Root nodules are found on theroots ofplants, primarilylegumes, that form asymbiosis withnitrogen-fixing bacteria.[1] Undernitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known asrhizobia.[2] This process has evolved multiple times within the legumes, as well as in other species found within theRosid clade.[3]Legume crops includebeans,peas, andsoybeans.
Within legume root nodules, nitrogen gas (N2) from the atmosphere is converted intoammonia (NH3), which is then assimilated intoamino acids (the building blocks of proteins),nucleotides (the building blocks ofDNA andRNA as well as the important energy moleculeATP), and other cellular constituents such asvitamins,flavones, andhormones.[citation needed] Their ability tofix 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 (a product ofphotosynthesis).Malate as a breakdown product ofsucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein calledleghaemoglobin, closely related to animalmyoglobin, to facilitate the diffusion of oxygen gas used in respiration.
Plants that contribute to N2 fixation include thelegume family –Fabaceae – with taxa such askudzu,clovers,soybeans,alfalfa,lupines,peanuts, androoibos. They containsymbiotic bacteria calledrhizobia within the nodules, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants, and this helps to fertilize thesoil.[4][5] The great majority of legumes have this association, but a few genera (e.g.,Styphnolobium) do not. In many traditional farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of a leguminous crop such as clover, in order to take advantage of this.
Although by far the majority of plants able to form nitrogen-fixing root nodules are in the legume familyFabaceae, there are a few exceptions:
The ability to fix nitrogen is far from universally present in these families. For instance, of 122 genera in theRosaceae, only 4genera are capable of fixing nitrogen. All these families belong to theordersCucurbitales,Fagales, andRosales, which together with theFabales form anitrogen-fixing clade (NFC) ofeurosids. In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen may beplesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basicgenetic andphysiological requirements were present in an incipient state in thelast common ancestors of all these plants, but only evolved to full function in some of them:
| Family: Genera Betulaceae:Alnus (alders) | ...... |
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Two main types of nodule have been described in legumes: determinate and indeterminate.[9]
Determinate nodules are found on certain tribes of tropical legume such as those of the generaGlycine (soybean),Phaseolus (common bean), andVigna. and on some temperate legumes such asLotus. These determinate nodules lose meristematic activity shortly after initiation, thus growth is due to cell expansion resulting in mature nodules which are spherical in shape. Another type of determinate nodule is found in a wide range of herbs, shrubs and trees, such asArachis (peanut). These are always associated with the axils of lateral or adventitious roots and are formed following infection via cracks where these roots emerge and not usingroot hairs. Their internal structure is quite different from those of thesoybean type of nodule.[10]
Indeterminate nodules are found in the majority of legumes from all three sub-families, whether in temperate regions or in the tropics. They can be seen inFaboideae legumes such asPisum (pea),Medicago (alfalfa),Trifolium (clover), andVicia (vetch) and allmimosoid legumes such asacacias, the few nodulatedcaesalpinioid legumes such aspartridge pea. They earned the name "indeterminate" because they maintain an active apicalmeristem that produces new cells for growth over the life of the nodule. This results in the nodule having a generally cylindrical shape, which may be extensively branched.[10] Because they are actively growing, indeterminate nodules manifest zones which demarcate different stages of development/symbiosis:[11][12][13]
This is the most widely studied type of nodule, but the details are quite different in nodules of peanut and relatives and some other important crops such as lupins where the nodule is formed following direct infection of rhizobia through the epidermis and where infection threads are never formed. Nodules grow around the root, forming a collar-like structure. In these nodules and in the peanut type the central infected tissue is uniform, lacking the uninfected ells seen in nodules of soybean and many indeterminate types such as peas and clovers.
Actinorhizal-type nodules are markedly different structures found in non-legumes. In this type, cells derived from the root cortex form the infected tissue, and the prenodule becomes part of the mature nodule. Despite this seemingly major difference, it is possible to produce such nodules in legumes by a singlehomeotic mutation.[14]
Legumes releaseorganic compounds assecondary metabolites calledflavonoids from their roots, which attract the rhizobia to them and which also activatenod genes in the bacteria to producenod factors and initiate nodule formation.[15][16] Thesenod factors initiateroot hair curling. The curling begins with the very tip of the root hair curling around theRhizobium. Within the root tip, a small tube called the infection thread forms, which provides a pathway for theRhizobium to travel into the root epidermal cells as the root hair continues to curl.[17]
Partial curling can even be achieved bynod factor alone.[16] This was demonstrated by the isolation ofnod 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 thenod factor itself, not the bacterium that causes the stimulation of the curling.[16]
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 theroot 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 amicrocolony. From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of theepidermis cell, and onwards into theroot cortex; they are then surrounded by a plant-derivedsymbiosome membrane and differentiate into bacteroids thatfix nitrogen.[18]
Effective nodulation takes place approximately four weeks aftercrop 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.[19]
Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene).Autoregulation of nodulation[20] 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 inLotus japonicus, SUNN inMedicago 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 theENOD40 gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3].
Root nodules apparently have evolved three times within theFabaceae but are rare outside that family. 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 toabscisic acid may enable the later evolution of root nodules.[21]
Somefungi produce nodular structures known as tuberculateectomycorrhizae on the roots of their plant hosts.Suillus tomentosus, for example, produces these structures with its plant hostlodgepole pine (Pinus contorta var.latifolia). These structures have, in turn, been shown to hostnitrogen fixingbacteria, which contribute a significant amount ofnitrogen and allow the pines to colonize nutrient-poor sites.[22]
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