Mycorrhizae are the symbiotic association between soil fungi and agricultural and forestry plants roots. Mycorrhiza is a Greek word that etymologically means fungus and root. This is a mutual association where each element takes advantage of mycorrhization: the fungus colonizes plant roots, providing water and nutrients absorbed from the soil through its external system of hyphae, while the plant provides organic matter to the fungus (like sugars, proteins, vitamins). Where mycorrhizae develop, plants are healthier, more vigorous and less prone to environmental stresses. There are many types of natural mycorrhizae because, during their evolution, plants adapted themselves to all sorts of environments. By the fungus ability to penetrate host plant root cells and by this unity appearance, we can identify three forms of mycorrhizae: ectomycorrhiza, endomycorrhiza, ectoendomycorrhiza.
Arbuscular endomichorrizae AM or VAM, (Vesícula arbuscular mycorrhizal), obviously represents the most common type of mycorrhizae, affecting the main herbaceous and tree crops all over the world. Regarding cultivated plants, this mycorrhizae can be found in cereals (wheat, maize, barley) and vegetables such as tomato and potato, in industrial crops (tobacco and sunflower), in tree crops (vine, peach tree, apple tree, pear tree, olive tree, etc…) and many other agricultural and forestry crops. They are called endomychorrizae because the fungus penetrates host cells. The spores on the soil germinate because of the presence of host roots as a result of root exudative. They grow until they reach the root and colonize it penetrating through both intercellular spaces and cells.
Inside the cells hyphae branch off to create branched structures called arbuscules. They are responsible for nutrient exchange between the two symbionts: the plant transfers the exceeding carbohydrates produced by photosynthesis while the fungus transfers the mineral salts absorbed from the surrounding soil. Another structure created by the fungal hyphae is the vescicule, which is an intracellular or intercellular round swelling, a system of fat granules accumulation and stock. The spores formed by these fungi are asexual and directly created by vegetative hyphae. The fungi mycelium, consisting of vegetative extra root hyphae from 8 to 20 micrometers, can be very extended and has the key role of exploring the soil around the roots to increase the absorbing efficiency of the root itself. The most important kind of endomycorrhiza is Glomus. The effect of mycorrhization is basically a wide multiplication of root surface and volume (up to 700-800 %); thanks to the external root growth the fungal mycelium explores inaccessible parts of the soil. Other benefits of the mycorrhization are: an increased plant resistance to water stress and salinity, an increased resistance and tolerance to soil pathogens, a greater absorption of microelements, an improved and more consistent production, a better plant growth, more efficient fertilizers, pesticides and irrigation, an improved plant adaptation to the environment, less tired soils.
PGPR (Plant Growth Promoting Rhizobacteria) promote plant growth. They are microorganisms that bio-stimulate root growth and increase their defenses. They act in different ways, always producing a great amount of useful enzymes stimulating the plant endogenous defenses. One of the most common methods that PGPR adopt to improve plant nutrients absorption is the alteration of vegetal hormones levels. This process changes root architecture: it increases root system mass through its branching and elongation and root hair amount. This leads to a root volume increase and to a better nutrients absorption. Moreover, PGPR stimulate plant growth, assuring prime conditions (like pH neutralization) in the rhizosphere (a part of the soil explored by the roots) so that the plant can quickly assimilate all the necessary nutrients. In addition, some of them can make insoluble phosphorus and iron (siderophores) available. Last but not least is the growth inhibition and competition for space and nutrients, which operate to the detriment of many pathogenic and fungal species. In brief, PGPR have three very interesting properties and can be used as: biofertilizers, bio-activators, bio-protectors.
Trichoderma is a fungus that plays a strong action against several phyto-pathogenic fungi in different ways:
– a rapid rhizosphere colonization through the competition for space and nutrients engaged with pathogenic fungi to delimit their development;
– a production of chitinolytic enzymes that help Trichoderma sp. penetrating the host, so the pathogen is used as nourishment;
– a production of fungistatic substances that prevent pathogens development limiting the occurrence of infections;
– a plant growth stimulation with an increased roots, leaves and stems production and an improved efficiency in both macro and micro-nutrients absorption.
Clonostachys is a fungus that acts as a saprophyte in the soil and is characterized by a particular spiral growth. This fungus, thanks to the production of toxic metabolites, plays a good antagonistic activity against several soil-borne pathogens. It is also able to occupy and remove forms of pathogens resistance, such as sclerotia.
Streptomyces belongs to Attinomycetes group, microorganisms with intermediate characteristics between fungi and bacteria. The members of this genus are commonly found in substrates and natural soils; they produce many useful plant substances, such as siderophores (organic substances that chelate iron), microelements, vitamins, amino acids and organic acids. The production of such substances directly promotes plant development. Some studies have also shown that some species of Streptomyces sp. are characterized by the ability of producing antibiotic substances (streptomycin) which control many root system pathogenic fungi (Pyrenocheta lycopersici, Fusarium oxysporum, Pythium ultimum, Verticillium dahliae, Phytophtora capsici, etc.). The antagonistic activity towards phytopathogens is not only managed by a direct process, but it is also performed by space occupation and competition for nutrients.
The species of Pseudomonas sp. genus are soil bacteria and belong to the group of PGPR (Plant Growth Promoting Rhizobacteria). The activity of growth promoting is carried out through several processes: one of the most significant is the release of hormone-like substances that stimulate proliferation, multiplication and root cells growth in the rhizosphere. Like Streptomyces sp., Pseudomonas sp. is also able to chelate iron and other microelements by producing organic acids known as siderophores. Several studies have shown that microorganism synergistic activity leads to an increased plant root development. Plants with wellstructured roots can more efficiently exploit water and nutrients.
The Bacillus sp. genus belongs to PGPR group and includes species of bacteria isolated from the rhizosphere with several beneficial activities. Many studies prove that these bacteria help the plants overcoming stresses caused by different phytopathogenic fungi that attack both the aerial system (Alternaria, Botrytis cinerea , Powdery mildew, Bremia and group Peronosporaceae, Moniliaceae) and the root system (Phytium , Rhizoctonia, Fusarium and Sclerotinia). This activity is carried out through several processes such as particular degrading enzymes production (chitinolytic, glucanolytic and proteolytic enzymes) secreted when bacteria approach the fungus; they stimulate plant self-defenses and compete with pathogens to achieve space and nutrients. In addition,
Bacillus sp. genus species promote plant growth by producing phytohormones and phytohormonal action substances. Other activities related to plant growth promotion are: production of organic matters called “siderophores” that facilitate plant absorption of iron and other microelements, production of organic acids that can dissolve unavailable phosphorus salts.
Pochonia sp. is a hyphamycete, a nematode eggs parasite. Its antagonist activity is linked to the production of proteolytic and chitinolytic enzymes that degrade nematodes cell structures, especially eggs and early stage female cells.
Arthrobotrys sp. is a nematode parasite fungus. The nematode bio-control activity is related to ring structures production that swell and trap nematodes passing through. Then the nematodes are enzymatically degraded and used as nourishment by the fungus.
A mixture of Pochonia sp. and Arthrobotrys sp. could be a significant biological mean of controlling pathogenic nematodes of Meloindogyne and Heterodera genus. Some tests have proved that the use of these fungi on soils (previously sterilized by chemicals, solarization, steam) maintained nematodes level below the damage threshold for more years compared to soils without fungi, thanks to the “biological vacuum” filling action.
Beauveria sp. and Metarhizium sp. are pathogen fungi that can penetrate into host insect through its exoskeleton by producing chitinolytic enzymes . Once the fungus is inside the host, it develops and eats it to death. Infected but still alive insects have limited movements and are unable to eat and they represent a source of infection to other insects.