20.3.3.7 Enterobacteriaceae
Several genera of the enterobacteriaceae identified from soil include diazotrophs,
particularly those from the rhizosphere of rice (Kennedy et al. 2004). It has been
speculated that the long agricultural use of organic matters such as manure fertilizer
for crops might have developed an enteric flora adapted to an animal-soil-plant
rhizosphere nutritional cycle (Kennedy et al. 2004). These enteric genera contain
some examples of diazotrophs with PGP activity including Klebsiella, Enterobacter,
Citrobacter, and probably several others not yet identified. Klebsiella pneumoniae,
Enterobacter cloacae, and Citrobacter freundii may pose concern for human safety,
or at least a need to show that no risk is present, when inoculant bacteria even
remotely related to others negatively associated with human health that are produced
in large quantities (Kennedy et al. 2004). This fact was recently noted for C. freundii
(Nguyen et al. 2003), although the level of risk is probably quite slight.
20.3.3.8 Burkholderia and Pseudomonas
These genera may also pose concern for human safety (Kennedy et al. 2004). A
concern exists on the use of Burkholderia vietnamensis in biofertilizer products
because its relative Burkholderia cepacia has been linked to cystic fibrosis. We must
consider the safety issues to develop biofertilizer of these bacteria, although these
bacteria commonly exist in most soil, and crop rhizospheres does not seem to have a
significant safety risk in relation to human health. The typical species of these
bacteria are Pseudomonas putida and P. fluorescens that are plant-associated
bacteria.
20.3.3.9 Herbaspirillum
These bacteria are gram negative, in general vibrioid, and sometimes helical with
one to three polar flagella. The Herbaspirillum are isolated from herbaceous seedbearing plants. Notably, Herbaspirillum colonize the roots of sugarcane, rice, maize,
sorghum, and other cereals as an endophyte (James et al. 2000). BNF with this
bacterium could provide 31–54% of total nitrogen of rice plants (30-d-old rice
seedlings) from the atmosphere (Baldani et al. 2000).
Herbaspirillum could fix nitrogen at 48–63 mg of dry weight per tube, and
95–107 mg of total nitrogen in rice plant per tube containing 12.5% atoms of
15
N
in a form of (NH 4 ) 2 SO 4 (Baldani et al. 2000 and Reis et al. 2000). Rice yield
increased significantly when inoculated with Herbaspirillum in a greenhouse experiment ( p ¼ 0.05) up to 7.5 g per plant (Mirza et al. 2000). Shoot and root length,
1,000-grain weight and grain yield of rice can also increase when inoculated with
Herbaspirillum seropedicae in field conditions (Arangarasan et al. 1998). Seed
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
411
Several genera of the enterobacteriaceae identified from soil include diazotrophs,
particularly those from the rhizosphere of rice (Kennedy et al. 2004). It has been
speculated that the long agricultural use of organic matters such as manure fertilizer
for crops might have developed an enteric flora adapted to an animal-soil-plant
rhizosphere nutritional cycle (Kennedy et al. 2004). These enteric genera contain
some examples of diazotrophs with PGP activity including Klebsiella, Enterobacter,
Citrobacter, and probably several others not yet identified. Klebsiella pneumoniae,
Enterobacter cloacae, and Citrobacter freundii may pose concern for human safety,
or at least a need to show that no risk is present, when inoculant bacteria even
remotely related to others negatively associated with human health that are produced
in large quantities (Kennedy et al. 2004). This fact was recently noted for C. freundii
(Nguyen et al. 2003), although the level of risk is probably quite slight.
20.3.3.8 Burkholderia and Pseudomonas
These genera may also pose concern for human safety (Kennedy et al. 2004). A
concern exists on the use of Burkholderia vietnamensis in biofertilizer products
because its relative Burkholderia cepacia has been linked to cystic fibrosis. We must
consider the safety issues to develop biofertilizer of these bacteria, although these
bacteria commonly exist in most soil, and crop rhizospheres does not seem to have a
significant safety risk in relation to human health. The typical species of these
bacteria are Pseudomonas putida and P. fluorescens that are plant-associated
bacteria.
20.3.3.9 Herbaspirillum
These bacteria are gram negative, in general vibrioid, and sometimes helical with
one to three polar flagella. The Herbaspirillum are isolated from herbaceous seedbearing plants. Notably, Herbaspirillum colonize the roots of sugarcane, rice, maize,
sorghum, and other cereals as an endophyte (James et al. 2000). BNF with this
bacterium could provide 31–54% of total nitrogen of rice plants (30-d-old rice
seedlings) from the atmosphere (Baldani et al. 2000).
Herbaspirillum could fix nitrogen at 48–63 mg of dry weight per tube, and
95–107 mg of total nitrogen in rice plant per tube containing 12.5% atoms of
15
N
in a form of (NH 4 ) 2 SO 4 (Baldani et al. 2000 and Reis et al. 2000). Rice yield
increased significantly when inoculated with Herbaspirillum in a greenhouse experiment ( p ¼ 0.05) up to 7.5 g per plant (Mirza et al. 2000). Shoot and root length,
1,000-grain weight and grain yield of rice can also increase when inoculated with
Herbaspirillum seropedicae in field conditions (Arangarasan et al. 1998). Seed
20 The Diazotroph as an Endophyte and How a Diazotroph Interacts with Its. . .
411