20.3.3.4 Azorhizobium
Azorhizobium is gram-negative and fix nitrogen in symbiosis with plants of Sesbania
rostrata of Leguminosae (Dreyfus et al. 1988). A. caulinodans ORS571 has been
fully sequenced (Lee et al. 2008). The species of rhizobia responsible for nitrogen
fixation in Sesbania rostrata is Azorhizobium caulinodans. Matthews et al. (2001)
reported that A. caulinodans could increase the dry weight of wheat in a greenhouse
experiment. In a following experiment, they used a N-fixing (nif
+ ) and non-N-fixing
(nif
À ) strains of A. caulinodans. However, both strains promoted wheat growth
compared with the non-inoculated plants. This result indicates that the beneficial
effect of inoculating wheat plants with A. caulinodans was attributed to plant growth
promotion (PGP) substance production rather than by BNF. Also, Saleh et al. (2001)
said that its inoculation could save up to 50% of the recommended rate of urea-N in
greenhouse trials under gnotobiotic (or sterile) conditions.
20.3.3.5 Azoarcus
The endophytic gram negative Azoarcus sp. BH72 was originally isolated from
Kallar grass (Diplachne fusca) growing in the typical saline-sodic soils in Pakistan
(Reinhold-Hurek et al. 1993). Azoarcus spp. also colonize grasses, such as rice, in
both the laboratory and the field (Hurek et al. 1994). In rice roots, the zone behind the
meristem was most intensively colonized. The rhizoplane in the zones of elongation
and differentiation is the preferential entry sites of the bacteria followed by the
colonization of the root interior both inter- and intracellularly. These bacteria were
found not only in the root cortex but also in the xylem. However, it is not known yet
that Azoarcus resided intracellularly in living plant cells. The influence of inoculation with Azoarcus sp. BH72 in aseptic systems on rice roots was cultivar-dependent
(Reinhold-Hurek et al. 2002).
20.3.3.6 Clostridium
Clostridium is a strictly anaerobic endospore-forming gram-positive that produces
butyric acid (Seki et al. 2003). Clostridium can be commonly found in soil. This
species was first reported for its contribution to the growth of winter wheat by
providing extra soil nitrogen in West Australian soils of light structure (Parker
1953). Clostridia are obligately anaerobic heterotrophs only capable of fixing N 2
in the absence of oxygen (Saralov and Babanazarov 1983; Kennedy and Tchan
1992). Clostridia can usually be isolated from rice soils (Khamas et al. 1994;
Elbadry et al. 1999) and their activity has been observed to increase after returning
straw to rice fields, to raise the C to N ratio (Kennedy et al. 2004). Inoculation with
clostridia can increase rice yields significantly in favorable conditions when C
containing compounds are incorporated to the soil (Mishustin et al. 1983).
410
S.-C. Chun
Azorhizobium is gram-negative and fix nitrogen in symbiosis with plants of Sesbania
rostrata of Leguminosae (Dreyfus et al. 1988). A. caulinodans ORS571 has been
fully sequenced (Lee et al. 2008). The species of rhizobia responsible for nitrogen
fixation in Sesbania rostrata is Azorhizobium caulinodans. Matthews et al. (2001)
reported that A. caulinodans could increase the dry weight of wheat in a greenhouse
experiment. In a following experiment, they used a N-fixing (nif
+ ) and non-N-fixing
(nif
À ) strains of A. caulinodans. However, both strains promoted wheat growth
compared with the non-inoculated plants. This result indicates that the beneficial
effect of inoculating wheat plants with A. caulinodans was attributed to plant growth
promotion (PGP) substance production rather than by BNF. Also, Saleh et al. (2001)
said that its inoculation could save up to 50% of the recommended rate of urea-N in
greenhouse trials under gnotobiotic (or sterile) conditions.
20.3.3.5 Azoarcus
The endophytic gram negative Azoarcus sp. BH72 was originally isolated from
Kallar grass (Diplachne fusca) growing in the typical saline-sodic soils in Pakistan
(Reinhold-Hurek et al. 1993). Azoarcus spp. also colonize grasses, such as rice, in
both the laboratory and the field (Hurek et al. 1994). In rice roots, the zone behind the
meristem was most intensively colonized. The rhizoplane in the zones of elongation
and differentiation is the preferential entry sites of the bacteria followed by the
colonization of the root interior both inter- and intracellularly. These bacteria were
found not only in the root cortex but also in the xylem. However, it is not known yet
that Azoarcus resided intracellularly in living plant cells. The influence of inoculation with Azoarcus sp. BH72 in aseptic systems on rice roots was cultivar-dependent
(Reinhold-Hurek et al. 2002).
20.3.3.6 Clostridium
Clostridium is a strictly anaerobic endospore-forming gram-positive that produces
butyric acid (Seki et al. 2003). Clostridium can be commonly found in soil. This
species was first reported for its contribution to the growth of winter wheat by
providing extra soil nitrogen in West Australian soils of light structure (Parker
1953). Clostridia are obligately anaerobic heterotrophs only capable of fixing N 2
in the absence of oxygen (Saralov and Babanazarov 1983; Kennedy and Tchan
1992). Clostridia can usually be isolated from rice soils (Khamas et al. 1994;
Elbadry et al. 1999) and their activity has been observed to increase after returning
straw to rice fields, to raise the C to N ratio (Kennedy et al. 2004). Inoculation with
clostridia can increase rice yields significantly in favorable conditions when C
containing compounds are incorporated to the soil (Mishustin et al. 1983).
410
S.-C. Chun