What is mycorrhiza: A mycorrhiza is a symbiotic association between a green plant and a fungus. The plant makes organic molecules such as sugars by photosynthesis and supplies them to the fungus, and the fungus supplies to the plant water and mineral nutrients, such as phosphorus, taken from the soil. Mycorrhizas are located in the roots of vascular plants.

Types of Mycorrhiza: Mycorrhiza are mainly divided into

  • Ectomycorrhiza
  • Endomycorrhiza

The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane. Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza, while arbutoid mycorrhizas can be classified as ectoendomycorrhizas. Monotropoid mycorrhizas form a special category.

Role of mycorrhiza in improving crop yield

Nutrient exchange: The major role of mycorrhiza is in exchange of nutrients with the crops. Glomeromycotan fungi are obligate symbionts and rely on the carbon provided by their plant hosts to complete their life cycle In return, the fungus provides nutritional benefits to the plant by delivering minerals, including the biologically essential nutrients phosphorus (P) and nitrogen (N). The majority of this nutrient exchange is believed to occur within root cortical cells containing highly-branched hyphal structures termed arbuscules.

Phosphate uptake: Phosphate acquisition via the mycorrhizal pathway begins with the uptake of free phosphate from soil by fungal extra-radical hyphae. These fungal hyphae extend beyond the host root system, allowing a greater soil volume to be exploited for phosphate uptake. Uptake at the soil hypha interface is mediated by fungal high-affinity phosphate transporters of the Pht1 family. Following fungal uptake, phosphate is transferred to the fungal vacuole where it is polymerized to form polyphosphate chains and translocated through the vacuole to the intraradical hyphae. The polyphosphate is then hydrolysed and phosphate released to the interfacial apoplast. From the interfacial apoplast, plant mycorrhizal Pht1 transporters guide the phosphate across the peri arbuscular membrane. Once in the plant cytosol, phosphate is translocated into the vasculature for delivery to all parts of the plant.

Nitrogen uptake: The extraradical mycelium of mycorrhizal fungi also absorb ammonium, nitrate and amino acids, and the role of mycorrhizal nitrogen delivery is becoming better understood. The majority of nitrogen is thought to be taken up in the form of ammonia via the action of fungal-encoded AMT1 family transporters such as the protein GintAMT1 characterized from Glomus intra radices. The transfer of nitrogen most probably occurs in form of amino acid arginine. Having been translocated to the intra-radical hyphae, amino acids may be delivered directly to the interfacial apoplast for plant absorption. However, there is also evidence for an alternative route whereby arginine is broken down by ornithine aminotransferase and urease to release free ammonium.

How does mycorrhiza enhance nutrient uptake in crops?

Mycorrhizal crops (crops with natural or applied mycorrhiza), can take up nutrients from the soil via two pathways:

The ‘plant pathway’- involves the direct uptake of nutrients from the soil by the root epidermis and its root hairs.

The ‘mycorrhizal pathway’- that involves the uptake of nutrients via the ERM of the fungus and the transport to the Hartig net in ECM interactions or to the IRM in AM interactions, and the uptake by the plant from the interfacial apoplast.

The uptake of nutrients from the soil via the plant pathway, however, is often limited by the low mobility of nutrients in the soil. The mobility of for example phosphate (P) is so low that its uptake leads rapidly to the development of depletion zones around the roots and limits the further P uptake via the plant pathway due to the low rate of diffusion.

What is the role of mycorrhiza in crop disease control?

Mycorrhiza and their associated interactions with plants reduce the damage caused by plant pathogens (Harrier and Watson, 2004). With the increasing cost of pesticides and the environmental and public health hazards associated with pesticides and pathogens resistant to chemical pesticides, mycorrhiza may provide a more suitable and environmentally acceptable alternative for sustainable agriculture.

An infestation in the roots of crops by plant parasitic nematodes or by soil fungi, may not be noticed (under soil), until the damage symptoms appear on the aerial part. Root colonization by mycorrhiza reduces the severity of diseases caused by plant pathogens. The reduced damage is a cumulative result of the following mechanisms working together or separately.

  • Changes in root growth and morphology.
  • Histopathological changes in host root.
  • Physiological and biochemical change within the plant.
  • Changes in host nutrition.
  • Mycorrhizosphere effects which modify microbial populations
  • Competition for colonization sites and photosynthates.
  • Activation of defence mechanisms.

How does mycorrhizae help to control abiotic stress?

Metal stress- soil contamination resulting from mining activities, industrial processes, agriculture, and military activities have resulted in high localized concentrations of heavy metals. Mycorrhiza play a significant role in soil remediation. Mycorrhiza ensure crop establishment in soil and high yield even in contaminated soil due to improved nutrition, water availability and soil aggregation properties.

Water scarcity- Mycorrhiza offer indispensable advantages to host plant subjected to water shortage, by, (a) drought mitigation strategy, whereby, mycorrhiza ensures enhanced water uptake in plants through extensive hyphae network. (b) drought tolerance strategy, which includes direct benefits of mycorrhiza, which improve crop’s innate ability to cope up with stress. (c) mycorrhiza increase water pressure (turgor) in plant tissue, thereby preventing or delaying wilting. This supports cell function, allowing growth and photosynthesis to continue.

How do mycorrhizae improve growth and grain production in crops? : Mycorrhiza benefit growth of many crops due to

  • The extensive hyphal network development in soil
  • More efficient exploitation of nutrients
  • Enhanced nutrient uptake
  • Increased resistance to biotic and abiotic stresses and reduces disease incidence.

Is the soil mycorrhiza population decreasing?

Yes, the soil mycorrhiza population is decreasing, mainly due to extensive use of pesticides, fungicides and fertilizers. Also, the use of agricultural machinery has huge impact on the soil mycorrhiza population. Hence the use of commercial mycorrhiza to improve crop yield and fight biotic and abiotic stress is very important.