Policing the legume-Rhizobium symbiosis: a critical test of partner choice

In legume-Rhizobium symbioses, specialised soil bacteria fix atmospheric nitrogen in return for carbon. However, ineffective strains can arise, making discrimination essential. Discrimination can occur via partner choice, where legumes prevent ineffective strains from entering, or via sanctioning, where plants provide fewer resources. Several studies have inferred that legumes exercise partner choice, but the rhizobia compared were not otherwise isogenic. To test when and how plants discriminate ineffective strains we developed sets of fixing and non-fixing strains that differed only in the expression of nifH - essential for nitrogen fixation - and could be visualised using marker genes. We show that the plant is unable to select against the non-fixing strain at the point of entry, but that non-fixing nodules are sanctioned. We also used the technique to characterise mixed nodules (containing both a fixing and a non-fixing strain), whose frequency could be predicted using a simple diffusion model. We discuss that sanctioning is likely to evolve in preference to partner choice in any symbiosis where partner quality cannot be adequately assessed until goods or services are actively exchanged

Bacteria-inducing legume nodules involved in the improvement of plant growth, health and nutrition

Bacteria-inducing legume nodules are known as rhizobia and belong to the class Alphaproteobacteria and Betaproteobacteria. They promote the growth and nutrition of their respective legume hosts through atmospheric nitrogen fixation which takes place in the nodules induced in their roots or stems. In addition, rhizobia have other plant growth-promoting mechanisms, mainly solubilization of phosphate and production of indoleacetic acid, ACC deaminase and siderophores. Some of these mechanisms have been reported for strains of rhizobia which are also able to promote the growth of several nonlegumes, such as cereals, oilseeds and vegetables. Less studied are the mechanisms that have the rhizobia to promote the plant health; however, these bacteria are able to exert biocontrol of some phytopathogens and to induce the plant resistance. In this chapter, we revised the available data about the ability of the legume nodule-inducing bacteria for improving the plant growth, health and nutrition of both legumes and nonlegumes. These data showed that rhizobia meet all the requirements of sustainable agriculture to be used as bio-inoculants allowing the total or partial replacement of chemicals used for fertilization or protection of crops

Ecological distribution conflicts and the vocabulary of environmental justice

Unidad de excelencia María de Maeztu MdM-2015-0552There is a fundamental clash between economy and the environment due to the growing social metabolism of industrial economies. Energy cannot be recycled. Therefore, the energy from the fossil fuels is used only once, and new supplies of coal, oil, and gas must be obtained from the "commodity extraction frontiers". Similarly, materials are recycled only in part, and therefore, even an economy that would not grow would need fresh supplies of iron ore, bauxite, copper, and paper pulp. The industrial economy is entropic. Meanwhile, permanent "funds" such as aquifers, forests, and fisheries are overexploited, the fertility of the soil is jeopardized and biodiversity is depleted. Thus, the changing social metabolism of industrial economies (including waste disposal such as the excessive production of carbon dioxide) gives rise to growing numbers of ecological distribution conflicts that sometimes overlap with other social conflicts on class, ethnicity or indigenous identity, gender, caste, or territorial rights. The term Ecological Distribution Conflicts (EDC) was coined to describe social conflicts born from the unfair access to natural resources and the unjust burdens of pollution. Such conflicts give birth to movements of resistance, to the point that we can speak already of a global movement for Environmental Justice