Root nodule

[1] Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia.

Within legume root nodules, nitrogen gas (N2) from the atmosphere is converted into ammonia (NH3), which is then assimilated into 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.

Legume nodules harbor an iron containing protein called leghaemoglobin, closely related to animal myoglobin, to facilitate the diffusion of oxygen gas used in respiration.

Plants that contribute to N2 fixation include the legume family – Fabaceae – with taxa such as kudzu, clovers, soybeans, alfalfa, lupines, peanuts, and rooibos.

All these families belong to the orders Cucurbitales, Fagales, and Rosales, which together with the Fabales form a nitrogen-fixing clade (NFC) of eurosids.

In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the last common ancestors of all these plants, but only evolved to full function in some of them: Betulaceae: Alnus (alders) Cannabaceae: Trema Casuarinaceae: ......

[9] Determinate nodules are found on certain tribes of tropical legume such as those of the genera Glycine (soybean), Phaseolus (common bean), and Vigna.

Another type of determinate nodule is found in a wide range of herbs, shrubs and trees, such as Arachis (peanut).

They earned the name "indeterminate" because they maintain an active apical meristem that produces new cells for growth over the life of the nodule.

In this type, cells derived from the root cortex form the infected tissue, and the prenodule becomes part of the mature nodule.

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.

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 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).

Suillus tomentosus, for example, produces these structures with its plant host lodgepole pine (Pinus contorta var.

A simplified diagram of the relation between the plant and the symbiotic bacteria (cyan) in the root nodules
Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N 2 ) to ammonia (NH 3 ) in a process called nitrogen fixation . Ammonia is then assimilated into nucleotides , amino acids , vitamins and flavones which are essential to the growth of the plant. The plant root cells convert sugar into organic acids which then supply to the rhizobia in exchange, hence a symbiotic relationship between rhizobia and the legumes.
Indeterminate nodules growing on the roots of Medicago italica
Diagram illustrating the different zones of an indeterminate root nodule (see text).
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.
Nitrogen-fixing nodules on a clover root.