Mycorrhizae of terrestrial orchids
If they are well known for their bright colors and pleasant scents, it is in their roots, well hidden from view, that orchids hide their most beautiful
treasures.
These take the form of microscopic fungi (Basidiomycetes) which colonize the roots of orchids by forming mixed organs
(half-plant, half-mushroom) called “mycorrhizae”.
While most plants are capable of forming symbioses with fungi, those of orchids are a bit peculiar. They form in the root cells of the latter small
clusters, called "platoons", which are a privileged place of exchange between the two partners. These fungi are also saprophytic in the soil, that is,
they feed on organic matter such as soil litter, and also able to colonize the roots of other plants. They provide the plant with the water and
mineral elements it needs. In exchange, the plant gives up some of its photosynthetic sugars to the fungi.
In orchids, the symbiosis begins as soon as the tiny seeds germinate which, devoid of reserves, require a supply of organic matter (via these fungi)
to develop. In some non-photosynthetic orchids, such as the bird's nest Neottie, it is even the fungus which, throughout the life of the plant,
provides it with all the nutrients it needs, including sugars. that it cannot manufacture itself.
So goes the life of orchids which, secretly, cherish their treasure the better to amaze us.

  

 

Mycorrhizae of epiphytic orchids
Of the 27,000 species of orchids estimated to date, nearly 80% are epiphytes, that is, they grow on another plant, usually in the tropics,
without causing it any particular harm.
Like terrestrial orchids, epiphytic ones form symbiotic associations with microscopic fungi. A stubborn legend claims the opposite, arguably
because seeds of epiphytes germinate better without fungus in the laboratory than seeds of terrestrial orchids. These fungi, which belong to
the same group as those found in terrestrial environments, the Basidiomycetes, colonize the roots on contact with the bark and form
intracellular structures also called "platoons". Only the parts of the roots applied to the bark are colonized; the green parts are not. The
exchanges which take place at the level of these platoons are not to date as directly known as in terrestrial orchids but it is very likely that
they are of the same nature: the fungus provides the plant with water and minerals. and receives in exchange sugars resulting from photosynthesis.
Although invisible to the naked eye, the bark of these trees is therefore covered with microscopic fungi that degrade dead organic matter in the
bark and help epiphytic orchids survive. Like terrestrial orchids, they are also necessary for seed germination, although the resulting protocorms
are, in epiphytic orchids, generally photosynthetic and therefore less dependent on their fungi than their terrestrial counterparts.

 

 

Marc-André SELOSSE, Professeur du Muséum national d'Histoire naturelle,Professor at Gdansk University (Poland) and Kunming University (China)

Remi Petrolli, du laboratoire du Muséum national d'Histoire naturelle

Marc-André SELOSSE, Professeur du Muséum national d'Histoire naturelle,Professor at Gdansk University (Poland) and Kunming University (China)

Remi Petrolli, du laboratoire du Muséum national d'Histoire naturelle

 

source : Facebook orchidzone 1001 orchidées