Mycorrhiza or also called mycorrhizal symbiosis

The production of hypogeous fruiting bodies of truffles is linked to the formation of a particular association between the mycelium of the fungus and the roots of certain trees: the mycorrhiza or also called mycorrhizal symbiosis.

Mycorrhizal symbiosis was first observed by an Italian, but it was a German who discovered the symbiotic function of these particular structures, which he named "mycorrhizae" ("mycos" = fungus and "rhiza" = root).

The mycorrhizal phenomenon is not exclusive to truffles, as many other fungi are mycorrhizal with different plants.

Based on morphological and anatomical characteristics, mycorrhizae are divided into three main groups:

• ectomycorrhizae,
• endomycorrhizae,
• and ectoendomycorrhizae.

Truffles, by joining with plant roots, exclusively form ectomycorrhizae; therefore, only this type of mycorrhiza will be described in more detail below.

General characteristics of ectomycorrhizae

The roots affected by ectomycorrhizae are the secondary roots, the shorter ones, normally equipped with root hairs, which have the primary function of absorbing nutrients from the soil.

The entire surface of the rootlets is involved in the mycorrhization process; when fungal colonization is complete, ectomychorrizal roots are recognized because they are devoid of root hairs, which are replaced in their function by the dense mycelial felt. Furthermore, the apex no longer has the typical elongated and pointed appearance, but appears swollen and rounded, club-shaped.

Observing a section of an ectomycorrhiza under an optical microscope, the mantle and its internal continuation, the Hartig net, are clearly visible. The mantle, composed of several layers of hyphae, can vary in thickness, texture, and color depending on the host species, the nature of the symbiotic fungus, environmental conditions, and the age of the mycorrhiza itself.

The hyphae of the outer layers are devoid of cytoplasm and rich in tannic material, while those of the deeper layers contain storage material and cytoplasm reduced to a thin strip. The Hartig net, on the other hand, is composed of living hyphae in direct contact with the root cells; the passage of elements between fungal hyphae and root cells occurs through their respective juxtaposed walls.

Vegetative cycle of ectomycorrhizae

In nature, truffle mycorrhizae have their own vegetative cycle, related to the biological cycle of the symbiotic plant. In spring, mycorrhizae are sites of intense metabolic activity; in this season, new hyphae develop from the mycorrhizae, colonizing both the surrounding soil and the new rootlets meanwhile emitted by the root system.

Throughout the summer, the growth and enlargement activity of the mycorrhizae continues at alternating rates depending on the soil's water conditions. In autumn, following the reduction and blockage of chlorophyll photosynthesis, the mycelium ceases its exclusively vegetative development and proceeds to the formation of the fruiting body.

Functions and utility of mycorrhizal symbiosis

Mycorrhizae represent a fundamental stage in the biological cycle of truffles, but they also offer an advantage for the plant. In fact, mycorrhizae ensure better absorption of water and mineral elements from the soil, especially the less noble ones. Mycorrhization allows for better absorption of;

• phosphorus,
• nitrogen,
• potassium,
• zinc,
• sulfur,
• copper.

In particular, mycorrhizal plants absorb three to nine times more phosphorus and two to six times more nitrogen than non-mycorrhizal plants. This effect is due to both the increased absorbing surface of the root system and an increase in the number and size of the root tips.

Mycorrhizal fungi play a significant role in the recycling of nutrients in the forest. The dense network of mycelium, invading the soil and litter, extracts nutrients which it transfers directly to the plant, minimizing the risk of losses due to leaching.

Truffle-mycorrhized plants, or truffle-producing plants, are also advantaged by the fact that their roots appear not only longer-lived but also more resistant to both adverse conditions and, especially, to certain root pathogens.

In fact, on the one hand, the mycorrhiza forms a physical barrier to the penetration of such pathogens, and on the other hand, the mycorrhizal roots themselves are capable of secreting certain antibiotics that selectively inhibit root pathogens.