Pierre Frendo

The redox state, a referee of the legume-Rhizobia symbiotic game.

The symbiosis between legumes andRhizobia leads to the formation of a new organ, the nodule, where nitrogen fixation takes place. The plant provides bacteria with energy and amicro-aerobic environment compatiblewithnitrogenase activity. In exchange, bacteria provide a nitrogen supply to the plant. Therefore, nodules represent a unique model for the study of developmental processes, plant–microbe, and carbon/nitrogen/oxygen metabolism interactions. Reactive oxygen species (ROS) have been described to be produced at every step of the symbiotic association: during the symbiosis establishment, during the nitrogen fixation and during the senescence of the nodule. ROS can be harmful provoking oxidative damage or can have a positive role in signalling processes. To deal with ROS production, nodules are fitted with a large panel of enzymatic and non-enzymatic antioxidantmechanisms. This review summarizes the present knowledge about ROS production and scavenging systems in legume nodules, and about their functional roles during the different steps of the symbiotic interaction.

The Legume Root Nodule: From Symbiotic Nitrogen Fixation to Senescence

Biological nitrogen fixation (BNF) is the biological process by which the atmospheric nitrogen (N2) is converted to ammonia by an enzyme called nitrogenase. It is the major source of the biosphere nitrogen and as such has an important ecological and agronomical role, accounting for 65 % of the nitrogen used in agriculture worldwide. The most important source of fixed nitrogen is the symbiotic association between rhizobia and legumes. The nitrogen fixation is achieved by bacteria inside the cells of de novo formed organs, the nodules, which usually develop on roots, and more occasionally on stems. This mutualistic relationship is beneficial for both partners, the plant supplying dicarboxylic acids as a carbon source to bacteria and receiving, in return, ammonium. Legume symbioses have an important role in environment-friendly agriculture. They allow plants to grow on nitrogen poor soils and reduce the need for nitrogen inputs for leguminous crops, and thus soil pollution. Nitrogen-fixing legumes also contribute to nitrogen enrichment of the soil and have been used from Antiquity as crop-rotation species to improve soil fertility. They produce high protein-containing leaves and seeds, and legumes such as soybeans, groundnuts, peas, beans, lentils, alfalfa and clover are a major source of protein for human and animal consumption. Most research concentrates on the two legume-rhizobium model systems Lotus-Mesorhizobium loti and Medicago-Sinorhizobium meliloti, with another focus on the economically-important Glycine max (soybean) -Bradyrhizobium japonicum association. The legume genetic models Medicago truncatula and Lotus japonicus have a small genome size of ca. 450 Mbp while Glycine max has a genome size of 1,115 Mbp, and all are currently targets of large-scale genome sequencing projects (He et al., 2009; Sato et al., 2008; Schmutz et al., 2010). The complete genome sequence of their bacterial partners has been established (Galibert et al., 2001; Kaneko et al., 2000; Kaneko et al., 2002; Schneiker-Bekel et al., 2011).

Characterisation of a cDNA encoding gamma-glutamylcysteine synthetase in Medicago truncatula.

A gamma-ECS cDNA from Medicago truncatula was isolated using an Arabidopsis thaliana cDNA as probe. The analysis of the amino acid sequence deduced from this cDNA revealed 80% identity with the gamma-ECS from A. thaliana and Brassica juncea and suggested a plastidial localisation for the enzyme. Gamma-ECS activity and high level of GSH were detected in the gamma-ECS-deficient E. coli strain expressing a fusion protein containing the M. truncatula gamma-ECS protein. Southern blot analysis suggests that gamma-ECS is encoded by a small multigenic family in M. truncatula and shows that homologous genes are present in two other leguminous plants, Medicago sativa and Pisum sativum. Gamma-ECS gene expression was analysed by Northern blot in seedlings, plantlets and mature plants.

Synthesis and Roles of Glutathione and Homoglutathione in the Nitrogen-Fixing Symbiosis

Glutathione (GSH) is a major antioxidant molecule in plants. It is involved in regulating plant development and responses to abiotic and biotic environment changes. In leguminous plants, a GSH homolog, homoglutathione is also found. Most legumes can develop a symbiotic interaction with soil bacteria of the rhizobium family under nitrogen deficiency. This symbiosis allows the reduction of atmospheric nitrogen by the bacteria in plant organs called root nodules. In this chapter, we summarize studies that describe the synthesis and the roles of GSH and hGSH in the nitrogen-fixing symbiosis.

Radical Formation During Soybean Nodule Senescence

Leghemoglobin (Lb) is a key protein in the symbiosis between Rhizobiaceae and leguminous plants. This hemoprotein ensures an adequate supply of O2 to the microsymbiont partner through the peribacteroid membrane (PBM) separating the Rhizobium from the host cell cytosol. The oxygenated form of Lb (oxyLb) can undergo a slow autoxidation to the ferric form (metLb) giving rise to the superoxide radical (O2 −) and hence hydrogen peroxide (H2O2) by O2 − dismutation. Ferrous Lb has been shown to react with H2O2 to form ferrylLb (LbIV), which is unable to transport oxygen.