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When you go for a walk and feel, perhaps, the soft “squish” of nature underneath
your feet, you are in fact walking along the home of plants and fungi and microbes.
The soil itself might just seem like dirt, common, ubiquitous, mundane.
But dirt is essential.
It is where everything you eat comes from.
It recycles waste.
It cleans water.
It makes your life better in millions of tiny ways you never notice.
And also...it is mysterious: housing a complex network of reactions between plants and microbes.
We can’t see it with our eyes, but it’s like an economy built on chemical signals
and nutrients and competition that take place in the microcosmos, but that shape the macro
world as well.
Now, microbes require water to thrive, which can be challenging in soil.
On this channel, we’ve seen many examples of the protective cysts that many microbes
form when the habitat around them dries out, like these ciliates that will remain in this
state until the conditions become right for them to emerge again.
But protective cysts are only a small part of the story.
Soil is itself a mix of organic material, minerals, water, and air.
And that fraction of water doesn’t just sustain the plant life we see above ground,
it provides a home to many, many microorganisms that remain hidden below.
Soil is also not some mound of dirt that would be uniform but for the interruption of animals
and plant roots.
That world underneath our feet is made up of layers and zones.
For example, there’s bulk soil, the area that is untouched by plant roots.
Bulk soil is home to microbes, but at the turn of the 20th century, a German scientist
named Lorenz Hiltner noticed that the closer the soil was to a plant’s root, the higher
bacterial activity he would observe.
And from this work, he defined not just a new term, but a whole new world: the world
of the rhizosphere.
When Hiltner defined the rhizosphere in 1904, he described it as the area of the soil that
most immediately surrounded a plant root.
But in time, this definition has been expanded to describe not just the soil around the roots,
but also the parts of the root that get overtaken by microbes.
But the rhizosphere isn’t just a location.
It’s a whole set of interactions between plants and microbes.
And with the complex ecological relationship that builds, the soil of the rhizosphere can
actually look quite different from bulk soil, from the types of nutrients to its pH.
But different plants have different needs, and so the rhizosphere is a dynamic and varied
thing that changes according to the microbial communities that inhabit it and the complicated
chemical language they share.
In the big picture, roots provide structure to plants.
But they also play an important role in the rhizosphere, sending out chemical signals
that attract microbes.
These chemicals can include everything from free oxygen to water to enzymes to mucilage
to proteins to all sorts of other things.
Collectively, these compounds are known as the “root exudate.” and they might be
part of a plant’s waste that gets excreted through the root and into the surroundings.
Or they might be released either actively or passively from root cells.
Very broadly, the role of root exudate is to attract microbes.
In some terms, you could consider it an invitation, but it’s an invitation that comes with a
cost for the plant.
Some of those exudates are nutrients the plant themselves could use, so why spend them on
an invitation?
What can the microbes bring in exchange?
Well, for one, microbes help plants access nutrients they might have trouble getting
on their own.
Above ground, plants can access carbon dioxide, water, oxygen, and light.
And below ground, they get structure and many of the additional mineral elements they need
to survive.
But some elements, like nitrogen, are pretty difficult for plants to get.
Nitrogen is essential for life, and it is also plentiful, but it is so difficult to
convert Nitrogen into the forms life needs and plants aren’t able to do that.
Instead, they rely on the bacteria of the rhizosphere to take Nitrogen and convert it
into ammonia and other compounds that they can actually use.
Some plants, like legumes, even form nodules on their roots to house those bacteria, creating
an even tighter symbiotic relationship.
Now, whenever you invite someone over, there’s a little bit of a risk to that, especially
when, you know, sometimes people misunderstand the nature of the invitation.
Releasing compounds that attract bacteria is all well and good until the pathogens also
catch wind of what’s going on.
Sometimes these pathogens make it to the plant and are able to infect it.
But then other times, the non-pathogenic bacteria can shield the plant, creating protective
biofilms or releasing toxic compounds that target the pathogens.
Like your good friends that help you kick that guy out, who I don’t think you even
invited.
But collecting such an active bacterial community is also a great way to attract predators.
Protozoa and nematodes alike can be found in the rhizosphere thanks to the buffet of
of bacteria that await them.
And yet this is a risk that plays somewhat into the plant’s interests as well: as these
predators graze on the bacteria, they excrete nitrogenous waste that can continue to contribute
to plant growth.
(Or, in some cases, the predator dies, and they become the source of nutrients.)
The composition of these microbial communities is shaped by the compounds released by the
plant’s roots.
Scientists are studying the ways these specific molecules in the root exudate can influence
the presence of specific bacteria and fungi.
As we learn more about the rhizosphere, we may come to understand not only how these
root exudates influence the ecology of microbes and plants alike, but how we can better interact
with it when growing our own crops.
These chemical signals and the interactions they shape form a dialogue between different
parts of the living world.
They speak a language whose basic vocabulary we may understand, but whose full syntax is
complex and wonderful and mysterious.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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