Cross section though a soybean (Glycine max. Essex) root nodule. The bacteria, Bradyrhizobium japonicum, infects the roots and establishes a nitrogen fixing symbiosis. Bradyrhizobium is a genus of Gram-negative Soil Bacteria, many of which fix nitrogen. This high magnification image shows part of a cell with single bacteroids within their symbiosomes. In this image, you can also see endoplasmic reticulum, dictysome and cell wall.

Root nodules occur on the roots of plants that associate with Vigna bacteria. This article is about the biological phenomenon for other uses see Symbiosis (disambiguation The term symbiosis (from the Greek The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have

Under nitrogen limiting conditions, plants from the pea family Fabaceae form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Fabaceae or Leguminosae is a large and economically important family of Flowering plants which is commonly known as the legume family, pea Rhizobia (from the Greek words rhiza = root and bios = Life are Soil bacteria that fix Nitrogen ( Diazotrophy

Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which then is assimilated by the plant to form the basis for amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. The nitrogen fixation property makes legumes an ideal agricultural organism as their requirement for nitrogen fertiliser is reduced. Indeed high nitrogen content blocks nodule development. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called hemoglobin, closely related to animal myoglobin, to facilitate the conversion of nitrogen gas to ammonia.

Classification

Two main types of nodule have been described.

Temperate legumes like Pisum, Medicago, Trifolium, and Vicia develop a cylindrical shaped nodule that is called "indeterminate" because it maintains an active apical meristem that produces new cells for growth over the life of the nodule. A legume is a Plant in the family Fabaceae (or Leguminosae or a Fruit of these specific plants Pisum is a genus of the family Fabaceae, native to southwest Asia and northeast Africa. Medicago (family Fabaceae, the Legume family is a genus of flowering plants commonly known as medick or burclover. Alsike redirects here Alsike Sweden is also a town in the Knivsta Municipality, Sweden. Vicia ( Vetches) is a large genus of about 140 species of Flowering plants in the family Fabaceae, native to Europe, A meristem is a tissue in all Plants consisting of undifferentiated cells ( meristematic cells) and found in zones of the plant where growth can take place The genus Lupinus is nodulated by the soil microorganism Bradyrhizobium sp. (Lupinus). Bradyrhizobia are encountered as microsymbionts in other leguminous crops (Argyrolobium, Lotus, Ornithopus, Acacia, Lupinus) of Mediterranean origin

Tropical (sub)legumes from the genera Glycine, Phaseolus, Lotus, and Vigna form "determinate" nodules, that lose meristematic activity shortly after initiation. Glycine is a genus in the Bean family Fabaceae. The most well known species is the Soybean ( Glycine max) Phaseolus ( Bean, Wild Bean) is a Genus in the Family Fabaceae of about fifty Plant Species, See Lotus for other uses including several other plant taxa bearing this name The vagina (from Latin, literally " Sheath " or " Scabbard " is a fibromuscular tubular tract leading from the Uterus Growth is due to cell expansion, and mature nodules are spherical in shape.

Nodulation

Legumes release compounds called flavonoids from their roots, which trigger the production of nod factors by the bacteria. The term flavonoid (or bioflavonoid refers to a class of Plant Secondary metabolites According to the IUPAC nomenclature they can be classified into Nodulation (Nod factors are Signaling molecules produced by Bacteria known as Rhizobia during the initiation of nodules on the root of Legumes 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 wind around the bacteria multiple times until it fully encapsulates 1 or more bacteria. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. A root hair is a tubular outgrowth of Root epidermal cells of Vascular plants They are found only in the region of maturation of the root The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through a structure called an 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 membrane and differentiate into bacteroids that fix nitrogen. The epidermis is the outer single-layered group of cells covering a Plant, especially the Leaf and young tissues of a Vascular plant including stems In Botany, the cortex is the outer of the stem or Root of a plant bounded on the outside by the epidermis and on the inside by the Endodermis Nitrogen fixation is the process by which Nitrogen is taken from its natural relatively inert molecular form (N2 in the atmosphere and converted into

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 develing 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.

Connection to root structure

Root nodules apparently have evolved three times within the Fabaceae but are rare outside that family. Fabaceae or Leguminosae is a large and economically important family of Flowering plants which is commonly known as the legume family, pea The propensity of these plants to develop root nodules seems to relate to their root structure. In particular, a tendency to develop lateral roots in response to abscisic acid may enable the later evolution of root nodules. Abscisic acid (ABA also known as abscisin II and dormin, is a Plant hormone. ^[1]

In other species

A sectioned Alder tree root nodule.

A whole Alder tree root nodule.

Root nodules that occur on non-legume genera like Parasponia in association with Rhizobium bacteria, and those that arise from symbiotic interactions with Actinobacteria Frankia in some plant genera such as Alnus, vary significantly from those formed in the legume-rhizobia symbiosis. Actinobacteria or actinomycetes are a group of Gram-positive bacteria with high G+C ratio. This article refers to the bacteria Frankia was also one of the names of the Frankish Empire. Alder is the common name of a Genus of Flowering plants ( Alnus) belonging to the Birch family (Family Betulaceae) In these symbioses the bacteria are never released from the infection thread.

References

1. ^ Yan Liang and Jeanne M. Harris (2005). "Response of root branching to abscisic acid is correlated with nodule formation both in legumes and nonlegumes". American Journal of Botany 92: 1675–1683.