● `color{Brown}"Nodule formation"` involves a sequence of `color{violet}"multiple interactions"` between `color{violet}"𝘙𝘩𝘪𝘻𝘰𝘣𝘪𝘶𝘮"` and `color{violet}"roots"` of the host plant.

● `color{violet}"Principal stages"` in the nodule formation are summarised as follows:

● `color{violet}"Rhizobia multiply and colonise"` the surroundings of roots and get attached to `color{violet}"epidermal"` and `color{violet}"root hair cells"`.

● The `color{violet}"root-hairs curl"` and the `color{violet}"bacteria invade"` the root-hair.

● An `color{violet}"infection thread"` is produced carrying the bacteria into the `color{violet}"cortex of the root"`, where they initiate the `color{violet}"nodule formation"` in the cortex of the root.

● Then the `color{violet}"bacteria are released"` from the thread into the cells which leads to the `color{violet}"differentiation"` of specialised `color{violet}"nitrogen fixing cells"`.

● The `color{violet}"nodule thus formed,"` establishes a direct `color{violet}"vascular connection"` with the host for exchange of nutrients.

● These events are depicted in Figure.



● The nodule contains all the `color{violet}"necessary biochemical components"`, such as the enzyme `color{Brown}"nitrogenase and leghaemoglobin"`.

● The enzyme `color{violet}"nitrogenase is a Mo-Fe protein"` and catalyses the conversion of `color{violet}"atmospheric nitrogen"` to `color{violet}"ammonia"`,the first stable product of nitrogen fixation.

● The reaction is as follows: –



● The enzyme `color{violet}"nitrogenase is highly sensitive"` to the `color{Brown}"molecular oxygen"`; it requires `color{violet}"anaerobic conditions"`.

● The nodules have adaptations that ensure that the `color{violet}"enzyme is protected"` from oxygen.

● To protect these enzymes, the nodule contains an `color{violet}"oxygen scavenger"` called `color{Brown}"leg-haemoglobin"`.

● It is interesting to note that these `color{violet}"microbes live as aerobes"` under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become `color{violet}"anaerobic"` (thus protecting the nitrogenase enzyme).

● You must have noticed in the above reaction that the `color{violet}"ammonia synthesis by nitrogenease"` requires a very `color{violet}"high input of energy"` (`color{Brown}"8 ATP"` for each `NH_3` produced).

● The `color{violet}"energy required"`, thus, is obtained from the `color{violet}"respiration"` of the host cells.

● At `color{violet}"physiological pH"`, the `color{violet}"ammonia is protonated"` to form `NH_4^+` `color{violet}"(ammonium) ion"`.

● While most of the plants can assimilate `color{violet}"nitrate as well as ammonium ions"`, the latter is `color{violet}"quite toxic"` to plants and hence `color{violet}"cannot accumulate"` in them.

● Let us now see how the `NH_4^+` is used to `color{violet}"synthesise amino acids"` in plants.

● There are `color{violet}"two main ways"` in which this can take place:

(i) `color{Green}"Reductive amination:"`

● In these processes, `color{violet}"ammonia"` reacts with `color{violet}"α-ketoglutaric acid"` and forms `color{violet}"glutamic acid"` as indicated in the equation given below :



(ii) `color{Green}"Transamination"`:

● It involves the `color{violet}"transfer of amino group"` from one amino acid to the `color{violet}"keto group"` of a keto acid.

● `color{violet}"Glutamic acid"` is the main amino acid from which the transfer of `NH_2` , the `color{violet}"amino group"` takes place and other amino acids are formed through `color{violet}"transamination"`.

● The enzyme `color{Brown}"transaminase"` catalyses all such reactions.

● For example,



● The two most `color{violet}"important amides"` – `color{Brown}"asparagine and glutamine"` – found in plants are a structural part of proteins.

● They are formed from `color{violet}"two amino acids"`, namely `color{violet}"aspartic acid"` and `color{violet}"glutamic acid,"` respectively, by `color{violet}"addition of another amino"` group to each.

● The `color{violet}"hydroxyl part"` of the acid is replaced by another `NH_2^–` radical.

● Since amides contain `color{violet}"more nitrogen"` than the amino acids, they are `color{violet}"transported to other parts"` of the plant via `color{violet}"xylem vessels."`

● In addition, along with the `color{violet}"transpiration stream"` the nodules of some plants (e.g., `color{violet}"soyabean"`) export the `color{Brown}"fixed nitrogen as ureides"`.

● These compounds also have a particularly `color{violet}"high nitrogen"` `color{violet}"to carbon ratio"`.