. Notably, the Rhizobia are well known as symbiotic bacteria which often act
by making nodules on the roots and even on the stems on of leguminous plants,
fixing atmospheric nitrogen and providing it to the plants (Singh et al. 2006).
However, it also has been reported that bacteria belonging to the beta-subclass of
proteobacteria such as Burkholderia and Ralstonia similarly could induce nodules
on legumes and can fix atmospheric nitrogen (Chen et al. 2001; Moulin et al. 2001;
Vandamme et al. 2002; Verma et al. 2004). Many nodule-forming bacteria are
collectively referred to as legume-nodulating bacteria (Zakhia et al. 2004).
Much of the atmospheric nitrogen fixed on earth is attributed to the symbiosis of
Rhizobia with legumes (Chaintreuil et al. 2000). Although these genera of Rhizobia
do not induce nodulation on the plants other than legumes, they can still fix nitrogen
from air and plants can utilize this nitrate. All rhizobial genera including
Azorhizobium, Bradyrhizobium, Ensifer, Mesorhizobium, Methylobacterium, and
Rhizobium belong to the alpha-subclass of proteobacteria (http://www.rhizobia.co.
nz/Rhizobia_Taxonomy.html). It has been much reported that rhizobia may occur as
root endophytes (colonize intercellular spaces of roots) in nonlegumes such as rice,
promoting the plants growth and crop productivity, in addition to those symbiotic
associations which rhizobia have with legumes (Singh et al. 2006; Biswas et al.
2000; Matiru and Dakora 2004; Peng et al. 2002; Perrine et al. 2001; Yanni et al.
1997, 2001).
The rhizobia and other diazotrophs reside inside or outside of the root surface and
fix nitrogen from the atmosphere, providing their product as nitrate that plants can
utilize. It is obvious that many of these endophytes can promote plant growth and
productivity. What has not been much studied is the possibility that these endophytic
rhizobacteria could induce disease and drought resistance in addition to supporting
plant growth promotion, producing IAA and nitrogen fixing (Ji et al. 2014a). The
endophytic green fluorescent protein gene (gfp)-tagged Bacillus subtilis CB-R05
could colonize well on the root tissues of rice (O. sativa Ilpum cv.) for 16 days of
growth after inoculation (Ji et al. 2014b). B. subtilis CB-R05 was originally isolated
from endophytic tissues of rice and demonstrated to promote plant growth and
induce disease resistance. In their works, B. subtilis CB-R05 was transformed with
gfp gene and inoculated into rice seeds through immersion into the suspension of
gfp-tagged B. subtilis CB-R05 for 1 h. The seeds were planted into the soil and
seedlings had grown for 16 days, being observed for their colonization to root
system with confocal fluorescent microscopy (Ji et al. 2014b).
Bowen and Rovira (1999) wrote a valuable review of research and described that
proper management of the rhizosphere could improve plant growth. They reviewed
studies on the increases in growth of tomatoes inoculated with Azotobacter (Brown
et al. 1964). Similar results were also obtained by Rovira (1965) for wheat, following
inoculation with Azotobacter chroococcum, Clostridium pasteurianum, and Bacillus
392
S.-C. Chun