Root Nodule Formation

Root nodules are commonly found in the roots of leguminous plants. They are formed due to association with a nitrogen-fixing bacteria, Rhizobium. Rhizobia is the general term used for different genera of nitrogen-fixing bacteria, e.g. Rhizobium, Bradyrhizobium, Azorhizobium, etc.
 
Plants cannot take atmospheric nitrogen directly. These bacteria fix atmospheric nitrogen to ammonia, which can be taken up by plants and utilized in the synthesis of essential macromolecules such as amino acids, nucleotides, etc.
 
It is an effective way to supplement the soil and increase its nitrogen content. It is used as a biofertilizer and reduces the use of chemical fertilizers. Plants are grown in rotation with legumes as once the legume plants die, nitrogen is released in the soil making it available for other plants.

Root nodules are commonly found in the leguminous plants or plants belonging to the family Fabaceae. Examples are peas, beans, soybean, alfalfa, clover, etc.

Some non-leguminous plants also develop root nodules such as Parasponia is nodulated by Rhizobia, alder and bayberry are nodulated by Frankia. Some of the genera of the Rosaceae family also contain root nodules.

Root Nodule Types

There are mainly two types of root nodules:

1. Indeterminate Root nodules

   They are characterized by persistent nodule meristem. They are elongated due to cell division in nodules. Indeterminate root nodules show development gradient. It shows different zones, which are due to different stages of development, these are:

   Zone I – Active meristem, where new tissues are formed.

   Zone II – Infectious zone, having infectious threads with bacteria.

   Zone III – The nitrogen-fixing zone, having bacteroids. Cells contain a large central vacuole. Leghaemoglobin present in the cells gives nodules the characteristic pink colour.

   Zone IV – Senescent zone, plant and bacterial cells are degraded here. The leghaemoglobin is broken down giving the greenish colour present at the base of nodules.

   This type of nodule is more common and found in pea, clover, alfalfa, etc.

2. Determinate Root nodules

   They are spherical in shape and do not possess active meristem after initiation. The nodule growth is by expansion rather than cell division as in indeterminate nodules. The gradient development of nodules is absent in the determinate type of nodules. Examples include soybean, bean, Lotus, peanut, etc.

Root Nodule Formation Steps

Root nodule formation is initiated, when the soil contains a low level of nitrogen. The two symbiotic partners use cell signalling for the association and developing nodules. Steps of nodulation are:

1. Roots of legumes secrete flavonoids, which attracts rhizobia towards the root. Rhizobia congregate around root hairs.
2. Rhizobia secrete nod factors or nodulation factors, which causes curling of root hairs around them.
3. Nod factors stimulate many developmental changes, e.g. membrane depolarization, curling of root hairs, cell division in the root cortex and intracellular calcium movement.
4. The nod factor attaches to receptors present on the plasma membrane of the root hairs, which leads to the formation of the infection thread. Rhizobia can also enter through cracks in the root cells.
5. Infection thread provides the passage to bacteria to enter epidermal cells.
6. Rhizobia then enter cortex cells, each bacterium gets surrounded by a plant-derived membrane known as symbiosome.
7. Nodule formation is initiated by chemicals produced by rhizobia. It is a result of calcium dependent signal transduction pathway, which triggers biochemical changes leading to cell division and nodule formation.
8. Cytokinin also plays an important role in nodules formation.
9. Within nodules, bacteria get differentiated into bacteroids, which fix nitrogen. Vascular tissues are developed for nodules for exchange of nutrients.

Factors Affecting Nodule Formation

Nodulation is affected by both external and internal factors.

• External factors include heat, acidity, nitrate content of the soil, etc.
• If soil is rich in nitrogen content, it interferes with the nodule formation and symbiotic association as plants already have enough nitrogen and they do not need more.
• Nitrogen fixation is an oxygen-sensitive process. The root nodules contain a heme pigment called leghaemoglobin, which facilitates the diffusion of oxygen.
• Nodule formation is autoregulated by leaf tissues. Plants have evolved defense mechanisms to check the infection.
• Ethylene also regulates nodule formation internally. Exogenous application of ethylene has shown to inhibit nodule formation.

Also Read: Important notes on Morphology of Flowering Plants

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