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Our microbiomes have become super famous in the last several years.
We’ve learned that a world of tiny organisms living inside of us
helps us digest food, produce vitamins, and regulate our immune system.
But humans aren’t the only life form playing host to entire ecosystems.
Other animals have their own complex microbiomes, and amazingly, so do plants.
And these tiny microbes may play a surprisingly
large role in plants’ survival and evolution.
Plant microbiomes are partly made up of fungi and bacteria called endophytes,
which live at least part of their lives inside plants.
Unlike the microbes that cause mildew and root rot, these don’t harm their hosts.
So plants and endophytes get along just fine.
In some cases, a plant and endophyte will spend their entire lives together,
almost as if they were a single organism.
For instance, obligate endophytes may actually start out in the seed
of their host plant, along with all the genetic information for a new, baby plant.
So they’re literally with their host plant before it even exists.
Then, as the seed grows into a plant,
they feed on its nutrients and take in its water.
The plant also protects them from dangerous UV light and extreme temperatures.
And safe inside the plant, endophytes don’t have to
compete for resources with other microorganisms.
All this doesn’t cost the plant much, so it continues to grow healthily.
Meanwhile, the endophytes multiply and spread,
colonizing the leaves and tips of the shoot where flowers will eventually bloom.
That way, they get incorporated into the egg, then the embryo, and eventually,
a seed, ready to start a new generation of plant-microbe friendship.
But not all endophytes spend their whole lives in a plant.
Others can live on their own just fine, but they’ve evolved to thrive inside plants.
These so-called facultative endophytes live outside a plant
for part of their life cycle, often in the soil or even in an insect.
But they’ll seize any opportunity to hop onboard a plant.
For instance, if they come into contact with the natural cracks in a plant’s roots,
they’ll make their way in and colonize the whole thing.
Meanwhile, passive endophytes never go looking to colonize a plant at all,
but sometimes they’ll still end up making a home there.
Like, if some bacteria and fungi in the soil encounter some open wounds
on a plant’s roots, they might just settle in.
But since these microbes aren’t specialized for living inside plants,
they never actually become symbiotic.
Endophytes don’t just reap the benefits of living in a plant, though;
many actually give a lot back.
Some of them secrete chemicals that protect
a plant from herbivores and harmful invaders.
Because, after all, the plant is their home, so it’s their best interest to protect it.
Some endophytes also regulate their host plants’ pores.
For instance, when the weather is hot and dry, endophytes can help
keep pores closed so the plant loses less water, protecting it from drought.
These pores also let carbon dioxide in and oxygen out.
Since these are inputs and outputs of photosynthesis,
regulating them can make a plant more efficient at converting sunlight into fuel.
And finally, host plants can also count on certain endophytes
to help them take in nutrients.
When these microbes colonize a root system, they increase the roots’
surface area, which lets the plant take up more water and nutrients from the soil.
Once inside a plant, endophytes can also break down nitrogen in the air and
turn it into other compounds the plant can use.
On top of all that, some will secrete hormones that can make
their host more efficient at absorbing nutrients.
With all these extra resources, a plant can grow bigger and produce
more fruits and flowers that spread seeds.
And sometimes, the endophytes in a plant’s seed might convert enough nitrogen
into nutrients to help its host to sprout in the first place, even in harsh conditions.
So, these tiny microbes can be incredibly important to plants’ survival.
And plants and endophytes seem to have been
working together for a long, long time.
Fossil records suggest that fungi may have colonized the very first plants
to live on land, 400 million years ago.
In fact, they might have actually helped plants evolve to survive here.
A paper in 2003 found that there’s at least one type of endophyte
in every single plant species ever discovered.
But they haven’t just lived together all these years.
It seems that plants and their endophytes have actually evolved together.
What’s amazing about that is that, historically, scientists thought natural selection
was pretty simple: Individual organisms with the best genes were the most likely
to survive and pass on their DNA.
And they thought this is what drove the evolution of a species.
But when researchers discovered endophytes,
they realized that may not be the whole story for plants.
Now, scientists think that natural selection
acts on a plant species plus its microbiome.
So, in survival of the fittest, fitness is determined by the genes of a plant
and the genes of all the microbes that call that plant home…
almost as if the whole ecosystem were a single organism.
At first, the relationship between a plant and an endophyte might seem like your
run-of-the-mill symbiotic relationship.
But plant microbiomes are incredibly complex, just like humans’.
And what we’ve learned so far shows that the relationships between plants
and their microbes are profound.
They may not affect just a single plant, but a species as a whole.
Which is a pretty major achievement for a microscopic world.
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