Handling Soybean ( Glycine max L.) Under Stress - Crop Improvement: New Approaches and Modern Techniques - page 426

cies were able to noldulate the soybean roots with high differences in nodulation,
nitrogenase activity, N uptake and plant growth (Zhang et al. 2002 ; Meghvanshi
et al. 2005 ). Inoculation with the bacterium significantly enhanced the growth and
N uptake of soybean plants.
The most sensitive stages of nodulation to acidity appear to be the early stages
including attachment (Howieson et al. 1993 ), root morphological changes (Munns
1968 ; Evans 1980 ; Miransari et al. 2006 ) and the formation of infection tread
(Evans 1980 ; Franco and Munns 1982 ). The followings may indicate the higher
sensitivity of the early stages of N-fixation to low pH: decreased rhizobial number
and growth, changes in the structure of plant roots adversely affecting bacterial rec-
ognition and higher uptake of some nutrients (Vassileva et al. 1997 ; Miransari et al.
2006 ; Miransari and Smith 2007 ; Ferreira et al. 2012 ).
The onset of N-fixation is with the exchange of signal molecules between the
bacteria and the host plant. Plant roots produce different products with nutritional
and non-nutritional values acting as secondary metabolites. Secondary metabolites,
which are necessary with a threshold level of 10 −12 M have different functioning in
plant including the activation of microbial genes during the process of symbiosis
(Boller 1995 ).
For the onset of N-fixation the specific host plant produces flavonoids, which are
able to trigger the bacterial chemotaxis response and movement toward the plant
roots. Subsequently, the nodulation genes (Nod genes) in bacteria are activated, re-
sulting in the production of lipochitooligosacharides (LCO) by bacteria (Subrama-
nian et al. 2004 ). LCO molecules are able to induce morphological changes in the
roots of their host plant by curling or bulging the root hairs, inducing cell cycling,
and stimulation of nodule formation. These stages are followed by the formation
of infection thread, which results in the entrance of bacteria into the plant roots.
Bacteria alter some of the root cellular, morphological and physiological activities,
which can increase the division rate of root cortical cells resulting in the formation
of root nodules (Aguilar et al. 1988 ; Long 2001 ; Miransari et al. 2006 , Miransari
and Smith 2007 , 2008 , 2009 ).
Miransari and Smith ( 2007 ) hypothesized that stress results in the disruption of
the signal molecule exchange between the bacteria and the host plant at the onset of
symbiosis under acidity. They also hypothesized and proved that preincubation of
Bradyrhizobium japonicum inoculum with the signal molecule genistein may par-
tially or completely inhibit the adverse effects of stress on the process of N-fixation
under greenhouse and field conditions. By addition of salt or sulfur to the field soil
they adjusted soil salinity and pH to the desired values, followed by the plantation
of soybean seeds, and inoculation with the bacteria preincubated with genistein at
µM levels. Different nodulation and soybean growth and yield parameters were
measured both under salinity and acidity treatments. According to the results, ge-
nistein could alleviate the stress of salinity and acidity on soybean nodulation and
hence N-fixation as well as soybean yield production. The enhancing effects of
genistein on nodulation could be due to increasing the possibility of infection, re-
sulting in the higher number of infections and nodulation (Zhang and Smith 1995 ).
Under stress the production of signal molecules by soybean root as well as the sen-