8  Mycorrhizal & other Fungai

Mycorrhizal fungi form a mutualistic relationship with the roots of most plant species. In such a relationship, both the plants themselves and those parts of the roots that host the fungi, are said to be mycorrhizal. Relatively few of the mycorrhizal relationships between plant species and fungi have been examined to date, but 95% of the plant families investigated are predominantly mycorrhizal either in the sense that most of their species associate beneficially with mycorrhizae, or are absolutely dependent on mycorrhizae. 

Sugar-water/mineral exchange

In this mutualism, fungal hyphae (E) increase the surface area of the root and uptake of key nutrients while the plant supplies the fungi with fixed carbon (A=root cortex, B=root epidermis, C=arbuscle, D=vesicle, F=root hair, G=nuclei).

The mycorrhizal mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose.[38] The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant's fungal partners. In return, the plant gains the benefits of the mycelium's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hairs, and partly because some such fungi can mobilize soil minerals unavailable to the plants' roots. The effect is thus to improve the plant's mineral absorption capabilities.

Unaided plant roots may be unable to take up nutrients that are chemically or physically immobilised; examples include phosphate ions and micronutrients such as iron. One form of such immobilization occurs in soil with high clay content, or soils with a strongly basic pH. The mycelium of the mycorrhizal fungus can, however, access many such nutrient sources, and make them available to the plants they colonize.[40] Thus, many plants are able to obtain phosphate without using soil as a source. Another form of immobilisation is when nutrients are locked up in organic matter that is slow to decay, such as wood, and some mycorrhizal fungi act directly as decay organisms, mobilising the nutrients and passing some onto the host plants; for example, in some dystrophic forests, large amounts of phosphate and other nutrients are taken up by mycorrhizal hyphae acting directly on leaf litter, bypassing the need for soil uptake. 

<iframe width="560" height="315" src="https://www.youtube.com/embed/n0XuBJZ1Zn0" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

Here are the typical stages of fungal growth:

1. Spore germination: 

2. Hyphal growth: 

3. Mycelium formation: 

4. Asexual or sexual reproduction: 

5. Sporulation: 

6. Dormancy: 

The growth stages of fungi can vary depending on the species and the environmental conditions. Some fungi can complete their growth cycle in a matter of days, while others may take months or even years.

Physiological properties

Azotobacter respires aerobically, receiving energy from redox reactions, using organic compounds as electron donors, and can use a variety of carbohydrates, alcohols, and salts of organic acids as sources of carbon.

Azotobacter can fix at least 10 μg of nitrogen per gram of glucose consumed. Nitrogen fixation requires molybdenum ions, but they can be partially or completely replaced by vanadium ions. If atmospheric nitrogen is not fixed, the source of nitrogen can alternatively be nitrates, ammonium ions, or amino acids. The optimal pH for the growth and nitrogen fixation is 7.0–7.5, but growth is sustained in the pH range from 4.8 to 8.5.[23] Azotobacter can also grow mixotrophically, in a molecular nitrogen-free medium containing mannose; this growth mode is hydrogen-dependent. Hydrogen is available in the soil, thus this growth mode may occur in nature.[24]

While growing, Azotobacter produces flat, slimy, paste-like colonies with a diameter of 5–10 mm, which may form films in liquid nutrient media. The colonies can be dark-brown, green, or other colors, or may be colorless, depending on the species. The growth is favored at a temperature of 20–30°C.[25]

In culture, B. bassiana grows as a white mould. On most common cultural media, it produces many dry, powdery conidia in distinctive white spore balls. 

As a species, Beauveria bassiana parasitizes a very wide range of arthropod hosts. However, different strains vary in their host ranges, some having rather narrow ranges, like strain Bba 5653 that is very virulent to the larvae of the diamondback moth and kills only few other types of caterpillars. Some strains do have a wide host range and should, therefore, be considered nonselective biological insecticides. These should not be applied to flowers visited by pollinating insects.[8] 

The artificially grown fungi's spores are also used as a natural pesticide. Certain strains are advised against use in food-growing fields and in close proximity to water sources due to risk of their contamination. 

Fungal growth involves several stages that are necessary for the fungus to complete its life cycle. Here are the typical stages of fungal growth:

1. Spore germination: Fungal growth begins with the germination of spores, which are produced by the fungus as a means of reproduction. Spores are dispersed by wind, water, or other means and when conditions are favorable, they germinate into hyphae.

2. Hyphal growth: Hyphae are long, thin, branching filaments that grow from the germinated spores. They are responsible for the expansion of the fungal colony. Hyphae can penetrate and digest organic material, allowing the fungus to obtain nutrients.

3. Mycelium formation: As the hyphae continue to grow and branch out, they form a network of interconnected filaments called mycelium. The mycelium is responsible for absorbing nutrients and water from the surrounding environment.

4. Asexual or sexual reproduction: Depending on the species, fungi can reproduce asexually or sexually. Asexual reproduction involves the production of spores that are genetically identical to the parent fungus. Sexual reproduction involves the fusion of two different types of fungal cells to create genetically diverse spores.

5. Sporulation: Eventually, the fungus will reach a stage where it produces spores that are released into the environment to start the growth cycle anew.

6. Dormancy: Fungi may enter a dormant stage in response to adverse environmental conditions, such as low nutrients or water availability. During this stage, the fungus will stop growing and conserve its resources until conditions become favorable again.

The growth stages of fungi can vary depending on the species and the environmental conditions. Some fungi can complete their growth cycle in a matter of days, while others may take months or even years.