Root Nodule formation in Plants- Fate of Ammonia

Some microbes make symbiotic association with the root of higher plants . The most common association on roots is  nodules. These nodules are small outgrowths on the roots. The microbes like  Frankia  produce nitrogen-fixing nodules on the roots of non-leguminous plants  like Alnus. 

👌👌Remember - Both Rhizobium and Frankia are free living in soil but they  fix atmospheric nitrogen as the symbiotic relationship.

When the nodules are formed then a swollen  central portion is formed that is red or pink in color. This  colour is due to the presence of leguminous haemoglobin or leg-haemoglobin.

Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. Rhizobium starts to reproduce and make colony at the roots and get associated with the  epidermal and root hair cells of the root . Now bacteria are infected with root-hairs. As a result the root hair becomes curled . Afterwards , The  infection  is spread  into the cortex of the root, where they form the nodule formation therefore the nodule formation takes place  in the cortex of the root. Finally  the bacteria are released from the thread and form some specialised nitrogen fixing cells. These nodules set up a direct vascular connection with the host for exchange of nutrients. The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia, 

The enzyme nitrogenase is highly sensitive to molecular oxygen. It requires anaerobic conditions to get functional . The nodules have mechanisms to get protection for nitrogenase from  oxygen. To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.But  these microbes are present in aerobic and free-living conditions  but during nitrogen-fixation, they become anaerobic.

During nitrogen fixation in root nodules, Ammonia is  formed  by nitrogenase enzymes. And eight molecules of  ATP  are produced at production of  each Molecule of ammonia.

At the suitable  pH, the ammonia is transformed into ammonium ion. Most of the plants can take nitrate as well as ammonium ions but the ammonium ions are toxic to plants therefore plant cannot accumulate the ammonium ions.

Moreover the ammonium ions are used to synthesise amino acids in plants. There are two methods by which plant synthesised amino acids from ammonium ions.  First method is Reductive amination  and second one is  Transamination.

In Reductive amination, Ammonia reacts with α-ketoglutaric acid and forms glutamic acid. whereas In Transamination, there is a transfer of amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid that  forms two  amino acids aspartic acid  by the transfer of amino groups through the process of  transamination. The enzyme transaminase is involved in these reactions.

The two most important amides asparagine and glutamine are also formed  in plants . These amides are  structural parts of proteins and  formed from two amino acids aspartic acid and glutamic acid, respectively. The hydroxyl part of the acid is replaced by amide. if  amides contain more nitrogen than the amino acids, they are transported to other parts of the plant through the xylem vessels.