Summary of Rhibozium field experiments in 1988

87M11, 87H12, 88KA77, 88ME97, 88GE32, 88ME98, 88GE33A, 88GE33B, 88KA86, 88ME86. Title, aim, background, treatments, tables, results, discussion, conclusions

Summary of Medic-Rhizobium field experiments

83ME9 - Nutrition of R. meliloti in acid soil. - Meaningful results only available in 1984/85 Summary of Medic-Rhizobium Field Experiments 1983/84. 83ME87 - Effect of pH of culture conditions on survival in, and colonisation of, an acid soil by R. meliloti. - Meaningful results available 1984/85. (82ME22, 82ME22A, 82ME23) Survival and colonisation of acid soils by 18 strains of R. meliloti in association with 6 Medicago hosts. - Results discussed in this summary

Summary of Medic-Rhizobium field experiments 1983 – 1984

Trials 83ME9, 83ME87, 82ME22, 82ME22A, 82ME23. Method. Comment

Summary of Rhizobium experiments.

Rotation trials, 84ME32, 84ME24, 84N18, 84M18, 85KA58. Cross row trials, 85M28. One year trials, 86M41, 86M42. On-going trials, 86M58, 85KA64, 86V3, 86AL. 84WH24

The Legume – Rhizobia Symbiosis. Does It Vary for the Tropics Relative to the Mediterranean Basin?

Symbiotic N fixation from legumes is one of the most important biological processes on the planet. It currently provides the majority of the N requirement in agriculture, yet will have to double if cereal crop production is to meet world demand by 2020 (Kennedy and Cocking 1997). To effectively harness the value of biological N fixation from legumes we need to more fully understand G2 x E; where G refers to the genotypes of both the legume (Gl) and its microsymbiont (rhizobia; Gr), and E refers to the edaphic environment in which the symbiosis is to function. In the Mediterranean basin, indigenous legumes are nodulated by specific rhizobial genotypes. Despite co-evolution of the symbionts, their relationship is not always optimal for N fixation. It has been proposed that rhizobial genotypes are differentially adapted to soil conditions and that it is this adaptation, rather than the relationship with their host, that most strongly governs outcomes relating to competition and persistence (Sprent 1994, Howieson 2000). Expressed in terms of the above formula, symbiotic effectiveness (G2) is unimportant to the persistence and success of rhizobia relative to adaptation of the rhizobia to soil and climate (Gr x E). In this paper we investigate whether rhizobia which have co-evolved with tropical legumes show similar sub-optimal patterns of N fixation. Tropical legumes differ from Mediterranean legumes in that many nodulate promiscuously, and often effectively, with a broad range of rhizobial genotypes (both rhizobia and bradyrhizobia). This introduces the possibility that impacts of soil and climate on the persistence and success of some tropical rhizobia (Gr x E) are less important for optimal N fixation (G2) than they are in the Mediterranean region. However, observations on N fixation in promiscuous tropical legumes are mostly based on agricultural species associated with rhizobia from outside the centre of origin of the host legume. We examine N fixation by tropical rhizobia which have co-evolved with their host. We show that sub-optimal N fixation can be improved using several research options, the choice of which depends on the nature of the limitation to the legume or the performance of the rhizobia. We describe three scenarios that might compromise N fixation: a) where the legume is sown into soil containing a high population of variably effective rhizobia, b) where the rhizobial population is low, and c) where there is no background population of rhizobia capable of nodulation with the legume. Each scenario has presented with it several research strategies for improving symbiotic N fixation. These strategies have application for both Mediterranean and tropical environments

The genetics of symbiotic nitrogen fixation: comparative genomics of 14 Rhizobia Strains by resolution of protein clusters.

The symbiotic relationship between legumes and nitrogen fixing bacteria is critical for agriculture, as it may have profound impacts on lowering costs for farmers, on land sustainability, on soil quality, and on mitigation of greenhouse gas emissions. However, despite the importance of the symbioses to the global nitrogen cycling balance, very few rhizobial genomes have been sequenced so far, although there are some ongoing efforts in sequencing elite strains. In this study, the genomes of fourteen selected strains of the order Rhizobiales, all previously fully sequenced and annotated, were compared to assess differences between the strains and to investigate the feasibility of defining a core ?symbiome??the essential genes required by all rhizobia for nodulation and nitrogen fixation. Comparison of these whole genomes has revealed valuable information, such as several events of lateral gene transfer, particularly in the symbiotic plasmids and genomic islands that have contributed to a better understanding of the evolution of contrasting symbioses. Unique genes were also identified, as well as omissions of symbiotic genes that were expected to be found. Protein comparisons have also allowed the identification of a variety of similarities and differences in several groups of genes, including those involved in nodulation, nitrogen fixation, production of exopolysaccharides, Type I to Type VI secretion systems, among others, and identifying some key genes that could be related to host specificity and/or a better saprophytic ability. However, while several significant differences in the type and number of proteins were observed, the evidence presented suggests no simple core symbiome exists. A more abstract systems biology concept of nitrogen fixing symbiosis may be required. The results have also highlighted that comparative genomics represents a valuable tool for capturing specificities and generalities of each genome.bitstream/item/74069/1/ID-34062.pd