polymyxa. Bowen and Rovira (1999) discussed the possible mechanisms for plant
growth-promoting rhizobacteria (PGPR) responses such as both positive and negative plant growth effects (by potential phytohormones), induced systemic resistance
to phytopathogens, siderophore production, phosphate solubilization, and root associated biological nitrogen fixation (BNF) (Kennedy et al. 2004).
Dobbelaere et al. (2003) reported on the diazotrophic benefit of PGPR, focusing
on the microbes mechanisms of action including BNF, plant growth promotion by
plant hormone produced such as auxins, cytokinins, gibberellins and ethylene, and
phosphate solubilization, increase in nutrient uptake, enhanced stress resistance,
vitamin production, and biocontrol. In this review, I will be presenting some updated
information on previously reported genetic studies on the diversity of diazotrophs,
and discussion focusing on interactions.
20.2 Background of Diazotrophs
A bacterium that fixes nitrogen in root nodules was discovered by Beijerinck in
1888. He isolated the bacteria but did not have the experimental conditions which
would allow the bacteria to fix nitrogen (Beyerinck 1888). He named the bacterial
strain Bacillus radicicola, and we currently know it as Rhizobium leguminosarum.
Beijerinck obtained the organism from root nodules of legumes that first had been
discovered to fix gaseous nitrogen by German scientists Hellriegel and Wilfarth in
the year 1888. Beijerinck (1901) also isolated an Azotobacter sp. that now is known
to fix gaseous nitrogen. Diazotrophs were first suggested to penetrate all tissues of
sugarcane (Dobereiner 1992), and subsequent studies have shown that these bacteria
are not restricted only to this crop (Reis et al. 2000) but are in fact widely present in
plants. Regardless of their non-endosymbiotic or endosymbiotic nature, many
diazotrophs have the genes for nitrogen fixation (nif genes) but not nodule inducing
(nod genes) like those of the leguminous Rhizobia.
Among the various N 2 -fixing endophytic bacteria, two types have been suggested
to classify these bacteria (Reis et al. 2000): the obligate endophytes Acetobacter
diazotrophicus, which cannot survive in the soil, and the facultative ones that include
the Azospirillum group (Baldani et al. 1997) and other many bacteria such as
Burkholderia vietnamiensis (Gillis et al. 1995), B. brasilensis (Baldani et al. 2000;
Hartmann et al. 1995), and B. subtilis (Ji et al. 2014a), which can survive in soil and
plants. These diazotrophs are genetically diverse. The biological traits for host
ranges, tissue colonized, and endosymbiosis are shown in Table 20.1 (Baldani
et al. 1997; Reis et al. 2000).
The crops of family Poaceae, previously known by the name Gramineae, are the
grasses such as rice and wheat, currently need to receive costly mineral fertilizer
(Döbereiner et al. 1995; Triplett 1996). Studies on long-term N-balance and
15
N
isotope dilution technique (Viera-Vargas et al. 1995) have shown that some of
Brazilian sugarcane (Saccharum spp.) varieties may in fact obtain up to 70% of
their N requirements by nitrogen fixation. This process seems to involve
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
393
growth-promoting rhizobacteria (PGPR) responses such as both positive and negative plant growth effects (by potential phytohormones), induced systemic resistance
to phytopathogens, siderophore production, phosphate solubilization, and root associated biological nitrogen fixation (BNF) (Kennedy et al. 2004).
Dobbelaere et al. (2003) reported on the diazotrophic benefit of PGPR, focusing
on the microbes mechanisms of action including BNF, plant growth promotion by
plant hormone produced such as auxins, cytokinins, gibberellins and ethylene, and
phosphate solubilization, increase in nutrient uptake, enhanced stress resistance,
vitamin production, and biocontrol. In this review, I will be presenting some updated
information on previously reported genetic studies on the diversity of diazotrophs,
and discussion focusing on interactions.
20.2 Background of Diazotrophs
A bacterium that fixes nitrogen in root nodules was discovered by Beijerinck in
1888. He isolated the bacteria but did not have the experimental conditions which
would allow the bacteria to fix nitrogen (Beyerinck 1888). He named the bacterial
strain Bacillus radicicola, and we currently know it as Rhizobium leguminosarum.
Beijerinck obtained the organism from root nodules of legumes that first had been
discovered to fix gaseous nitrogen by German scientists Hellriegel and Wilfarth in
the year 1888. Beijerinck (1901) also isolated an Azotobacter sp. that now is known
to fix gaseous nitrogen. Diazotrophs were first suggested to penetrate all tissues of
sugarcane (Dobereiner 1992), and subsequent studies have shown that these bacteria
are not restricted only to this crop (Reis et al. 2000) but are in fact widely present in
plants. Regardless of their non-endosymbiotic or endosymbiotic nature, many
diazotrophs have the genes for nitrogen fixation (nif genes) but not nodule inducing
(nod genes) like those of the leguminous Rhizobia.
Among the various N 2 -fixing endophytic bacteria, two types have been suggested
to classify these bacteria (Reis et al. 2000): the obligate endophytes Acetobacter
diazotrophicus, which cannot survive in the soil, and the facultative ones that include
the Azospirillum group (Baldani et al. 1997) and other many bacteria such as
Burkholderia vietnamiensis (Gillis et al. 1995), B. brasilensis (Baldani et al. 2000;
Hartmann et al. 1995), and B. subtilis (Ji et al. 2014a), which can survive in soil and
plants. These diazotrophs are genetically diverse. The biological traits for host
ranges, tissue colonized, and endosymbiosis are shown in Table 20.1 (Baldani
et al. 1997; Reis et al. 2000).
The crops of family Poaceae, previously known by the name Gramineae, are the
grasses such as rice and wheat, currently need to receive costly mineral fertilizer
(Döbereiner et al. 1995; Triplett 1996). Studies on long-term N-balance and
15
N
isotope dilution technique (Viera-Vargas et al. 1995) have shown that some of
Brazilian sugarcane (Saccharum spp.) varieties may in fact obtain up to 70% of
their N requirements by nitrogen fixation. This process seems to involve
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
393