Rhizobia are soil bacteria that form important symbiotic associations with legumes, and rhizobial surface polysaccharides, such as K-antigen polysaccharide (KPS) and lipopolysaccharide (LPS), might be important for symbiosis. Previously, we obtained a mutant of Sinorhizobium fredii HH103, rkpA, that does not produce KPS, a homopolysaccharide of a pseudaminic acid derivative, but whose LPS electrophoretic profile was indistinguishable from that of the wild-type strain. Also, we previously demonstrated that the HH103 rkpLMNOPQ operon is responsible for Pse5NAc7(3OHBu) production and is involved in HH103 KPS and LPS biosynthesis and that an HH103 rkpM mutant cannot produce KPS and displays an altered LPS structure. Here, we analyzed the LPS structure of HH103 rkpA, focusing on the carbohydrate portion and found that it contains a highly heterogeneous lipid A and a peculiar core oligosaccharide composed of an unusually high number of hexuronic acids and containing β-configured 5-acetamido-3,5,7,9-tetradeoxy-7-(3-hydroxybutyramido)-L-glycero-L-manno-nonulosonic acid [β-Pse5NAc7(3OHBu)]. This pseudaminic acid derivative, in its α-configuration, was the only structural component of the S. fredii HH103 KPS and, to the best of our knowledge, has never been reported from any other rhizobial LPS. We also show that Pse5NAc7(3OHBu) is the complete or partial epitope for a monoclonal antibody, NB6-228.22, that can recognize the HH103 LPS, but not those of most of the S. fredii strains tested here. We also show that the LPS from HH103 rkpM is identical to that of HH103 rkpA, but devoid of any Pse5NAc7(3OHBu) residues. Notably, this rkpM mutant was severely impaired in symbiosis with its host Macroptilium atropurpureum.

China is the geographical origin of cultured soybeans and is where they have been cultivated for more than 5,000 years. Soybean is grown in nearly all provinces of China, the exceptions being Qinghai and the Tibetan Plateau. Based on climatic differences, China can be divided into five main geographical regions as regards soybean cultivation. As a complementary classification, Chinese soybean varieties have also been classified according to the sowing date; they were divided into winter-, spring-, summer-, and autumn-sowing varieties. In spite of the soybean acreage in China (9.4 million ha in 2001), which ranks fourth in the world, China has also become the largest soybean importer in the world. Although China has greatly improved soybean productivity in the last 20 years, its total production (or average seed yield) is still clearly lower than that reached in western countries. The symbiotic interaction, which soybean forms with different rhizobia, is a key factor in increasing soybean productivity in China within the context of agricultural sustainability. Research efforts in China have mainly concentrated on the identification and characterisation of biological material, both soybean accessions and rhizobial strains that effectively nodulate soybeans. As a result of this activity, a large collection of soybean germplasm is available and the existence of a broad spectrum of bacteria that nodulate soybeans (which we will collectively call "soybean-rhizobia") has been demonstrated.However, although soybean

Split-root systems (SRS) constitute an appropriate methodology for studying the relevance of both local and systemic mechanisms that participate in the control of rhizobia-legume symbioses. In fact, this kind of approach allowed to demonstrate the autoregulation of nodulation (AON) systemic response in soybean in the 1980s. In SRS, the plant main root is cut and two lateral roots that emerge from the seedlings after root-tip removal are confined into separate compartments. After several days of growth, these plants have two separate roots that can be inoculated with the same or with different bacteria, at the same or at different times. In this work, we have used a non-destructive SRS to study two different competitiveness relations between rhizobial strains in soybean roots. One of them is the competition for nodulation between two soybean-nodulating rhizobia: the slow-grower Bradyrhizobium japonicum USDA110 and the fast-grower Sinorhizobium fredii HH103. The second is the competitive blocking of S. fredii 257DH4 nodulation in the American soybean Osumi by Sinorhizobium fredii USDA257, which is unable to nodulate American soybeans. Our results showed that the competitiveness relationships studied in this work are mitigated or even avoided when the competitive strains are spatially separated in different compartments containing half-roots from the same plant, suggesting that competitive relations are more related to local plant responses. In our opinion, split-root systems are an appropriate approach to further study competitive relations among rhizobial strains.

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