Tuesday, 5 November 2019

Importance of Mycorrhiza

Importance of Mycorrhiza (poster)


In this article, we gave information about the importance of mycorrhiza, types, functions, its significance and how it gives the benefit to plants. Nature is home to innumerable organisms, a place where they interact in a coordinated fashion and form a complex environment. A variety of benefits, as well as harmful organisms, are associated with plants and they exert different quantitative and qualitative impacts on plant production and yield.  Infield, a plant is exposed to various pathogens and their association with the plant may not necessarily end up in causing disease. The population of one major pathogen dominates all others and becomes the main disease-causing agent resulting in a reduction of crop yield. This complex association has been shown to cause distinct biological changes in both,  the host and the pathogen. Plants known to be resistant to one particular pathogen become susceptible to it when attacked by some other pathogen. Furthermore, the presence of one organism may inhibit or stimulate the growth of another organism.


Plant-parasitic nematodes are most important pests of a medium number of agricultural crops worldwide, especially in tropical and subtropical countries,  where environmental conditions favor their multiplication, survival, and dispersal.,  Variety of deleterious nematodes feed on plant roots, buds, stem, crown, leaves, seeds, rhizome, sucker, seedlings, and tuber. Nematode infected root cannot efficiently uptake the nutrients and moisture present in the soil thus creating different disease symptoms in plants. The general underground symptoms caused by plant-parasitic nematodes are root galling, root lesions, root pruning and cessation of root growth whereas chlorosis, stunting and wilting are few above-ground symptoms.

Types of Mycorrhiza


1. Ectomycorrhiza (Ectotrophic Mycorrhiza)


This is an important group of fungi where fungus grows predominantly intercellular in root tissues, especially in the cortical region. The fungus by penetrating the epidermis secretes proteolytic enzymes and develops extensively outside the root, forming a network of hyphae, which is commonly known as Harting net or fungus mantle.  This fungus mantle or Harting net saves the plant feeder roots from the attack of different soil-borne pathogens including plant-parasitic nematodes. Such AM fungi also produce some growth-promoting substance like cytokinins which enhances the different growth parameters like root/  shoot plant weight, length, yield. The ectomycorrhizae generally form a cover on root surfaces as well as in between the root's cortical cells.  It has been reported that ectomycorrhizae are found in  10%  of plant families, mostly the woody plant species.

2. Endomycorrhiza (Endotrophic Mycorrhiza)


Endomycorrhiza (Endotrophic Mycorrhiza) diagram

Endomycorrhiza is associated with most of the plants belonging to family Graminae. Initially, this type of mycorrhizal fungi grows between the cortical cells and later penetrates the cells forming Arbuscules and vesicles. The hyphae of these mycorrhizal fungi produce glomalin, which may be one of the major stores of carbon in the soil. The most interesting part of this fungus is that its hyphae are restricted to the cortical region of the root and never penetrate the stele. Arbuscules in the cortical cells are supposed to be a major site of material exchange between host and fungus as they provide intimate contact between the plasmalemmae of the two symbiotic partners. Arbuscules are transient in nature and live for 1-15 days. Some of the species of this group form vesicles within the root cells. Vesicles are lipid-filled, terminal swelling of the hypha with storage function.

3. Ecto-Endo Mycorrhiza


Ecto-Endo Mycorrhiza diagram

This is an association of the fungus and roots of the plant representing a condition where typical Ectotrophic intercellular infection is accompanied by intracellular penetration of the hyphae. where the infection is typically Ectotrophic along with endotrophic penetration of hyphae.

Mycorrhizae Function


1. Nutrient Uptake and Exchange


It is well established that arbuscular mycorrhizal fungi are obligate symbionts as they require the living root tissues for their proliferation and development. Root tissues are the major source of carbon and nitrogen for mycorrhizal fungi. There is a complex type of interaction between plant and mycorrhizal fungi as AM fungi take up the plant host's photosynthesis products in the form of  \0  "Hexose" hexoses. The transfer of carbon from the plant to the fungi may occur through the Arbuscules or hyphae. AM fungi are mostly dependent on the plant for their nutrition. Inside the mycorrhizal fungi, a hexose is converted to \0  "Trehalose" trehalose and \0  "Glycogen" glycogen and they further get degraded in the form sugar resulting in the enhancement of sugar concentration in root tissues. The hexose in the root tissues' enters the oxidative  \0  "Pentose phosphate "pentose phosphate pathway which produces \0 "Pentose" pentose for nucleic acids synthesis.

The benefit of mycorrhiza to plants is mainly attributed to increased uptake of nutrients, especially phosphorus. This increase in uptake of nutrients from the soil may be due to the increase in surface area of mycorrhizal fungi. The available phosphorus concentration in the root zone can be increased by mycorrhizal activity. Mycorrhiza lowers the rhizosphere  \0  "PH" pH due to the selective uptake of NH4 + (ammonium ions)  and the release of  H+  ions. Decreased soil pH increases the solubility of phosphorus precipitates. The hyphal uptake of N ~ + also increases the flow of nitrogen to the plant as NH4 + is adsorbed to the soil's inner surfaces and must be taken up by diffusion. A recent study has reported the transfer of nitrogen by AM fungi, which is an encouraging aspect for agriculturists all over the world.  Mycorrhiza modifies the chemistry and biology of the rhizosphere in a way that alters the nutrient cycling and suppresses the activity of the root pathogen.

2. Stress Tolerance


There is evidence that the plants with mycorrhiza are more tolerant of stress such as soil salinity, alkalinity, acidity, and drought conditions. Moreover, due to their exploitation of medium soil volume, extended root growth, and increased absorptive area, the mycorrhizal plants exhibit better growth than the non-mycorrhizal ones especially in the arid and semi-arid region where low moisture and high temperature are very critical for survival and growth of the plants. Recently glomalin related soil protein (GRSP) is shown to have a high binding capacity for some heavy metals like CU, Cd, and  Pb.

3. Increased Soil Quality


In general, the mycorrhizal fungi effectively increase the availability of soil nutrients and bind the soil effectively. There is evidence that this enhancement of soil aggregate stability is due to the production of a soil protein is commonly known as glomalin. Glomalin contains tightly bound iron from  0.04%-8.8%.  Glomalin has been identified by monoclonal antibodies that had been raised by crushing the spores of AM fungi. This glomalin is referred to as glomalin related soil protein (GRSP).A strong correlation h ~ s been found between GRSP  and soil aggregate water stability in a wide variety of soils where organic material is the main binding agent. Mycorrhizal soils maintained better soil structure, especially soil water-stable aggregates and soil proteins, which are important for maintaining soil porosity, stability against wind and soil erosion, increased storage of carbon obtained from microbial degradation. According to recent research findings AM fungus inoculation could increase soil peroxide (POD), decrease Catalase (CAT), and not affect polyphenol oxidase (PPO) level in the soil, suggesting that reactive oxygen groups produced in the soil are in negligible amount when colonized by AM fungi.


Significance of Mycorrhiza


1. Management of Pathogen Through the use of AM Fungi


The rhizosphere is a complex environment, where several microorganisms interact with each other. The association of AM fungi with root brings about several changes in the plants as these fungal organisms absorb nutrients and induce resistance against several plant pathogens. The AM fungi may produce metabolites when interacting with roots and may be toxic to different plant pathogens including plant-parasitic nematodes.

The AMF colonizes the root system and makes a thick fungal mat around the root, therefore, alter the pathogen to infest the colonized root system. These fungi may change the physiology of the root system or compete with other organisms for root colonization. There is strong evidence that there is an accumulation of defensive plant compounds upon mycorrhization. The presence of reactive oxygen species, activation of phenylpropanoid metabolisms and accumulation of hydrolytic enzymes such as chitinases and glucanases have been reported in mycorrhizal roots. Mycorrhization makes the plant defense system stronger by inducing resistance, the mechanism specifically known as  MIR  (Mycorrhiza Induced Resistance).

2. Mycorrhiza and  Phytonematode Management


Mycorrhizal plants are often more resistant to diseases, such as those caused by soil-borne pathogens including plant-parasitic nematodes. Arbuscular mycorrhizal fungi do have the potential to be a biocontrol agent for nematode management when both groups of organisms occur simultaneously in the root and rhizosphere of the same plant. The plants heavily colonized with AM  fungi are able to grow well in spite of the presence of a damaging level of nematodes, thus promoting tolerance to nematodes. The favorable effect of mycorrhiza in decreasing the nematode disease intensity has been demonstrated in various crops.

3. Mycorrhiza and Nematode Interaction


Nematodes and arbuscular mycorrhizal (AM) fungi often occur in the root of rhizosphere plants therefore frequently encounter each other. Plant-parasitic nematodes generally interfere with these functions. Generally, arbuscular mycorrhizal fungi do not colonize the root tissues infected by endoparasitic nematodes, and nematodes avoid to infect root tissues that are colonized by VA fungi. Nematode - mycorrhizal interactions appear to be very specific and highly dependent on the particular association of plant cultivar, fungal and nematode species. Thus, the nature of interaction varies to neutral, positive or negative.

On the other hand, early colonization by mycorrhiza is not always beneficial for plant growth in the presence of nematodes and mycorrhizal establishment influenced by nematode invasion of cortical tissues.  It there is no single response in the interaction mycorrhiza-nematode and that the response depends on 011  various factors like availability of nutrient in the soil, soil types, mycorrhization, nematode infection, host plant, etc.

4. Molecular Approaches in Mycorrhizal Research


Molecular techniques have been used to further understand the signaling pathways which occur between arbuscular mycorrhizae and the plant roots.  In the presence of plant root exudates, the mycorrhizal fungi undergo some physiological changes which allow it to colonize the host root. The colonized plant root exudates generally triggered and upregulate the expression of the gene responsible for transcription of some antinematode protein.  In experiments, there was an increase in the transcription rate of 10 genes after  O.Sh  of exposure of root to the exudates and an even greater rate after  1h  exposure.  In the mycorrhizal fungi, the genes were isolated and found to be involved in mitochondrial activity and enzyme production. The mycorrhizal fungal respiration rate was measured by the O 2  consumption rate and increased by  30% three hours after exposure to root exudates.

Conclusion


It can be concluded that future research programs need to be more inclined towards sustainable and eco-friendly approaches for the management of plant-parasitic nematode in Agri crops including medicinal and aromatic crops. Indiscriminate use of pesticides to protect crops against noxious nematode has caused a global concern on bio-degradation, groundwater contamination,  human and animal health hazard. To mitigate such a colossal loss, genuine efforts are needed to be made in proper and fullest utilization of the potentiality of mycorrhiza as a biocontrol agent. The present thrust on research on biological control of nematode needs to be intensified. The obligate nature of these symbionts is a major bottleneck to its mass culture production. Nowadays, the emphasis has been given on bulk production and utilization of arbuscular mycorrhizal fungi on a medium scale and mass production of AMF by root-organ culture technique is one of the upcoming methods. Inhibition of nematode attack in the plant by inducing the ability to produce some kind of toxin or by activating the Nemahooal gene products may be one of the successful approach to combat with nematode problem in Agri crops.


FAQ (Frequently Asked Questions)


Question: What are mycorrhiza and its benefits?

Answer: Mycorrhiza is a type of fungal rhizome (fungal colony) that acts symbiotic with plant roots and provides food to each other.

Question: How are mycorrhizae beneficial to plants?

Answer: Mycorrhiza easily absorbs phosphorus, salts, etc. which are used in plant growth, in association with plant roots. Absorption of these nutrients is reduced by plant roots in small amounts and some plants are not able to do roots, thus they are useful for plants.

Question: 
Why are mycorrhizae economically important?

Answer: Mycorrhiza attaches to plant roots and absorbs foods necessary for plant growth such as phosphorus from the soil. In this way plant growth is rapid. We can use it for economically important plants.

Question: How are mycorrhizae produced?

Answer: Mycorrhiza fungi produce arbuscular mycorrhizae by reproduction. Arbuscular mycorrhiza is thick-walled, produce spores, and occur as extra-radicular hype. They are buried deep in the soil and can live long.

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