Agriculture Reference
In-Depth Information
Within the Eubacteria, the most important groups involved in forming symbiotic
associations with plants are species of the genus
Rhizobium
and related genera (known
collectively as 'Rhizobia'). These bacteria form close symbiotic associations almost
entirely with legumes in which the bacteria inhabit nodules associated with the roots,
although not all legumes form such associations. As discussed below, N-fixing associa-
tions occur with other plant groups, notably grasses, although they do not involve
the formation of nodules. Members of the actinobacterial genus
Frankia
fix N in large
nodule-like structures ('actinorhizae') that form on the roots of plants from a number
of families and genera. Certain cyanobacteria such as
Anabaena
and
Nostoc
also form
N-fixing associations with plants (Table IV.10). Rhizobia and
Frankia
only form nodu-
lating N-fixing associations with the relatively primitive dicotyledons. Such associations
do not appear to occur in more recent dicotyledons and monocotyledons (Dommergues
et al.,
1985).
The 'Rhizobia' are symbiotic bacteria that initiate and occupy nodules on the roots of
many legumes. Dinitrogen from the atmosphere is reduced by nitrogenase enzymes
(N-fixation) into forms available to the host. Rhizobia belong to four genera:
Rhizobium,
Bradyrhizobium, Sinorhizobium
and
Azorhizobium
(Martinez-Romero, 1994) including
approximately 13 species. Although they are widespread in soils, indigenous Rhizobia
may not ensure effective nodulation and N-fixation, because of the specificity of many
host legumes (Lie
et al.,
1987).
Bradyrhizobium
forms root nodules in an efficient
N-fixing symbiosis with pioneer species of the genus
Parasponia
(Ulmaceae) which is
distributed from the Malay Archipelago to Polynesia (Soepadmo, 1977) ; this is the only
known case of nodulation with a non-legume. The substantial N fixation abilities of
this symbiosis provides these pioneer plants with a capacity to invade disturbed sites
effectively (Trinick and Hadobas, 1988).
Nodulation can be impeded by low pH, Al toxicity, nutrient deficiencies, salinity or
genetic incompatibility with the host (Dan, 1974; Munns, 1978; Singleton and Bohlool,
1983). Rhizobia are facultative symbionts with some saprophytic ability and can survive
in soils and proliferate within the rhizospheres of both host and non-host plants (Giller
and Wilson, 1991). Rhizobia are stimulated by the presence of the host plant and this
leads to infection, nodulation and nodule senescence, a series of events which serves to
maintain or increase rhizobial populations (Woomer and Bohlool, 1989). Rhizobia enter
the roots of the host plant through the root hairs and through cracks in the epidermis
(Giller and Wilson, 1991).
In commercial environments, the legume-rhizobia symbiosis is ensured before
planting by applying inoculum to the exterior of seeds as liquid formulations or pellets
(FAO, 1984). Nitrogen fixation rates as high as 584 kg have been reported for
Leucaena (
Leucaena leucocephala
) and 354 kg for Cowpea (
Vigna unguicu-
lata
) (FAO, 1984); however, the more usual range of rates reported lies between 50 and
200 kg (see Giller and Wilson, 1991).
N-fixing Actinobacteria belonging to the genus
Frankia
form so-called actinorhizal
nodules on roots of about 200 plant species belonging to 29 genera and eight families
(Table IV.10) (Huss-Danell, 1997).
Casuarina
and
Alnus
are well-known examples of
plant genera with species infected by
Frankia.
Nodules may remain active for several
years and grow to more than 20 cm in diameter. N-fixation in these associations may be
