An underground resistance network.
For several millennia, humans have exchanged brightly colored flowers, in a so-called language of flowers — primarily to communicate their love. Of course, flowers are also the means by which plants are pollinated by insects and thus reproduce. We now know that plants use yet another language, that of roots rather then flowers, for communication and self-preservation.
It is a very ancient language, yet its existence was only demonstrated in recent years. It is mediated by fungus living in symbiosis with the roots of the plants.
An apt analogy is with the microbiome: bacteria we carry — on our scalp, ears, mouth and nose, skin, armpits, intestine, anus, urinary-genital tracts, — that are highly beneficial. Wanton destruction of these symbionts, due to obsessive phobia of ‟germs‟, is deleterious. Why ? Especially because of the very ancient coupling between the two genomes, in our case, the human genome and that of the microbiome. It is a mutualism, i.e., a form of symbiosis beneficial to both partners. The human microbiome is an essential component of our immunity. We ought to relearn how to live in harmony with nature, rather than wantonly destroying it. But let us return to the hard facts of plant life, there is much to be learned from them
Indeed, the relationship of fungi and higher plants is comparable to that between humans and bacteria — only few of which are pathogenic. It is a symbiotic interaction: fungi benefit from plant photosynthesis, they absorb up to 30 % of its output, the sugars the plant produces from atmospheric CO2. In exchange for these carbon-containing organic molecules, fungi pass on to the plant nitrogen, phosphorus, potassium and other mineral nutrients.
Is such symbiosis frequent or rare? Not at all. As many as 95 % of all plant species enjoy such relationships with fungi. Moreover, the plant could not survive in their absence. For example, the so-called ectomycorrhizas (EcM), typically involve Ascomycota, Basidiomycota and Zygomycota fungi, thriving on mostly woody plants such as the birch, eucalyptus, oak, pine, rose and orchids as well. An individual tree may have 15 or more different fungal EcM partners at one time.
The fungus develops a tiny arborescent network of filaments, known as an arbuscule, whose purpose is to carry nutrients in both directions, from the fungus to the plant and vice-versa. In a plant root, within an interacting cell, the membrane fits over the arbuscule like a rubber glove over your hand. Thus the fungus never comes into direct contact with the root cell nucleus, mitochondria or other cell structures. The exchange of nutrients between the two partners, minerals from fungus to plant and sugars from plant to fungus, takes place at the cell membrane-arbuscule interface.
Moreover, these tiny underground networks not only connect fungi and plant roots, they also interconnect plants. An instance is the feeding of carbon, i.e., sugars, from paper birches (Betula papyrifera) as donors to Douglas-firs (Pseudotsuga menzies) as acceptors.
In their struggle to survive attacks by herbivores, plants also exchange defense signals through such networks. For instance, pathogen-infected tomato plants (Lycopersicon esculentum) can communicate such invasion to nearby healthy tomato plants, thus stimulating their defense mechanisms, i.e., enhanced production of enzymes, to attack and destroy the invaders.