Agriculture Reference
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significantly transform agriculture by enabling on-farm generation of N food crops that
are not legumes (e.g., rice, wheat, sorghum, millet).
A wide range of root endophytic bacteria that can fix N 2 have been identified, includ-
ing Arhizobium , several Burkholderia spp., Gluconacetobacter , Herbaspirillum , Azoarcus,
Acetobacter , Arthrobacter , Azospirillum , Azotobacter , Bacillus , Beijerinckia , Bradyrhizobium ,
Derxia , Enterobacter , Erwinia , Herbaspirillum , Klebsiella , Methylobacterium , Mesorhizobium ,
Pseudomonas , Ralstonia , Rhizobium , Sinorhizobium , Stenotrophomonas , Zoogloea , and a large
number of cyanobacterial genera (Werner, 2007; Bergmann et al., 2007; Gillis et al., 1995;
Palus et al., 1996; Chelius and Triplett, 2000; Rosenblueth et al., 2004; Jha and Kumar, 2009;
Zhang et al., 2002; Lodewyckx et al., 2002; Barraquio et al., 2000; James et al., 2000).
The physical location of diazotrophs seems to be important in controlling rates of N
fixation. The interior of plant tissues may be a more favorable niche for N fixation because
of lower partial oxygen pressure (pO 2 ) and more direct plant uptake of N fixed by bacteria
(James and Olivares, 1998).
A number of Azospirillum diazotrophic species have been shown to fix N (Lodewyckx
et al., 2002). This diazotrophic bacterium has been widely studied and found on the rhi-
zoplane, rhizodermis, apoplast, rhizodermal cells, and root hairs of a number of crops
using techniques such green fluorescent protein (GFP) labeling, strain-specific antibod-
ies, and fluorescence in situ hybridization (FISH) (Scholter et al., 1997; Assmus et al., 1995;
Rothballer et al., 2003). Yield increases of 5-30% with Azospirillum inoculation have been
found on wheat, sorghum, and maize (Dobbelaere et al., 2001). However, it should be noted
that, typically, endophytic organisms produce phytohormones (e.g., auxins), which are
likely important for them to colonize plant surfaces and cells and have several implica-
tions. This improves early root and shoot development and increases lateral roots, root
hair density, and root branching (Dobbelaere et al., 2001). Thus, inoculation with diazo-
trophic organisms can also cause phytohormone yield responses. However, 15 N dilution
studies of rice have shown that up to 30% of the N uptake can be attributed to Azoarcus sp.
BH72 and Gluconacetobacter iazotrophicus PAL3 (Hurek et al., 2002; Hurek and Piche, 2002;
Selesi et al., 2005).
In another N dilution study, Mia et al. (2007) showed that adding N fertilizer sup-
pressed N 2 fixation N by Azospirillum and Bacillus spp. inoculated to banana. This same
research group showed that inoculation ( Azospirillum brasilense strain Sp7 and Bacillus
sphaericus strain UPMB10) of banana root seedlings optimized root and shoot growth, root
proliferation, photosynthetic rate, and yield when N fertilizer was applied at 33% of the
recommended N rate (Mia et al., 2010). In addition, besides providing a significant level
of atmospheric derived N, inoculated plants had increased calcium (Ca) and magnesium
(Mg) uptake.
Mia et al. (2007) showed that addition of inorganic N can affect diazotrophic N fixation
rates. They found that at 0 ppm N addition 38% of N uptake of 45-day-old seedlings was
derived from N fixation compared to only 5% when 50 ppm N was added to soil. This is
notable as it showed that addition of inorganic N suppressed N fixation in diazotrophs,
which is well established for the symbiotic N fixation of rhizobium in legumes. Therefore,
N fertilizer management must be considered in the development of diazotrophic-based N
fixation strategies.
Another consideration for optimizing diazotrophic N fixation is plant genotype. This
was shown by Naher et al. (2009) on rice. They found that strains of Rhizobium sp. (Sb16)
and Corynebacterium sp. inoculated to rice seedlings significantly increased biomass of
45-day-old seedlings compared to the control, 35 kg N ha −1 , but there was an interaction
between microbial strain and rice genotype. The N content was highest for the rice cultivar
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