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If you take a seed from your orange and plant it in the ground, it’ll grow into a tree.
But, to get fruit, you might have to wait as long as 15 years.
Which is kind of a bummer for the impatient types among us.
You can’t blame the plant, since it has to grow up and go through the plant version
of puberty before it can start making flowers and then fruit.
But don’t despair, OJ lovers, because there’s an age-old trick called grafting that
can shorten the wait to about three years.
And it can do all kinds of other things, too.
All you do is smoosh part of one plant into part of another
plant and you get a super-plant.
Which seems awfully easy.
But even weirder, it turns out to be a low-tech way of tinkering with a plant’s genes.
It just goes to show that long before anyone invented modern gene editing,
humans have been finding ways to manipulate plants into feeding us
on our own terms.
Grafting is as simple as taking a root from one plant and sticking it with a
shoot from another.
With enough pressure and moisture, the two should fuse into one.
It almost always works with plants of the same species,
like between apple varieties.
But it can sometimes work with different species.
In fact, it can sometimes work between plants of different genuses,
like pear and quince.
If you take an older shoot, from a tree that’s already producing fruit,
and merge it with a younger root, the shoot maintains its mature state.
This works with even miniscule shoots.
In vitro grafting takes a shoot from a mature tree that’s just
one-and-a-half millimeters long, and grafts it onto the roots
from a just-sprouted seedling.
It’s not only a shortcut to getting earlier fruit,
it’s also a way to get rid of citrus-infecting viruses.
That’s because viruses and other pathogens usually don’t infect the very
tips of a plant’s shoots.
This micro-grafting is a pretty recent development.
But grafting in the more conventional sense is far from new.
People have been using it for at least 2,500 years.
No one is sure how fruit growers first came up with the idea,
but they may have taken inspiration from a similar process that can happen
naturally when the roots or branches of neighboring trees meet and fuse.
But regardless of how grafting started, over the centuries people have come up
with all kinds of cool ways to use it.
These range from building fanciful and artistic forms, to manipulating a tree’s size
and fruit characteristics, to engineering plants that resist cold,
drought, or deadly diseases.
In the late 1800s, grafting even saved the entire wine industry from being
wiped out by an insect pest.
This pest came from the Americas, and most European
grapes had no resistance to it.
But growers found although American grapes were no
good for making wine, their roots resisted the pests.
So grafting shoots from European wine grapes onto roots from
American grapes made the entire plant resistant.
Today, nearly all wine grapes are grown this way.
But by far, grafting’s main use, both early on and into today, is asexual propagation.
Basically, it’s a way to take a tree that produces great fruit and clone it,
over and over.
For many types of fruit trees,
scientists think this was an important early step in domestication.
That’s because most fruit trees don’t grow true from seed.
That means if you plant a seed from your Red Delicious apple,
it’ll grow into a tree.
But it’ll probably make a totally different kind of apple.
And, after waiting a decade or so for it to start making fruit,
you may find that it doesn’t even taste good.
It’s easier to develop true-breeding varieties of plants with shorter life cycles,
because we have the patience to do that.
But for slow-maturing plants like apple trees,
it’s much faster to pick a specimen that makes good fruit and clone it.
Plus, if someone invents a better variety, or if you’re a fruit grower and consumer
tastes change, you can try grafting new shoots onto your established trees
to get back in the game faster.
But it’s not just good fruit we’re after.
A lot of the work over the centuries has gone into developing
the plants that provide the roots,
also known as rootstock.
And for the root, you can focus on a totally different set of traits than
you would for a tree’s fruit-making half.
Like, there are citrus rootstocks that are cold tolerant,
and grape rootstocks that can tolerate drought and salt,
and even thrive in different types of soil.
A common use for rootstocks is to limit a tree’s size.
This can work a number of ways, but it means that by mixing and matching
rootstocks, you can grow your favorite fruit varieties in practically any size, from
dwarf for easy harvesting to full-sized.
And beyond fruit trees, grafting is becoming more common in non-woody plants
too, like tomatoes, peppers, and melons.
Because every efficiency-loving gardener needs a Pomato.
Potatoes on the bottom, tomatoes on top.
It’s the mullet of plants, and also, I need one!
Now for a root and a shoot to make a good life together
it takes a lot more than just smooshing them together.
On the part of the plant, that is.
They need to be able to communicate with each other and share resources.
Only then can they work together as one.
Setting all that up takes quite a bit of work from both plant halves.
And we now have a pretty decent understanding of how it works at the
molecular, cellular, and organismal levels.
The initial fusion process works a lot like wound healing,
like when a branch breaks or gets a cut.
Or sort of like how a broken bone knits itself back together.
Soon after root and shoot are mashed together,
cells at the cut edges de-differentiate.
Basically, mature cells lose their specialized characteristics and become stem cells,
with the potential to become any number of cell types.
Then they divide to form a mass called a callus.
With the help of hormones and other molecules,
the cells differentiate to form various types of tissues, including vascular tissue.
In a plant, these are phloem and xylem,
basically tubes that carry materials from one part of a plant to another.
And they’re essential for joining the top and the bottom into a functional whole.
Phloem is made up of living cells that mostly carry things like sugars
made in the leaves down to the roots.
And xylem is made of dead cells that mostly carry things like water and
nutrients from the roots to the top.
At the graft junction, adjacent cells also form structures called plasmodesmata,
channels that let materials flow between them.
All these communication networks are like a superhighway that plants use to
seamlessly share materials and information,
sort of like our nerves and blood vessels.
And plants use these channels to share not only nutrients and water,
but also signaling molecules that affect gene expression,
like hormones, proteins, and even RNA.
These signals originate in both root and shoot and travel in both directions,
which is what makes a grafted plant more than the sum of its parts.
Plant hormones are crucial for establishing a
connection between a root and a grafted shoot.
A lot like the hormones in our bodies, these small molecules can affect the activity
levels of large numbers of genes,
traveling across large distances to regulate growth and development.
And hormones and other molecules from disease-resistant rootstock can make
the entire plant resistant.
Proteins can travel in a plant’s vascular system too,
with similar broad-reaching effects.
In tomatoes, a protein made in the shoot can stimulate root growth.
And in grapes, proteins made in the root help control bacterial and fungal
growth in the shoot.
And a big area of discovery in recent years is the regulation of gene expression
through small RNA molecules, via a mechanism called RNA interference.
It uses short RNA molecules, up to about 30 building blocks long.
They include microRNAs and small interfering RNAs, or siRNAs.
And they can move through a plant’s vascular system to affect the entire plant.
Often they affect other genes, by dialing the genes’ activity up or down,
causing more or less of these genes’ protein products to be made.
Depending on the small RNA, they can work in a couple ways.
One way is as a type of natural RNA interference.
They pair with the RNA molecules that direct the production of proteins,
then destroy or modify that RNA before it can do that.
This is a way of temporarily dialing down the production of certain proteins.
Other small RNAs work by silencing genes at the DNA level,
through changes to the plant’s epigenome.
The small RNAs signal to cellular machinery to come to specific genes and add
molecular tags called methyl groups to them.
Genes that are tagged this way are less active, which can be
beneficial in the right circumstances.
Not only that, small RNAs often regulate genes whose
products regulate other genes.
So one small RNA can have an amplifying effect that
ultimately controls large networks of genes.
And complex processes, like plant puberty.
At least two small RNAs have key roles in regulating this process:
microRNA156 and microRNA172.
Studies of microRNA156 have shown that its levels are higher in
younger plants, and go down as the plants reach maturity.
In two studies from 2012, artificially increasing the level of this microRNA
slowed maturation and flowering.
Other experiments have shown that microRNA156 responds to environmental
conditions, and it controls protein production from a lot of genes
that are important for a variety of processes throughout the life of a plant.
Not just flower production, but also things like embryonic development,
root growth, and pigment distribution.
And it’s been shown to move across grafts.
Scientists are working hard to learn more about how this and other small RNAs
affect plant genes, and how manipulating these powerful
regulators can help to improve plant traits.
One possible application is to use gene editing to engineer rootstocks
with improved traits like disease or stress resistance.
It will take more research to figure out how to make something like this work.
But even without doing any direct, deliberate gene editing,
it’s pretty amazing what humans have been able to accomplish
with grafting over a few thousand years of trial and error.
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