Molecular Soil Biology 2015, Vol.6, No.4, 1-12
3
fiber, fuel, timber, medicines, forages, biodiesel
fuel, and chemicals.
Grain legumes are an integral part of African farming
systems, covering usually large parts of the farmlands
in the regions (Akibode, 2011). For legume-based
farms, the effectiveness of the fertility maintenance
depends upon the balance of N-fixing legumes and
N-depleting non-legumes in the rotation. The amount
of N
2
fixed by a legume crop varies widely because it
depends on the rhizobial strain, legume genotype and
the environment. Assessing BNF is essential to
manage N turnover in the soil and maximize crop
yields while minimizing losses of reactive N to the
environment (Lupwayi et al., 2011). Thus, quantifying
BNF is a key factor for both economic viability and
environmental performance of low-input farming
systems. In spite of several reviews examining
BNF by crops and pasture legumes (Herridge et
al., 2008; Jensen et al., 2010; Unkovich et al., 2010;
Yang et al., 2010; Unkovich, 2012), there is still a
strong need to estimate different rhizobia strains
and host-legume interaction effect on N fixation
in order to achieve full environmental potential
and resource benefits of protein crops.
Rhizobia
BNF is carried out by a small number of diazotrophic
prokaryotic microorganisms, belonging to a wide
range of eubacteria and archaebacteria. Diazotrophs
are usually divided into free-living and symbiotic
forms, though some cyanobacteria are able to fix N
either independently or in symbiotic association.
Symbiotic diazotrophs include a number of genera of
the Rhizobiaceae, which form symbiosis with legumes
(Unkovich et al., 2008), where nitrogen fixation takes
place in specialized organs, the nodules. These soil
bacteria are called rhizobia. Most of rhizobial species
form nodules on legume roots, but some like
Azorhizobium caulinodans, Azorhizobium dodereinereae
and
Azorhizobium oxalatiphilum
are able to form both
root and aerial (stem) nodules (Dreyfus et al
.,
1988).
Azorhizobium
caulinodans
ORS571 also shows N
2
fixing ability in the free living state that most of the
other rhizobia do not (Dreyfus et al., 1983).
Because of their ecological and economic importance,
the diversity and taxonomy of rhizobia have been
extensively studied. In 1982, two groups of root
nodule bacteria were described: fast growing types
assigned the generic name
Rhizobium
and slow
growing types referred to as
Bradyrhizobium
(Jordan,
1982). In 1996, seventeen species, in four genera
(
Rhizobium
,
Bradyrhizobium
,
Sinorhizobium
and
Mesorhizobium
) were described (Young and Haukka,
1996). Currently, rhizobia include at least 14 genera
comprising more than 98 species (Weir, 2013).
On the basis of the 16S ribosomal DNA sequence,
rhizobia species belong to three main distinct
phylogenetic subclasses:
,
and
-
Proteobacteria
(Table
1). The majority of species belong to the
-
Proteobacteria
which comprises more than 95
species grouped in 11 genera:
Rhizobium
,
Mesorhizobium
,
Bradyrhizobium
, Azorhizobium, Ensifer
(formerly
Sinorhizobium
) ,
Me thy lobac terium
,
Devosia
,
Microv irga
,
Ochrobac trum,
Phy llobac ter ium
and
Shinella
(Zakhia et al
.,
2004). Two genera,
burkholderia
and
Cupriavidus
(formerly
ralstonia
)
(Moulin et al
.,
2001) belong to the
-
Proteobacteria
(Nogom, 2004) whereas one genus belong to the
-
Proteobacteria
(Benhizia et al., 2004). Han et al.
(2005); Liu et al. (2005); Velázquez et al. (2005);
Pulawska et al. (2012) revealed that some species
previously named
Agrobacterium
are able to nodulate
leguminous plants, and hence included in genus
Rhizobium
, namely
R. radiobacter
,
R. rhizogenes
,
R.
rubi
,
R. vitis
and
R. nepotum
. Increase in advanced
study of legume’s new species in different geographical
regions opens new perspectives to isolate and characterize
more rhizobial species/strains.
Source: Barrada and Fikri-Benbrahim (2014)
Rhizobium-Legume Symbiosis
The interaction between legume and rhizobia leads to
the development of a nitrogen fixing symbiosis
(Ohyama et al
.,
2009). The legume-rhizobia symbiosis
is the single most important source of biologically
fixed nitrogen in agricultural systems (Graham and
Vance, 2000). Between one-third and one-half of the
total N added to agricultural land is attributable to the
legume-rhizobia symbiosis (Herridge et al., 2008).
Sprent (2001) documented species in the legume
family that have the ability to form intimate
intracellular associations with diverse rhizobial species.
Generally, the relation between rhizobia and legume is
a selective one: each rhizobia species has a distinct
host range allowing nodulation of a particular set of
