Formation Of The Symbiosis

The establishment of the mutualistic relationship between rhizobia and legumes is accomplished via a series of developmental stages, all of which are mediated by a chronological cascade of physiological signals from both participants. The process of establishing the symbiotic relationship is highly specific (i.e., a specific bacterial species with one, or a limited number of, legume species). At each stage of the formation of this relationship, the chemical signals released by the plant and the bacterium reciprocally induce unique genetic programs that lead to the formation of a nodule and result in BNF.

Stage 1. Rhizobia exist primarily as soil saprophytes that are widely distributed and are found in the rhizospheres of plant roots. They initiate nodule formation by

TABLE 14.7 Genera and Species of the Root Nodule Bacteria of Legumes and Examples of Their Hosts

Allorhizobium

A. undicola (Neptunia) Azorhizobium

A. caulinodans (Sesbania) Bradyrhizobium

B. canariense (genistoid legumes) B. japonicum (soybean)

B. elkanii (soybean) B. liaoningense (soybean) B. spp. (Vigna, Lupinus, etc.)a Mesorhizobium

M. amorphae (Amorpha) M. chacoense (Prosopis) M. ciceri- (Cicer, chickpea) M. huakii- (Astragalus, milkvetch) M. loti- (Lotus, trefoil) M. mediterraneum (Cicer, chickpea) M. plurifarium (tropical trees) Rhizobium

R. etli (bean, Phaseolus) R. galegae (Galega) R. gallicum (bean, Phaseolus) R. giardinii (bean) R. huakuii (Astragalus) R. huautlense (Sesbania) R. indigofera (Indigofera)

R. leguminosarum (three biovars nodulate (i) clovers; (ii) peas, lentils, and vetch; (iii) bean R. mongolense (Medicago) R. tropici (bean, Phaseolus, Leucaena) Sinorhizobium

S. americanum (Acacia) S. arboris (tree legumes) S. fredii (soybean) S. kostiense (tree legumes) S. kummerowiae (Kummerowia) S. medicae (annual medics) S. meliloti (alfalfa) S. morelense (Leucaena) S. saheli (Sesbania) S. teranga (Sesbania)

aAlthough many legumes are nodulated by bacteria of the Bradyrhizobium genus, the latter have not received official species designation. They are referred to by the name of the legume host from which they were isolated (e.g., Bradyrhizobium [Lupinus]).

penetrating a legume root either by infection of root hairs (e.g., clover and pea) or, more rarely, by entry at the sites of lateral root emergence (peanut) or through penetration of root primordia found on the stems of some legumes such as Sesbania and Aeschynomene. In the case of root-hair infection, the bacteria attach

FIGURE 14.4 Medicago root hair curling and infection thread invasion by rhizobia upon inoculation with Sinorhizobium meliloti. Courtesy of Rene Geurts; used by permission.

to the cell wall of the root hair and trigger a series of morphological and physiological changes in the latter. Inhibition of cell expansion on one side of the root hair causes it to curl back on itself with the rhizobia attached to the inside surface of the curled root hair wall.

Stage 2. Rhizobia pass down the root hair inside a structure known as an infection thread (or tube), which is an invagination of the root hair wall and is clearly visible within 2 days of seedling exposure to rhizobia (Fig. 14.4). The infection thread branches and penetrates into the root cortex, where cortical cells divide and enlarge to form a prenodule in response to the rhizobial invasion.

Stage 3. As the infection thread gets near the dividing cortical cells it forms various side branches, each of which eventually comes to a halt at the cell wall of a cortical cell. At the point of contact, the cell wall dissolves and the rhizobia (10-100 bacterial cells) are delivered into the nodule cell by endocytosis.

Stage 4. Rhizobia are released from the infection thread into root cortical cells and are enclosed within a plant-derived membrane called the peribacteroid membrane. They remain physically isolated from the host cell cytoplasm. Each membrane-enclosed bacterium is referred to as a "symbiosome." As the plant cortical cells divide and become infected to form the nodule structure, the vascular strands of the plant extend into the nodule to permit exchange of nutrients.

Stage 5. The rhizobia undergo further pleiomorphic and biochemical changes in the symbiosome and ultimately commence BNF.

Root nodules differ in appearance and structure depending upon the host plant species. Some plants develop determinate nodules that are generally spherical in shape, reach a finite size, and possess no active meristems, e.g., Phaseolus (beans), Vigna (cowpea), and Glycine (soybean). Other legumes produce cylindrically shaped, indeterminate nodules with persistent active meristems, which enable the nodule to grow, enlarge, and produce new cortical cells in response to plant growth, e.g., Medicago (alfalfa), Trifolium (clover), Vicia (vetch). In indeterminate nodules, nonbacteroid rhizobia remain in the infection threads and they infect new cortical cells as the meristem of the nodule continues to divide.

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