Symbiosis genes, symbiovars and mechanisms of specificity in the legume-rhizobium symbiosis

Most of the ~ 23,000 legume (Fabaceae) species can fix atmospheric nitrogen (N2) via symbiotic bacteria (‘rhizobia’) in root nodules. N2-fixing legume species are components of most of the world's main vegetation types and agricultural systems and as such, legume N2 fixation plays an important role in the global N cycle. Currently, ~260 rhizobium species within 18 bacterial genera have been formally described. Legume species exhibit specificity in their rhizobial symbionts and vice versa. The symbiosis genes that enable rhizobial strains to induce N2 fixing nodules on legumes (nod, nif and fix genes) are encoded on transferable plasmids or integrative and conjugative elements and horizontal transfer of these genes can occur between bacterial strains within and across genera. Symbiovars are defined as strains of the same or different rhizobial species that establish an N2 fixing symbiosis with distinct legume species and can additionally be distinguished by specific symbiosis gene phylogenies.This Research Topic focusses on specificity in the legume-rhizobium symbiosis. Generally, nodulation is initiated by legume production of a rhizobium-specific mix of compounds from roots into the rhizosphere. These compounds diffuse to compatible rhizobia and activate their nodulation protein D (NodD) and consequently, a range of genes including nodABC which encode the enzymes required for synthesis of the core Nod factor structure, the signal molecule from the rhizobium that induces nodule morphogenesis in compatible legumes. Nod factors cause changes to legume root hairs that allow rhizobial entry via root hair infection. Here, rhizobium cells are released from the tips of infection threads into nodule primordia (symbiosomes), where they synthesise the N2 fixing enzyme nitrogenase and differentiate into their N2-fixing form (bacteroids) in mature nodules that can be indeterminate or determinate in growth depending on legume host. Rhizobial nif and fix genes are involved in the synthesis and function of nitrogenase. There are exceptions to this general nodulation process. For example, nodulation can be independent of Nod factors and infection threads, and rhizobia enter the root intercellularly. Also, specificity can occur at the N2 fixation stage (nodules form but don’t fix N2) and nodule maturity as for sanctioning of rhizobial ‘cheaters’.We invite submission of research articles, reviews, case studies and opinion papers that increase our knowledge and understanding of specificity in the legume-rhizobium symbiosis. Research areas covered include:• Description of novel legume-rhizobium symbioses, novel rhizobia and novel symbiovars• Detailed characterisation of formally described symbiovars• Signal exchange between legume and rhizobium e.g. flavonoids from the legume, Nod factors and exopolysaccharides from the rhizobium• Additional factors promoting the legume-rhizobium symbiosis eg rhizobium type III secretion systems, exopolysaccharide production, flagellae• Mechanisms of specificity in the legume-rhizobium symbiosis from nodule initiation to their maturity• Mechanisms of transfer of sym genes between bacterial strains• Consequences of transfer of sym genes between bacterial strains• Introduction of legume crops with specific rhizobium symbionts into agricultural systems• Establishment/ re-establishment of legumes with specific rhizobial symbionts in ‘natural’ systems, e.g. reforestation• Rhizobial strain selection for improved crop production Topic editors: Mitchell Andrews, Gustavo Santoyo, Maren Friesen Source: https://www.frontiersin.org/research-topics/79465/symbiosis-genes-symbiovars-and-mechanisms-of-specificity-in-the-legume-rhizobium-symbiosis