Vesicular Arbuscular Mycorrhiza and its Role in Plant Nutrition

The term mycorrhiza comes from a Greek word that means the root of a fungus. It is a symbiotic association between higher plants and certain higher fungi. In a natural ecosystem, it is very common to find mycorrhizal associations. The fungi in mycorrhizae are associated with 80% of vascular plants which includes Angiosperms, Gymnosperm, Pteridophytes, and some Bryophytes.

In mycorrhizae, the fungi provide mineral nutrients to the plants from the soil through the absorption process to the higher plants. The fungus in return gets the sugars produced by the plants. This colonization is restricted to the root cortex of the higher plants and does not enter into the vascular cylinder.

Vesicular-Arbuscular Mycorrhiza (VAM) is a type of mycorrhiza which is also called Endomycorrhiza. Endomycorrhiza is the most common type of mycorrhizal association found in a natural ecosystem. Root of some of the major crops, tropical and temperate tree species like wheat, maize, potato, tomato, soybean, apple, orange, mango, etc. are associated with fungus. The Fungus in this symbiotic association belongs to a new phyletic group Glomeromycota. These are Glomus, Gigaspora, Archaeospora, etc. There is no morphological variation observed in the roots associated with VAM from the normal root system. There is no external sheath around the root, only inter and intracellular fungal hyphae are present within the root. In the cortical region of the root VAM form two special structures which are Arbuscules and Vesicles.

Role of Mycorrhiza in plant nutrition

Several factors are responsible for plant nutrition and its yield. Other than the plant genome itself, various environmental factors such as nutrient availability in the growing media, water availability and uptake, and the ability of the plant to properly utilize what is available in the soil all play an integral part in plant growth.

The importance of mycorrhizal colonization in higher plants has long been considered for the uptake of mineral nutrition from the soil. Among several roles of mycorrhizae in nutrition uptake, most of the studies are with phosphorous, since this element normally exists in the form of insoluble calcium phosphate or organic phosphate in soil and is therefore unavailable to plant roots.

The mycorrhizal association is also known to improve the uptake of nitrogen in plants. This happens due to the ability to transfer immobile ammonium from the soil to plant roots due to the increased absorption surface of VAM fungal hyphae. This association is even known to enhance fixation rates of nitrogen-fixing bacterial association of legumes. Therefore, symbiosis of leguminous plants with Rhizobium and VAM fungi helps the plant to acquire two vital elements-nitrogen and phosphorus.

Arbuscular Mycorrhiza develops structures called vesicles. These vesicles perform the function of storage sites within the plant roots and store the absorbed minerals and lipids. These stored nutrients act as reserves of food material and become accessible by the host plant when their supply is limited.

Mycorrhizae are known to provide other types of benefits also to the plants and the environment like:

  1. Increase plant establishment and survival at seeding or transplantation. 

  2. Increase yields and crop quality

  3. Improve drought tolerance

  4. Enhance flowering and fruiting

  5. Optimize fertilizers use, especially phosphorus

  6. Increase tolerance to soil salinity 

  7. Reduce disease occurrence

  8. Contribute to maintaining soil quality and nutrient cycling

Mycorrhiza is a truly beneficial relationship for the plant and the environment. This association not only helps in increasing water absorption, uptake of nutrients, availability of nutrients that are unavailable to the plant’s roots but also protects the plant from excess salts or harmful minerals.