Nodule Formation: Formation Steps and Factors Affecting Nodule Formation

Root nodule formation is an interesting example of mutualistic symbiosis between legumes and nitrogen-fixing bacteria, especially those of the genus Rhizobium.

• Root nodules are found on the roots of plants, especially legumes, and form a symbiotic relationship with nitrogen-fixing bacteria.
• In nitrogen-limiting conditions, capable plants create a symbiotic association with a host-specific strain of bacteria known as rhizobia.
• This method has evolved several times in legumes as well as other plants in the Rosid group.
• Legume crops consist of beans, peas, and soybeans.

Key Terms: Root nodules, Leguminous plants, Nitrogen-fixing bacteria, Amino acids, Formation of root nodules, Rhizobia, Non-Leguminous plants, Epidermal cells, Symbiosome

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What are Root Nodules?

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Root nodules are structures created by the symbiotic interaction between leguminous plants and nitrogen-fixing bacteria.

• The formation of root nodules is due to the requirement for nitrogen for the synthesis of amino acids, energy molecules such as ATP, and nucleotide assimilation.
• Because plants are unable to use air nitrogen directly, they depend on Rhizobia, to form specialized structures known as root nodules.
• Rhizobia is a general term for nitrogen-fixing bacteria.

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Why are Root Nodules Useful for Plants?

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The nodule is a specialized organ or outgrowth on the roots of the host plant, especially those of the family Fabaceae (Leguminosae), that contains bacteria preferably Rhizobium.

• Rhizobia bacteria can convert nitrogen gas (N₂) to ammonia (NH₃) in a process called nitrogen fixation.
• Any symbiotic microorganism, generally a diazotrophic bacterium, reduces N₂ to ammonium.
• The ammonia is subsequently converted to nitrates, amino acids, and other cellular constituents such as vitamins, flavones, and hormones.
• These constituents are essential to the growth of the plant.
• Sugar which is formed through photosynthesis is converted into organic acids by the plant root cells.
• This serves the bacteria the needed energy for nitrogen fixation.

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Leguminous and Non-Leguminous Plants

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The flowering plant family of the plant kingdom is said to include both leguminous and non-leguminous plants.

• In contrast to non-leguminous plants, legumes have the ability to fix atmospheric nitrogen due to the presence of Rhizobium especially in their roots.
• The nitrogenase complex found in these diazotrophs helps in the conversion of nitrogen into ammonia.
• However, several non-leguminous plants have been shown to fix atmospheric nitrogen.

Leguminous Plants

Leguminous plants are a kind of angiosperm that enriches soil with nitrogen-rich compounds through the process of biological nitrogen fixation. Leguminous plants can also enhance soil quality and can prevent erosion.

Leguminous Plants Examples

Leguminous plants are an essential component of our diet. The most common legumes include beans, cowpeas, chickpeas, kidney beans, alfalfa, and groundnuts. The leaves of the plant are prescribed and compounded. The fruits are the pods.

Non-Leguminous Plants

Except for Fabaceae, non-leguminous plants are classified into several classes. 

• These plants do not produce pods, nor do they have Rhizobium sp. associated with their roots.
• These plants lack the ability to replace the soil with nitrogen.
• Instead, they deplete the soil's nitrogen.

Non-Leguminous Plants Examples

Non-leguminous plants include Tal or Asian palm (B.flabellifer Sorisha) and black mustard (B.nigra). Other non-leguminous plant examples include rose, mango, and ficus.

Legumes Plant

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Types of Root Nodules

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There are two types of root nodules. These are

• Indeterminate Nodules
• Determinate Nodules

Indeterminate Nodules

They are found in most of them such as peas, alfalfa, clover, vetch, etc., They have an active meristem that is responsible for the growth of new cells as the nodule grows. They are generally found to have a cylindrical shape and have long branches.

Their growth is classified zone-wise.

Zone I

This zone is the Active meristem zone, where a new nodule is formed.

Zone II

Infection zone that has infection threads full of bacteria and the plant cells get larger.

Zone III

This zone is known as the Nitrogen Fixation Zone. Bacteroides fix nitrogen actively. These cells are pink in color which is due to leghemoglobin supplied by the plant.

Zone IV

This zone is known as the Senescent Zone. The plant and bacterial cells are degraded here. The greenish color present at the base of nodules is due to the breakdown of leghemoglobin.

Determinate Nodules

They are found in tropical legumes such as soybeans, Common beans, etc., and on some temperate legumes like Lotus.

• The nodules here are spherical in shape.
• After initiation, these nodules lose meristematic activity.
• Their growth is only due to cell expansion.
• Determinate modules are found in a wide range of shrubs, trees, and herbs.

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Root Nodule Formation Steps

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The root nodule formation begins when nitrogen levels in the soil are low. Cell signaling is used by the two symbiotic partners to associate and develop nodules. The steps of nodulation are:

• The roots of legumes release flavonoids, which attract rhizobia. Rhizobia gathers around root hairs.
• Rhizobia produces nodulation factors, which cause root hairs to curl around them.
• Nod factors stimulate a variety of developmental processes, including membrane depolarization, root hair curling, cell division in the root cortex, and intracellular calcium transport.
• The nod factor binds to receptors on the plasma membrane of the root hairs, resulting in the formation of the infection thread. Rhizobia can also enter through gaps in root cells.
• Infection threads allow germs to enter epidermal cells.
• Rhizobia then enters cortex cells, and each bacteria is surrounded by a plant-derived membrane known as a symbiosome.
• Rhizobia produces chemicals that lead to nodule formation. The calcium-dependent signal transduction pathway causes biochemical changes that contribute to cell division and nodule formation.
• Cytokinin also plays a significant function in nodule formation.
• Bacteria within nodules develop into nitrogen-fixing bacteroids. Vascular tissues form around nodules to facilitate nutrient exchange.

Formation of Root Nodules

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Factors Affecting Nodule Formation

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Nodulation is influenced by both external and internal factors.

• External factors include heat, acidity, soil nitrate level, and so on.
• If the soil is rich in nitrogen, it interferes with nodule formation and symbiotic relationships since plants have enough nitrogen and do not require more.
• Nitrogen fixation is an oxygen-sensitive process.
• Leghaemoglobin, a heme pigment found in root nodules, increases oxygen transport.
• Leaf tissues regulate the formation of nodules independently.
• Plants have evolved defensive mechanisms to prevent infection.
• Ethylene also controls nodule formation internally.
• The exogenous application of ethylene has been found to inhibit nodule formation.

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Things to Remember

• Root nodules are structures created by the symbiotic interaction between leguminous plants and nitrogen-fixing bacteria.
• Legumes have the ability to fix atmospheric nitrogen due to the presence of Rhizobium especially in their roots.
• Rhizobia bacteria can convert nitrogen gas (N₂) to ammonia (NH₃) in a process called nitrogen fixation. 
• Leguminous plants are a kind of angiosperm that enriches soil with nitrogen-rich compounds through the process of biological nitrogen fixation.
• There are two types of root nodules: Indeterminate Nodules and Determinate Nodules
• The root nodule formation begins when nitrogen levels in the soil are low.

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