Mycorrhiza
The word mycorrhiza, of Greek origin, defines the symbiosis between a fungus (mycos) and the roots (rhizos) of a plant. As in many symbiotic relationships, both participants obtain benefits. In this case, the plant receives mainly mineral nutrients and water from the fungus, and the fungus obtains carbohydrates and vitamins from the plant that it is unable to synthesize on its own while she can do it thanks to photosynthesis and other internal reactions. It is estimated that between 90 and 95% of the families of terrestrial plants (80% of the species) present mycorrhizae on a regular basis.
The symbiotic association is established between the roots of plants and the hyphae of fungi of the divisions Basidiomycota and Ascomycota and the class Glomeromycetes. At the beginning of colonization, the fungus forms a mantle made up of fungal hyphae that surround the apex of the root; then other hyphae penetrate the intercellular space between the root cells, forming what is known as Hartig's network. It is here in the Hartig network where the exchange of nutrients, minerals and water takes place: the fungus absorbs water and minerals that it then translocates to the plant and in return the plant provides sugars and other products of photosynthesis to the fungus. Among several of the positive effects that ectomycorrhizal fungi provide to their host, the most important is attributed to the extraradical mycelium that increases the amount of dissolved mineral intake.
The mobilization of nutrients can occur through an enzymatic pathway that allows the fungus to use organic nitrogen and phosphorus, or through the release of organic acids mobilizing calcium, magnesium, and potassium, among others. The hyphae mainly excrete oxalic acid which helps wear away rock surfaces; In addition, the diameter of the apex of a hypha compared to the apex of a root gives the plant a great advantage, since it allows it to explore substrates that it could not reach without the association with its ectomycorrhizal fungus.
It is possible for a fungus to form mycorrhizae with more than one plant at the same time, thus establishing a connection between different plants; this facilitates the existence of mycoheterotrophic plants (parasitic plants of fungi); some of which do not even carry out photosynthesis (such as those of the genus Monotropa), which extract everything they need from the mycobiont fungus and the other plants with which it also establishes symbiosis. Likewise, several fungi (sometimes from different species) can mycorrhize the same plant at the same time.
Advantages of mycorrhization
The benefits provided to plants by mycorrhization are numerous. Thanks to it, the plant is able to explore more soil volume than it reaches with its roots, as the hyphae of the fungus join in this work; it also more easily captures certain elements (phosphorus, nitrogen, calcium and potassium) and water from the soil. The protection provided by the fungus also makes the plant more resistant to certain environmental stresses that affect the soil, such as salinity, temperature changes, and soil acidification derived from the presence of sulfur, magnesium, and aluminum. As if all this were not enough, some physiological reactions of the fungus induce the root to remain active for longer than if it were not mycorrhized.
All of this results in a greater longevity of the plant: in fact, it has been proven that some trees, such as pines, can live longer than pines without mycorrhization after having been mycorrhized. In other species, this union is so close that without it the plant cannot survive, as is the case with orchids. Plants whose seeds lack endosperm (reserve food substances) are completely dependent on the fungus for food and subsequent germination.
Infection of the roots by the fungus occurs from propagules present in the soil. They can be spores and pieces of hyphae of the fungus and also already mycorrhized roots. In order to ensure the success of the company, the sowing of most edible or decorative plants and the reforestation that is currently carried out accompany the new plants and shoots with fragments of the most suitable fungus to establish mycorrhizal associations. with each species to be cultivated.
Types of mycorrhizae
Most land plants have mycorrhizae, and most likely the rest are descended from mycorrhizal plants that have secondarily lost this characteristic. In the case of fungi, most of the 5000 species identified in mycorrhizae belong to the Basidiomycota division, while in more exceptional cases members of Ascomycota are observed. The third division that has been observed forming mycorrhizae is Glomeromycetes, a group that is only known in mycorrhizal association and whose members die when deprived of the presence of roots.
Depending on their morphology, mycorrhizae are divided into different groups, including two main ones: ectomycorrhizae and endomycorrhizae.
Ectomycorrhizae
Ectomycorrhizae are characterized by the fact that the hyphae of the fungus do not penetrate inside the root cells, but are located on and between the separations of these. They can be seen with the naked eye and present the so-called Hartig's Network. This type of mycorrhization is the one that predominates among trees in temperate zones, being especially characteristic of beeches, oaks, eucalyptus and pines. Fungi are both Basidiomycota and Ascomycota.
Martin et al. published in Nature in 2010 the genome of Tuber melanosporum or black truffle, the most complex and longest genome sequenced to date, in order to understand better the biology and evolution of ectomycorrhizae. Below we will list three findings that help to understand the molecular basis of evolution, reproduction and establishment of the most popular member of this group.
The types of protein families responsible for nutrient exchange and colonization of the apoplast in the root were found within the transcriptome. In any case, in number they turned out to be much less than those found in pathogenic and saprophytic fungi of the Ascomycota group and even in Laccaria bicolor, another ectomycorrhiza. However, 64 membrane protein is overexpressed, suggesting that the flux of carbohydrates, oligopeptides, amino acids, and polypeptides at the symbiotic interface is greater.
Additionally, the genome turned out to be more compact, that is, with fewer genes within the gene families, which may be the product of selection for host specialization for T. melanosporum. For his part, L. bicolor has these expanded families, since it is directed by selection of the host and substrate to be exploited, which are diverse. The sequence Me128 was found, which contains the HMG locus; and the other mating type was identified in a natural isolate, confirming that T. Melanosporum is heterothallic and an obligate crossbreed. This is a great contribution to truffle cultivation, which will be enhanced by the use of host plants harboring truffle strains of opposite mating types.
Endomycorrhizae
In endomycorrhizae, on the other hand, there is no outer layer that can be seen with the naked eye. The hyphae are initially introduced between the root cells, but then penetrate inside them, forming food vesicles and arbuscules. For this reason, they are also known as VAM mycorrhizae or vesicular arbuscular mycorrhizae. Fungi belong to the Glomeromycota division and occur on all types of plants, although with a predominance of herbs and grasses. They abound in poor soils such as those of the prairies and steppes, the high mountains and the tropical forests. In the Atlantic forest they appear together with the ectomycorrhizae.
In addition to these two large groups, the following minor types can be distinguished:
- Ectendomicrrows: they have external mantle, such as ectomyrrhices, but they also penetrate the inside of cells, such as endomicrhes. There are no gallbladders or busses. They are seen both in Basidiomycota and Ascomycota and are more abundant in angiospermas than in gymnospermas. Little specific.
- Orchidoids or micorrizas de ovillo: Micorrizas de orquídeas, indispensable for its development and youth life. In an adult state, the plant can become independent of the fungus in some cases.
- Ericoides: simpler and simpler type. Penetra in the cells to form ovilles.
- Arbutoids: external mantle and cell penetration, where they form Russ.
- Monotropoids: The form of penetration in cells is something different.
Discovery
The first to observe mycorrhizae and baptize them with the name they currently bear was the German botanist Albert Bernhard Frank, in 1885, after detecting their presence in several fruit trees. In 1900, the Frenchman Bernard discovered its extreme importance in the life and development of orchids. In 1910 he began to extend his study of plants used in agriculture and gardening.
However, it was not until 1955, with the publication of Mosse's first studies in England, that mycorrhizae ceased to be considered as exceptions and their real importance and generality was accepted. In more recent times, numerous fossil findings have allowed us to determine that the origin and presence of mycorrhizae are enormously old, since Glomeromycota spores have been found in strata up to 460 million years old, belonging to the Ordovician period. Arbuscular forms are already quite widespread by the time the first land plants appeared in the fossil record, 400 million years ago. These plants, like the species Aglaophyton major, lacked true roots, presenting only an underground stem or rhizome from which several aerial stems protruded. The absorption of nutrients, therefore, fell almost exclusively on the mycorrhizal fungus, so it can be said that the presence of these was essential for the extension of plant life to the mainland, after which the animals would later arrive.
Role of mycorrhiza in nutrient absorption
Nutrient utilization by plants is mainly determined by the root absorption capacity and by the diffusion of nutrients and subsequently by the release of elements into the soil solution. The root morphology and the external mycelium of arbuscular fungi determine the use of ions with a low diffusion rate such as phosphorus, zinc and molybdenum.
The main function of the mycorrhiza is the increase in the volume of the explored soil, for the use of nutrients and thus favor the efficiency of their absorption from the soil solution. The most plausible explanation for this high utilization efficiency is that the fungal hyphae extending from the root are able to take up and transport phosphate from the soil to the host root, the absorptive surface offered by the branching of the outer mycelium around the hyphae. Mycorrhized roots allow the plant to take advantage of phosphate in the soil beyond the zone of depletion at the root surface.
The improvement of mineral nutrition is the result of the mycorrhizal association and is reflected in increased survival, growth and productive capacity of the plant. It is recognized that the growth response of the plant is often the result of increased phosphorus nutrition and other nutrients such as copper, zinc, magnesium, manganese, calcium and nitrogen.
Importance of arbuscular mycorrhiza in pathogen control
Arbuscular mycorrhiza has received considerable attention in recent years because plants receive various benefits from symbiosis. Protection against root pathogens is a benefit that has been intensively studied in various parts of the world. There are several reviews of mycorrhizal-pathogen interaction that emphasize the potential for biological control of root diseases, however. There are few studies regarding diseases on stems and leaves, but, in general, mycorrhized plants are less susceptible than non-mycorrhized ones.
Regarding the interaction between arbuscular mycorrhiza and pathogenic bacteria, they demonstrated that tomato plants in mycorrhizal association with Glomus mosseae were protected against Pseudomonas syringae; In fruit trees, no investigations have been carried out in relation to this, except that reported by Perrin (1990), of the interaction between mycorrhizal fungi and Pseudomonas syringae in peach, however the effect was null.
Economic relevance of the use of arbuscular mycorrhiza in fruit nurseries
Fruit trees are high value crops and their production often involves expensive practices such as fumigation, sophisticated fertilization, high labor requirements, specialized equipment, greenhouses, nurseries, etc. One of the most frequent inadequate management factors in fruit orchards is the planting of plants propagated in nurseries with nutritional deficiencies, this is because it is a common practice to fumigate with pesticides to obtain pathogen-free plants. In this way, beneficial fungi are also eliminated, which results in a poor development of the plant, also manifesting chlorosis and dwarfism.
In nurseries it is common to use artificial growth media such as vermiculite, perlite or other materials that do not have mycorrhizal fungi, so it is necessary to introduce them.
Nursery plants without mycorrhizae, when transplanted to permanent or definitive sites, have a high probability of dying, especially in adverse environmental conditions. In addition, in nurseries it is necessary to fertilize abundantly, to partially eliminate deficiency symptoms; in this way the production costs of fruit trees are increased. A reliable alternative in nursery technology is the use of endomycorrhizal fungi, which are capable of increasing the growth rate and improving the mineral nutrition of the plant, mainly in poor soils.
The special circumstances of establishing plants under controlled conditions allow mycorrhizal technology to be incorporated into these production systems. The increase in growth due to inoculation with endomycorrhizal fungi is desirable for the nurseryman, because it accelerates the cycle of fruit trees. This increases productivity with less labor, fertilization and water per plant unit. In addition, mycorrhized plants are usually more vigorous and have a better size, color, etc.; that is why they are of great attraction for fruit growers. The incorporation of mycorrhiza in crop production can be carried out using spores, soil and segments of colonized roots as a source of inoculum.
Many horticultural plants and most fruit and forest trees are first established in seedbeds or maintained during the early stages of development in nurseries before being transplanted to the field, so mycorrhizal inoculation can be performed either in the seedbeds or at the time of transplanting.
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