Nodule Formation

Root Nodules are structures that are formed by the symbiotic relationship between leguminous plants and nitrogen-fixing bacteria. The root nodule formation is because of the need for obtaining nitrogen for the synthesis of amino acids, energy molecules - the phosphates such as ATP and assimilating in the nucleotides. And since the plants are themselves incapable of utilising atmospheric nitrogen directly they take the help of Rhizobia - general term for nitrogen-fixing bacteria and form the specialised structures known as root nodules.

Why are Root Nodules Useful for the Plants?

Plants or any living organism for that matter require nitrogen which forms an essential part of the biomolecules like nucleotides, amino acids, energy molecules such as phosphates. These biomolecules carry out important biological functions such as protein synthesis which require amino acids, the formation of hereditary material such as DNA whose one of the basic components are the nucleotides and various biological processes that require energy such as the activity of enzymes. This requirement of plants is usually obtained when it is present in absorbable forms such as ammonia or urea inside the soil. During cultivation by farmers, this soil nitrogen is supplemented by using nitrogenous fertilizers. But when nitrogen is not available in sufficient quantities, symbiotic relationship between the leguminous plants and nitrogenous fertilizers leads to root nodule formation. 

This happens so because the plants are incapable of directly utilizing the atmospheric nitrogen (N2). Root nodules of leguminous plants help the plants to obtain atmospheric nitrogen with the help of Rhizobia bacteria such as Rhizobium. They absorb the atmospheric nitrogen and provide it to the plants in the form of ammonia which in turn can be utilised by the plants. So, when asked why are root nodules useful for the plants, the answer is because they provide atmospheric nitrogen to the plants in the form of ammonia. 

The root nodule function is pictorially depicted below:

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Root Nodule Formation Steps

What are root nodules and how are they helpful, has been explained above. Let us proceed to understand how root nodule formation takes place. Given below are the root nodule formation steps:

1. Root nodules function is to provide plants with an absorbable form of nitrogen. Thus, in absence of sufficient nitrogen, legumes release certain organic compounds called flavonoids which attract the Rhizobia towards the roots. 

2. The Rhizobia reach to the tip of the root hairs and start to colonise the surrounding region. The flavonoids along with attracting the bacteria to the roots, also activate nod genes in the bacteria which leads to the release of the nod factors which initiate root nodule formation and the root nodule formation steps follow.

3. The nod factors induce root-hair curling around the bacteria. The bacteria invade the root-hair and enter the root hair through a small tube opening known as the infection thread as it continues curling and following the infection thread the Rhizobia travel to the root cortex. 

4. The bacterial cells when released from the infection thread into the root cortex, differentiate into rod-shaped specialised nitrogen-fixing cells called Bacteroides and are surrounded by the plant-derived symbiosome membrane. 

5. The nodule thus formed, creates a vascular network with host plants for exchange of nutrients.

The entire process of root nodule formation is shown in the below diagram:

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Types of Root Nodules

There are two types of root nodules as explained below:

Indeterminate Root Nodules: These types of root nodules are continuously growing nodules. They contain an active meristem. The cells at the tip of the nodules divide continuously. This leads to segmentation or zones in the structure of the root nodule.

There five such zones formed which are as follows:

• Zone I - The active meristem zone where the cells continuously divide.

• Zone II - This is the infection zone as it contains infection threads full of bacteria. The plant cells here are larger than Zone I and the cell division is halted.

• Zone III -The nitrogen fixation zone. The bacteroids are present in this region which continuously fix nitrogen. Since they are sensitive to oxygen, because of specific enzymes for fixing nitrogen, the plant cells provide them with leg hemoglobins which absorb the surrounding oxygen and provide an anaerobic environment. The pink colour of the nodule is because of these leghemoglobin.

• Zone IV - It is the senescent zone as the plant cells with their bacteroid contents are degraded here.

Determinate Root Nodules: These are the types of nodules which do not have active meristem. The meristematic function is stopped after initial development. Instead of continuously dividing the cells in this nodule grow by expansion. 

Factors Affecting Nodule Formation in Legumes

Having gone through why are root nodules useful for plants and hence understood the importance of root nodule function it becomes clear the plants need to regulate root nodule formation. The factors affecting nodule formation in legumes are given below:

1. Nodulation is affected by a variety of factors both external and internal. External factors are heat, soil acidity, drought and availability of nitrate compounds. Internal factors include autoregulation of root nodule formation via ethylene and other factors. 

2. A soil rich in nitrogen sources inhibits the nodule formation and symbiotic relationship between Rhizoids and plants because nitrogen is available in abundance to the plants.

3. The leaf tissues autoregulated the nodule formation. They have specific molecules that send them signals whenever there is an opening of infection thread. And thus they regulate by sending signals for stopping from opening more infection threads and so on. 

4. Ethylene internally inhibits the root nodule formation.

The above-given article shares a brief yet comprehensive detail regarding the root nodules: what are root nodules, root nodule formation and the factors affecting nodule formation in legumes. These all underline the importance of the root nodules function and hence why are root nodules useful for the plants.