Rhizobia Legume Symbiosis

Most Leguminosae (about 90%) can establish a symbiotic association with aerobic diazotrophic Gram-negative bacteria commonly referred to as rhizobia. This symbiosis takes place in roots and brings about the formation of nodules in which N2 fixation occurs. The large contribution made by these symbioses to the nitrogen availability for agronomically important legumes is well known. Medicago sativa, for instance, can fix 300kgNha~ yr~ and Vicia faba over 500 kgN ha-1 yr-1. This makes biological nitrogen fixation a major component of sustainable agricultural systems, since it has the potential to greatly limit the use of chemical nitrogen fertilizers.

Numerous species belonging to the family Leguminosae are also abundant in natural ecosystems, such as the forests of tropical regions (e.g., those in Brazil and Guyanas), where they can represent over 50% of all trees. Tropical forests often grow on substrates poor in mineral nutrients, and thus the continuous supply of nitrogen through biological N2 fixation acquires an essential role to maintain large nitrogen pools in these ecosystems.

The rhizobia forming symbiosis with legume roots belong to five different genera: Rhizobium, Azorhizobium, Mesorhizobium, Sinorhizobium, and Bradyrhizobium. A given species of bacterium establishes symbiosis with one or few species of legumes (Table 2).

This is due to the host-symbiont recognition occurring in the rhizosphere through the exchange of molecular signals. The first event of the root nodule formation is the chemotactic movement of the bacterium toward the root of the host plant in response to chemical attrac-tants, usually specific flavonoids or betains secreted by the root under nitrogen-starvation conditions. These substances induce the expression of host-specific bacterial genes (nod genes) coding for Nod factors (lipochitin-oligosaccharides) that, in turn, induce plant responses and trigger the nodule developmental program.

Root infection starts with the bacterium-induced curling of a root hair, bacterium attachment to the hair surface, cell wall degradation, and formation of the infection thread. This is an internal tubular extension of the hair plasma membrane that carries out the proliferating rhizobia from the root surface into the root cortex. Concomitantly, some cortical cells undergo rapid divisions that give rise to the nodule primordium. When the branched infection thread reaches target cells within the developing nodule, its tip vesiculates releasing bacteria packaged in a membrane derived from the host cell plasmalemma. The rhizobia undergo some divisions but very soon they stop dividing and differentiate into diazotrophic bacteroids. Bacteroids and surrounding peri-bacteroid membrane form the symbiosome (Figure 3b),

Table 2 Some examples of associations between rhizobia and legumes

Rhizobia

Host plants

Bradyrizobium japonicum

Glycine, Vigna

Sinorhizobium meliloti

Medicago, Trigonella,

Melilotus

Sinorhizobium fedii

Glycine, Vigna

Azorhizobium caulinodans

Sesbania

Rhizobium leguminosarum biovar.

Phaseolus

phaseoli

Rhizobium leguminosarum biovar.

Trifolium

trifolii

Rhizobium leguminosarum biovar.

Vicia, Pisum, Cicer

viciae

Mesorhizobium loti

Lotus, Lupinus, Anthillis

(a) Root Cortical cylinder

/Stele Endodermis

Nodule

To shoot From shoot

Nodule endodermis

Nodule vascular bundle

Sclerenchyma

Nodule vascular bundle

Sclerenchyma

Peribacteroid membrane

Bacteroid

Figure 3 Schematic drawings of: (a) a determinate root nodule of a rhizobia-legume symbiosis and (b) a part of an infected cell with symbiosomes.

Peribacteroid membrane

Bacteroid

Figure 3 Schematic drawings of: (a) a determinate root nodule of a rhizobia-legume symbiosis and (b) a part of an infected cell with symbiosomes.

which is the site of N2 fixation. In the mature nodule (Figure 3 a) specialized structures are developed around the infected tissue: an endodermis and a vascular system continuous with the root stele, and a layer of cells hampering O2 diffusion to the root nodule interior. Some leguminous species such as soybean, peanut, and bean form spherical determinate nodules with a nonpersistent meristem (Figure 3 a). Others, such as pea, clover, and alfalfa, form cylindrical indeterminate nodules with a persistent terminal meristem.

Different mechanisms take place to obtain the micro-aerobic environment appropriate for maintaining ATP production in host cells and bacteroids and for preserving, at the same time, nitrogenase activity in N2-fixing tissue. The first hindrance to the entry of oxygen into the infected cells is the mechanical diffusion barrier in the nodule parenchyma. Moreover, leghemoglobin is synthesized in the cytoplasm of the host cells. This oxygen-binding protein plays a major role in delivering oxygen to the bacteroid surface and accounts for the characteristic pink color of N2-fixing tissue. Efficient bacteriod respiration also restricts oxygen penetration into the cytoplasm. Finally, in most rhizobia the activity of an uptake hydrogenase is an additional help for protecting nitrogenase against the O2-poisoning.

o-Ketoglutarate

Infected cell cytoplasm t

Carbohydrates

Amino acids and transport compounds

LeghOem2oglobin

Leghemoglobin

Oxidative metabolism

Plastid

Amino acids and transport compounds

Oxidative metabolism o-Ketoglutarate

Glutamate

Figure 4 A simplified diagram showing nitrogen fixation and ammonia assimilation in an infected cell of a legume root nodule. GOGAT glutamate synthase; GS, glutamine synthetase; N2ase, nitrogenase.

Glutamate

Amino acids and transport compounds

Symbiosome

Figure 4 A simplified diagram showing nitrogen fixation and ammonia assimilation in an infected cell of a legume root nodule. GOGAT glutamate synthase; GS, glutamine synthetase; N2ase, nitrogenase.

Bacteroids do not have enzymes for the ammonia assimilation. For this reason, the NH3 obtained from N2 reduction is released into the root cell where the assimilation occurs via GS-GOGAT pathway (Figure 4). This leads to production of glutamine, glutamate, and, successively, of other nitrogenous transport compounds. Some of these organic compounds are returned to the bacter-oids, but most are exported to the plant shoot via xylem.

In order to sustain N2 fixation, the host plant must supply the bacteroids with a carbon source, which arrives to the root nodule via phloem as sucrose. However, this sugar is metabolized in the host cell and converted to C4 dicarboxylates, principally malate. The dicarboxylates, in fact, are transported across the peribacteroid membrane, becoming the primary carbon source for the N2-fixing organisms.

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