K.G. Nandasena

Phylogenetic relationships of three bacterial strains isolated from the pasture legume Biserrula pelecinus L.

Three bacterial strains (WSM 1283, WSM 1284, WSM 1497) isolated from root nodules of the pasture legume Biserrula pelecinus L. growing in Morocco, Italy and Greece, respectively, were studied in order to determine their phylogenetic relationship to the other members of the family Rhizobiaceae. A polyphasic approach, which included analyses of morphological and physiological characteristics, plasmid profiles, symbiotic performance and 16S rRNA gene sequencing, indicated that these strains belong to the genus Mesorhizobium.

Diverse Mesorhizobium spp. with unique nodA nodulating the South African legume species of the genus Lessertia

Background and aimsLegumes of the genus Lessertia have recently been introduced to Australia in an attempt to increase the range of forage species available in Australian farming systems capable of dealing with a changing climate. This study assessed the diversity and the nodulation ability of a collection of Lessertia root nodule bacteria isolated from different agro-climatic areas of the Eastern and Western Capes of South Africa.MethodsThe diversity and phylogeny of 43 strains was determined via the partial sequencing of the dnaK, 16srRNA and nodA genes. A glasshouse experiment was undertaken to evaluate symbiotic relationships between six Lessertia species and 17 rhizobia strains.ResultsThe dnaK and 16S rRNA genes of the majority of the strains clustered with the genus Mesorhizobium. The position of the strains at the intra-genus level was incongruent between phylogenies with few exceptions. The nodA genes from Lessertia spp. formed a cluster on their own, separate from the previously known Mesorhizobium nodA sequences. Strains showed differences in their nodulation and nitrogen fixation patterns that could be correlated with nodA gene phylogeny. L. diffusa, L. herbacea and L. excisa nodulated with nearly all the strains examined while L. capitata, L. incana and L. pauciflora were more stringent.ConclusionRoot nodule bacteria from Lessertia spp. were identified mainly as Mesorhizobium spp. Their nodA genes were unique and correlated with the nodulation and nitrogen fixation patterns of the strains. There were marked differences in promiscuity within Lessertia spp. and within strains of root nodule bacteria.

Phenotypic and genetic diversity among rhizobia isolated from three Hedysarum species: H. spinosissimum, H. coronarium and H. flexuosum

Cultural, physiological and biochemical properties of 18 strains of rhizobia isolated from root nodules of the forage legume H. spinosissimum were compared with those of rhizobia from the related species H. coronarium (15 strains) and H. flexuosum (four strains). On the basis of 43 characteristics the 37 strains of Hedysarum rhizobia could be divided into two groups by numerical analysis. The H. spinosissimum rhizobia formed the first group and the second group comprised the strains from H. coronarium and H. flexuosum. The reference Rhizobium leguminosarum bv. viceae strain 250A was clustered with the rhizobia from H. coronarium and H. flexuosum. By contrast Bradyrhizobium sp. (Arachis) reference strain 280A was not clustered with any of the strains tested, indicating that the H. spinosissimum rhizobia differ from both Rhizobium and Bradyrhizobium. Serological data also discriminate between H. spinosissimum and H. coronariumrhizobia but not between the latter and H. flexuosum strains. The strains tested exhibit a high degree of specificity for nodulation and nitrogen fixation. We also determined the16SrRNA gene sequence of H. spinosissimum rhizobia (four strains), H. coronarium (two strains) and H. flexuosum (two strains) and found that the four H. spinosissimum isolates share a 98% identity among each other in this region but they showed less than 92% identity to the H. coronarium and H. flexuosum isolates. The H. spinosissimum isolates were closely related to both Mesorhizobium loti and M. ciceri, sharing 97% identity with each species.

Gene Transfer in the Environment Promotes the Rapid Evolution of a Diversity of Suboptimal and Competitive Rhizobia for Biserrula pelecinus L.

The emergence of biodiversity in rhizobia after the introduction of exotic legumes and their respective rhizobia to new is a challenge for contemporary rhizobiology. Biserrula pelecinus L. is a pasture legume species that was introduced to Australia from the Mediterranean basm and which is having a substantial impact on agricultural productivity on acidic and sandy soils of Western Australia and New South Wales (Howieson et al., 2000). This deep-rooted plant is also valuable in reducing the development of dryland salinity This legume is nodulated by a specific group of root-nodule bacteria that belongs to Mesorhizobium (Nandasena et at , 2001, 2007).

New Look at Old Root-Nodule Bacteria: Molecular Techniques Uncover Novel Isolates

Exotic pasture legumes and their associated microsymbionts are important in providing biological nitrogen fixation in Australian agricultural systems. Southern African species of Lotononis from the Listia section can potentially provide sustainable agricultural productivity in systems affected by increasing dryland salinity and climate change. There are eight species in the Listia section: L. angolensis, L. bainesii, L. macrocarpa, L. marlothii, L. minima, L. subulata and L. solitudinis (Van Wyk, 1991). They are perennial, stoloniferous and collar-nodulated. The root-nodule bacteria (RNB) isolated from several of these species are pigmented and the symbiosis between these RNB and their hosts is highly specific (Yates et al., 2007). Pioneering work on L. angolensis, L. bainesii and L. listii isolates was performed in Africa in the 1950–60s by Botha (Kenya), Sandman (Zimbabwe) and Verboom (Zambia) and in Australia (Norris, 1958).