Molecular Soil Biology 2015, Vol.6, No.4, 1-12
5
Figure 1 Nitrogen fixation and N
2
-fixing agents
reserved the ability to form an effective symbiosis
with classical host plants (pea, bean and clover) (Weir,
2006).
Rhizobium galegae
strains isolated from
Galega
orientalis
or
Galega officinalis
form effective symbioses
with their original hosts, whereas strains isolated from
G.
officinalis
form ineffective nodules on
G. orientalis
and vice versa (Andronov et al
.,
2003).
Rhizobia often fail to fix nitrogen when they
encounter atypical hosts. When rhizobia isolated from
one host species are inoculated onto different legumes,
invariably only a subset of such cross-inoculated
strains can nodulate the novel host, and only a smaller
subset efficiently fix nitrogen on the novel host. This
pattern suggests that many rhizobia that have been
identified as ineffective might be highly effective if
inoculated onto a more suitable host. Host-symbiont
mismatches might be widespread in agricultural
settings where plants, bacteria, and even soils are
transported among sites. On the other hand, though
rhizobia are defined by their ability to infect legumes,
studies of multiple rhizobial lineages have revealed
that much diversity within these groups is represented
by strains that do not exhibit the ability to infect
legumes (Sachs et al
.,
2010). The N
2
-fixing symbiosis
on N fixation is characterized by legume x rhizobium
specificity. Some host varieties are superior to others
in their ability to fix N
2
, and in turn, some rhizobil
strains have similar superior capability. One
hypothesis that has been posed in many studies is
that the appearance of rhizobial exploitation is
the result of mismatched host-symbiont pairs
(Sachs et al
.,
2010; Friesen, 2012).
FactorsAffecting Symbiotic Nitrogen Fixation (SNF)
SNF efficiency depends on rhizobial strain, plant host,
environmental factors, soil and their interaction. A
number of biotic and abiotic factors affect SNF. The
most important abiotic factors include drought,
salinity, waterlogging, temperature, soil acidity,
