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 harbour Rhizobium bacteria (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– Rhizobium symbiosis 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 termed Rhizobium . However, other (related) groups of flowering plants form a root nodule symbiosis with filamentous actinobacteria of the genus Frankia . Some of the invasion processes adopted by Rhizobium and Frankia are shared with a mineral‐scavenging symbiosis involving arbuscular mycorrhizal fungi that originated over 400 Ma. Colonisation of host cells by Rhizobium usually 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.

Rhizobium-Linked Nutritional and Phytochemical Changes Under Multitrophic Functional Contexts in Sustainable Food Systems

“…Scarcity of organic carbon and nitrogen in the rhizosphere favor the establishment of endophytic populations of symbiotic rhizobia in root nodules of host plants (Coba de la Peña et al, 2018). Besides providing shelter for rhizobia, root nodules also supply the symbiotic bacteria with organic carbon and other nutrients (Brewin, 2010). Rhizobium in the root nodules fix atmospheric N 2 with the help of nitrogenase enzyme.…”

The Microbe-Free Plant: Fact or Artifact?

“…Multiple reviews have been published on every group of microorganisms that are regularly associated with plants. We refer to the reviews on fungal endophytes (Muller and Krauss, 2005 ; Schulz and Boyle, 2005 ; Hartley and Gange, 2009 ; Rodriguez et al, 2009 ; Gao et al, 2010 ; Saikkonen et al, 2010 ; Eaton et al, 2011 ), fungal entomopathogens (Vega et al, 2009 ), bacterial endosymbionts (Weyens et al, 2009 ), mycorrhiza (Smith and Read, 2008 ; Bonfante, 2010 ; Koltai and Kapulnik, 2010 ; Ercolin and Reinhardt, 2011 ), and Rhizobia (Kiers and Denison, 2008 ; Oldroyd and Downie, 2008 ; Markmann and Parniske, 2009 ; Brevin, 2010 ). Here, we only provide some short paragraphs mainly aimed at defining the various terms and biological groups and highlighting the most important resistance effects that form the focus of the present article.…”

“…The second classical and highly investigated textbook example of a plant–microbe mutualism is formed by plants of the family Fabaceae (“Legumes”) and bacteria ( Rhizobium and related genera, all belonging to the order Rhizobiales) that infect plant roots where they induce typical morphological structures called “nodules”(Kiers and Denison, 2008 ; Oldroyd and Downie, 2008 ; Markmann and Parniske, 2009 ). Within these nodules, the formerly mobile bacteria convert themselves into immobile bacteroids that exclusively live on plant-derived assimilates and in turn provide the plant with nitrogen that has been fixed from the atmosphere (Oldroyd and Downie, 2008 ; Brevin, 2010 ).…”

The Microbe‐Free Plant: Fact or Artifact

“…Multiple reviews have been published on every group of microorganisms that are regularly associated with plants. We refer to the reviews on fungal endophytes (Muller and Krauss, 2005;Schulz and Boyle, 2005;Hartley and Gange, 2009;Rodriguez et al, 2009;Gao et al, 2010;Saikkonen et al, 2010;Eaton et al, 2011), fungal entomopathogens (Vega et al, 2009), bacterial endosymbionts (Weyens et al, 2009), mycorrhiza (Smith and Read, 2008;Bonfante, 2010;Koltai and Kapulnik, 2010;Ercolin and Reinhardt, 2011), and Rhizobia (Kiers and Denison, 2008;Oldroyd and Downie, 2008;Markmann and Parniske, 2009;Brevin, 2010). Here, we only provide some short paragraphs mainly aimed at defining the various terms and biological groups and highlighting the most important resistance effects that form the focus of the present article.…”

“…The second classical and highly investigated textbook example of a plant-microbe mutualism is formed by plants of the family Fabaceae ("Legumes") and bacteria (Rhizobium and related genera, all belonging to the order Rhizobiales) that infect plant roots where they induce typical morphological structures called "nodules" (Kiers and Denison, 2008;Oldroyd and Downie, 2008;Markmann and Parniske, 2009). Within these nodules, the formerly mobile bacteria convert themselves into immobile bacteroids that exclusively live on plant-derived assimilates and in turn provide the plant with nitrogen that has been fixed from the atmosphere (Oldroyd and Downie, 2008;Brevin, 2010).…”