Microbial Symbiosis: The Keystone of Plant Growth
Let’s start by taking a step back to what most of us understand about plants, to ask ourselves a simple question, how do they grow? Many will immediately think of a diagram they were shown in a textbook years back describing the fundamentals of botanical structure. Imagining the layout of a flower or tree, visual growth above the ground with the stem, branches, leaves and even flowers. Yet as above isn’t always as below, the root structure of plants is often much more diverse and sprawling than the growth we notice. Tree’s for example typically spread out their root diameter 2-3 times the height of the tree itself. Roots do serve the purpose of absorbing nutrients from the soil, but what is the actual process happening? What does the root zone actually look like?
Firstly let’s define it, the area surrounding plant roots has been scientifically named the Rhizosphere. Charted through a series of gradients, it describes an area extending around 4mm out from a plant root. Within this very small zone are colonies of microorganisms, ranging from simple beings such as bacteria to more complex societies of mycorrhizae and paramecium. How these relationships first developed is a particular mystery, yet the purpose they serve is not. Each of these organisms has evolved particular functions that can be correlated with beneficial plant growth or vitality, accomplishing such things as nutrient cycling, atmospheric nitrogen fixation, water retention, raising organic matter, pathogen suppression and more.
These organisms aren’t just laying around by chance, they’re actually invited in through an enticing array of exudates and hormones. Exudates are the outermost layers of root hairs that have sheared off throughout their movement in the soil, during which the root begins to release a chemical mixture of beneficial substances to the microbes; such as amino acids, sugars, enzymes and even organic acids that adjust the pH to what the microbes desire. This surplus draws the populations towards the roots, giving them an ideal living condition within the sanctuary of the root. In exchange the microbes or fungi now associated with the plant will work towards supplying a proportion of their nutrients towards their new partner.
That process in particular is referred to as rhizophagy, or “root eating”. Certain species of bacteria form colonies directly on the tips of root hairs, slowly working their way into the roots’ periplasmic space, the small gap between a cell wall and cell plasma membrane. Once inside, the plant begins to dissolve the cell walls of the bacteria inside by releasing superoxide molecules. A process that would eventually be lethal if these bacteria haven’t also evolved to produce an enzyme aptly named, superoxide dismutase. Slowing down the dissolving of their cell wall to endure the length of time the plant takes to get what it needs. Eventually the used up bacteria accumulate a little further down in the root hairs, growing in density until the hair must literally burst open and spew out all the cycled bacteria. Soon after, the microbes develop their membranes again and begin their trek back towards the front of the root, thus continuing the cycle.
Fungi, particularly a superbly beneficial species called Arbuscular Mycorrhizal Fungi, have worked out a much more appealing deal for themselves. Their networks are sprawling beyond imagination, connecting plants from all throughout their environment and with a stunning amount of interconnection, a single teaspoon of healthy soil can contain multiple kilometers of fungal hyphae. Similar to their microbial companions, fungi wait until a specific hormone has been released by a nearby and suitable plant. When detected, the hyphae begin to grow towards the root and release a hormone of their own to signal to the plant they’re ready for part two. Now the plant essentially inhibits its immune response in the signaled area, allowing the fungi to grow directly into the root. However the root gently travels between the layers of the first few cell walls until it reaches an interior cell membrane. Only then will it grow into the cell itself and form what is called an Arbuscule, a direct connection between the fungal network and the plant for nutrient exchange.
These connections are the essential points of plant vitality, allowing for all further processes to continue. Photosynthesis only produces one of the many vital compounds needed for life, and without other supplied nutrients that process begins to break down as well. Cell division, photosynthesis, maintaining homeostasis, protein production; all of these require nutrients that a plant cannot solely produce. These symbiotic relationships are necessary and yet so easily diminished. The smallest changes on our scale are a biblical event on the microscopic. Industrial agriculture and urbanization has changed our soil environment so that it rarely harbors the biodiversity it once did, stunting our potential without us even knowing. But there’s no reason it can’t change back.
