Root Nodules (Legume–RhizobiumSymbiosis)

Abstract Root nodule symbiosis enables nitrogen‐fixing bacteria to convert atmospheric nitrogen into a form that is directly available for plant growth. Biological nitrogen fixation provides a built‐in supply of nitrogen fertiliser for many legume crops such as peas, beans and clover. Legumes (Fabales) interact with single‐celled Gram‐negative bacteria, collectively termed rhizobia, whereas members of three other Rosid orders (Fagales, Cucurbitales and Rosales) interact with Gram‐positive filamentous actinobacteria of the genus Frankia . In legumes, infection proceeds through intercellular and trans‐cellular channels termed infection threads. At the same time, cells in the root cortex are induced to divide and generate the tissues of the nodule. Nitrogen fixation normally takes place within specialised bacteroid cells enclosed within organelle‐like cytoplasmic compartments termed symbiosomes. The anatomy and physiology of root nodules both reflect a high degree of structural and metabolic integration between plant and microbial symbionts. Key Concepts: Legumes (family Fabales) develop root nodules that harbourRhizobiumbacteria (rhizobia). Endosymbiotic bacteria (bacteroids) convert nitrogen to ammonia (biological nitrogen fixation). Legume crops restore fertility to agricultural soils by capturing nitrogen from the atmosphere. The legume–Rhizobiumsymbiosis provides one‐fifth of all nitrogen inputs into global agriculture. Symbiosis is based on metabolic exchange for mutual benefit: exchanges of oxygen, carbon and nitrogen are tightly regulated. Legumes only form a nitrogen‐fixing symbiosis with single‐celled bacteria collectively termedRhizobium. However, other (related) groups of flowering plants form a root nodule symbiosis with filamentous actinobacteria of the genusFrankia. Some of the invasion processes adopted byRhizobiumandFrankiaare shared with a mineral‐scavenging symbiosis involving arbuscular mycorrhizal fungi that originated over 400 Ma. Colonisation of host cells byRhizobiumusually involves specialised invasion structures termed ‘infection threads’ and specialised organelle‐like compartments termed ‘symbiosomes’. During evolution, there has been horizontal transfer of the genes that specify the capability for nodulation and symbiotic nitrogen fixation between diverse groups of soil bacteria.