Biological Nitrogen Fixation by Legumes - Nitrogen Management in Crop Production - page 285

subsequently allocated to the various sinks within the plant. The nodule and its subtending
root system represent one of the strongest sinks, receiving an estimated 15-30% or more of the
net photosynthate of the plant (Schubert, 1986). This supply of photosynthate transport via the
phloem is used for the energy-yielding substrate and carbon skeletons to support the growth and
maintenance of the nodule tissue. In addition, it is also used for the energy-consuming reactions
associated with the reduction of N 2 in the endophyte and the assimilation of the NH 4 + produced
in the host cytosol and the synthesis of N-containing organic compounds for export from the
nodule (Schubert, 1986).
Establishment of the legume- Rhizobium symbiosis involves infection of the host root and the
subsequent formation of nodular growth containing approximately equal weights of root and bacte-
rial cells (Bauer, 1981). Dart (1977) reported that Rhizobium cells can become attached to the host
root surface within seconds or minutes after inoculation. Hence, attachment of Rhizobium cells to
legume roots is likely to be one of the first steps in the required sequence of interactions leading
to infection and nodulation (Bauer, 1981). Legume roots infected with rhizobia induce curling and
branching of root hairs and the subsequent formation of a tubular structure called the infection
thread. The infection thread develops inward from its point of origin near the most acutely curled
region of the hair. Rhizobia are carried within the thread, usually single file, as the tip of the thread
follows the movement of the nucleus toward the base of the hair cell. The infection thread passes
through the wall of the hair cell and the adjacent cortical cell and branches into many newly divided
cortical cells (Bauer, 1981). Rhizobia are released from the tips of the infection threads into the
cytoplasm of the host cells, where they are surrounded by envelopes of the host plasma membrane.
The enzyme nitrogenase is synthesized in the bacteria and converts dinitrogen into ammonia at the
expense of the host plant photosynthate (Bauer, 1981).
The legume roots may be infected by N-fixing bacteria any time after root hairs are present on
them. Infection normally occurs through young root hairs, although infection through the epidermal
cell wall (Nutman, 1959) and, in peanut ( Arachis hypogaea ), through cells at the junction of the root
hair cells and the epidermal and cortical cells (Chandler, 1978) is reported. The bacteria in the soil
are attracted by secretions from the root, especially of tryptophan, which they convert into indole
acetic acid (IAA) (Cobley, 1976). The nodules increase in size for about 3 weeks after infection,
and vascular tissues derived from undifferentiated cells of the root cortex develop in it until they
become continuous with the stele of the root. After 8-10 weeks, the nodule begins to break down
and eventually falls from the root and disintegrates (Cobley, 1976).
Table 7.1 shows examples of host preference among bacteria species. Data in Table 7.1 show that
bacteria of the genera Rhizobium and Bradyrhizobium provide the major biological source of fixed
N in agricultural soils. The genus Rhizobium contains fast-growing, acid-producing bacteria, while
Bradyrhizobia are slow growers that do not produce acid (Brady and Weil, 2002). The host plant
provides carbohydrates for the N-fixing bacteria, and bacteria in exchange provide atmospheric

TABLE 7.1
Bacteria Species and Their Host Legumes

Bacteria Species Preferred Host Legume

Rhizobium leguminosarum Vetch, peas, lentils, and sweet pea

Rhizobium trifolii Clovers

Rhizobium phaseoli Dry bean and runner bean

Rhizobium meliloti Sweet clover, alfalfa, and fenugreek

Rhizobium loti Trefoils, lupinas, and chickpea

Bradyrhizobium japonicum Soybean

Bradyrhizobium species Cowpea, peanut pigeon pea, kudzu, crotolaria, and many other tropical legumes