page_title: Root nodule
section_title: Nodulation
hierarchical_section_title: Root nodule / Nodulation
caption_attribution_description: English: Transmission electron microscope image of a cross section though a soybean (Glycine max.Essex) root nodule (endophyte. The bacteria, Bradyrhyzobium japonicum, infects the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteriods (bacterium-like cell or modified bacterial cell) within their symbiosomes. In his image, you can also see endoplasmic reticulum, dictysome and cell wall. Français&#160;: Vue au microscope électronique à transmission d'une section transversale d'un nodule racinaire bactérien endophyte dans une racine de soja (Glycine max.Essex). La bactérie (Bradyrhyzobium japonicum) a colonisé l'intérieur des racines du soja et y a établi une symbiose fixatrice d'azote. Cette image (en fort grossissement) montre le réticulum endoplasmique, le dictysome et la paroi cellulaire.
context_page_description: Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans, peas, and soybeans.
  Within legume root nodules, nitrogen gas from the atmosphere is converted into ammonia, which is then assimilated into amino acids, nucleotides, and other cellular constituents such as vitamins, flavones, and hormones. Their ability to fix gaseous nitrogen makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed, high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf. Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive.
context_section_description: Legumes release organic compounds as secondary metabolites called flavonoids from their roots, which attract the rhizobia to them and which also activate nod genes in the bacteria to produce nod factors and intitiate nodule formation.  These nod factors initiate root hair curling. The curling begins with the very tip of the root hair curling around the Rhizobium.  Within the root tip, a small tube called the infection thread forms, which provides a pathway for the Rhizobium to travel into the root epidermal cells as the root hair continues to curl. 
  Partial curling can even be achieved by nod factor alone. This was demonstrated by the isolation of nod factors and their application to parts of the root hair.  The root hairs curled in the direction of the application, demonstrating the action of a root hair attempting to curl around a bacterium.  Even application on lateral roots caused curling.  This demonstrated that it is the nod factor itself, not the bacterium that causes the stimulation of the curling.
  When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: cell division is triggered in the root to create the nodule, and the root hair growth is redirected to curl around the bacteria multiple times until it fully encapsulates one or more bacteria. The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; they are then surrounded by a plant-derived symbiosome membrane and differentiate into bacteroids that fix nitrogen.
  Effective nodulation takes place approximately four weeks after crop planting, with the size, and shape of the nodules dependent on the crop. Crops such as soybeans, or peanuts will have larger nodules than forage legumes such as red clover, or alfalfa, since their nitrogen needs are higher. The number of nodules, and their internal color, will indicate the status of nitrogen fixation in the plant.
  Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). Autoregulation of nodulation controls nodule numbers per plant through a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly developing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean (Glycine max); HAR1 in Lotus japonicus, SUNN in Medicago truncatula) are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany the loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked. Investigations into the mechanisms of nodule formation showed that the ENOD40 gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3].
caption: Cross section through a soybean root nodule. The bacterium, Bradyrhizobium japonicum, colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their symbiosomes. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.
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