Identification of the carotenoid pigment canthaxanthin from photosynthetic bradyrhizobium strains.

Canthaxanthin (4,4(prm1)-diketo-(beta)-carotene) is produced as the major carotenoid pigment by orange- and dark-pink-pigmented bacteriochlorophyll-containing Bradyrhizobium strains isolated from stem nodules of Aeschynomene species. These two new pigmentation groups differ from the well-studied strain BTAi1, which accumulates spirilloxanthin as the sole carotenoid.

Indigenous Arbuscular Mycorrhizal Fungi Associated with Acacia albida Del. in Different Areas of Senegal.

The influences of seasons, plant age, and physicochemical properties of the soil on surface and deep biological arbuscular mycorrhizal fungus parameters associated with Acacia albida were assessed in different areas of Senegal. More indigenous arbuscular mycorrhizal propagules were found in the localities of the Sudano-Guinean zone (Djinaki and Kabrousse) than in those of the Sahelian zone (Louga and Diokoul), and species belonging to the genera Glomus, Gigaspora, Acaulospora, and Sclerocystis prevailed. The numbers of total and viable spores increased more during the rainy season than during the dry season (about 108% more total spores and 262% more viable spores). Similarly, both total and viable spores were more prevalent around young Acacia trees than old trees. However, the intensities of root colonization did not differ in each ecoclimatic zone.

Bradyrhizobium Populations Occur in Deep Soil under the Leguminous Tree Acacia albida.

Soil cores were drilled under the leguminous tree Acacia albida growing in two different ecoclimatic zones of West Africa: the Sahelian area (100 to 500 mm of annual rainfall) and the Sudano-Guinean area (1,000 to 1,500 mm of annual rainfall). Soil samples were collected at different depths from the surface down to the water table level and analyzed for the presence of rhizobia able to nodulate A. albida. In both areas, population densities of rhizobia were substantially greater near the water table than near the surface. In the Sahelian area, rhizobia were present as deep as 34 m at a concentration of 1.3 x 10/g of soil. In the Sudano-Guinean area, population densities at 0.5 to 4.5 m depth were higher than in the Sahelian area and, at several depths, comparable to that of temperate soils supporting legume crops (10 rhizobia per g of soil). Surface and deep soil isolates from all four sites were found to be slow-growing rhizobia (Bradyrhizobium sp.). The proportion of effective isolates was almost the same within surface and deep soils.

Initial stages in the morphogenesis of nitrogen-fixing stem nodules of Sesbania rostrata.

Morphogenesis of stem nodules in Sesbania rostrata was studied over a period of 6 days after inoculation with an appropriate species of Rhizobium. Nodulation sites were initially slightly raised, circular areas 0.3 to 0.6 mm in diameter and 4 to 5 mm apart in vertical rows along the length of the stem. Each site was underlaid by an adventitious root primordium. A site became susceptible to infection by a specific Rhizobium sp. when the root primordium broke through the epidermis, leaving a fissure. Rhizobia multiplied within this fissure and colonized the exposed intercellular spaces. The infection extended inward as narrow, branched intercellular threads moved into a cortical meristematic zone, where cell division was initiated, and invagination of infection thread branches into adjacent plant cells followed. Rhizobia were released into the plant cells and surrounded immediately by plant membrane. Intracellular rhizobia divided actively, leading to bacteroid-filled cells. Infected areas enlarged and coalesced as the nodule matured.

Free-living Rhizobium strain able to grow on n(2) as the sole nitrogen source.

A Rhizobium strain isolated from stem nodules of the legume Sesbania rostrata was shown to grow on atmospheric nitrogen (N(2)) as the sole nitrogen source. Non-N(2)-fixing mutants isolated directly on agar plates formed nodules that did not fix N(2) when inoculated into the host plant.

Genetic analysis of nitrogen fixation in a tropical fast-growing Rhizobium.

The Rhizobium strain ORS571, which is associated with the tropical legume Sesbania rostrata, has the property of growing in the free-living state at the expense of ammonia or N(2) as sole nitrogen source. Five mutants, isolated as unable to form colonies on plates under conditions of nitrogen fixation, were studied. All of them, which appear as Fixin planta, are nif mutants. With mutant 5740, nitrogenase activity of the crude extract was restored by addition of pure Mo-Fe protein of Klebsiella pneumoniae. A 13-kb BamHI DNA fragment from the wild-type strain, which hybridized with a probe carrying the nifHDK genes of K. pneumoniae, was cloned in vector pRK290 to yield plasmid pRS1. The extent of homology between the probe and the BamHI fragment was estimated at 4 kb and hybridization with K. pneumoniae nifH, nifK, and possibly nifD was detected. The pRS1 plasmid was introduced into the sesbania rhizobium nif mutants. Genetic complementation was observed with strain 5740(pRS1) both in the free-living state and in planta. It thus appears that biochemistry and genetics of nitrogen fixation in this particular Rhizobium strain can be performed with bacteria grown under non-symbiotic conditions.

Nodulation of acacia species by fast- and slow-growing tropical strains of Rhizobium.

Thirteen Acacia species were classified into three groups according to effective nodulation response patterns with fast- and slow-growing tropical strains of Rhizobium. The first group nodulated effectively with slow-growing, cowpea-type Rhizobium strains; the second, with fast-growing Rhizobium strains; and the third, with both fast- and slow-growing Rhizobium strains. The Rhizobium requirements of the Acacia species of the second group were similar to those of Leucaena leucocephala.