Agriculture Reference
In-Depth Information
Chapter 4
Bacteria Nodulating Legumes
Early on in the study of bacteria nodulating legumes, it was realised that they were not
all the same. Two categories were identified, fast-growing and slow-growing, and these
were subsequently called
Rhizobium
and
Bradyrhizobium
, respectively (Jordan, 1982).
Further differences include production of acid or alkali, respectively, when grown on
suitable media and location of the nodulation (
nod
) and nitrogen-fixing (
nif
) genes. In
Rhizobium
and
Sinorhizobium
these are located on extrachromosomal DNA (plasmids),
in
Bradyrhizobium
are chromosomal and in
Mesorhizobium
are on symbiotic islands
located within the chromosome (see Section 4.6). There are many other differences
between genera and species, such as preferred carbon substrates, salt tolerance and
antibiotic tolerance. These features form part of the specific descriptions and can be
found in the relevant references.
All the bacteria that are known to nodulate legumes are in the gram-negative Pro-
teobacteria. This group is divided into five sections, coded by Greek letters. Only the
-and
-groups have members proven to nodulate legumes. Reports from the
-group have not been authenticated and may represent casual nodule occupancy,
a problem discussed later. The
-Proteobacteria contain seven orders, with all of the
known nodulating genera being in Rhizobiales. Of the eleven families within this order,
six are currently known to contain nodulating species. In some genera, e.g.
Bradyrhizo-
bium
, all species are able to induce nodulation, but in others, e.g.
Phyllobacterium
, nodu-
lating ability is restricted to one or a few species. The nomenclatural situation is further
complicated by the suggestion that the largely plant-pathogenic genus
Agrobacterium
should be incorporated into
Rhizobium
. Farrand et al. (2003) discuss this and argue
the case for retaining
Agrobacterium
as a separate genus, and this position is adopted
here. On the basis of analysis of its
nodC
gene, a strain of
Bosea
(Bradyrhizobiaceae),
isolated from nodules in Spanish gypsum-rich soils, is thought to nodulate various pa-
pilionoid legumes (Rinc on et al., 2008). The
-Proteobacteria contain six orders, with
all of the known nodulating members being in Burkholderiales, and within this or-
der, in family Burkholderiaceae. Further details of bacterial taxonomy can be found
in Garrity et al. (2004). This publication is revised regularly, but the current edition is
