MARIA HARRISON: The plants grow down. Obviously they have to go down to obtain water and nutrients from the soil. But the soil's a very difficult environment. If you live in Ithaca, it's full of rocks. And the roots need to be able to negotiate around these rocks in order to grow down. If you meet a rock and you're unable to grow through it, it's necessary to have some kind of root growth behavior that would allow you to explore whether there's an edge over which you might be able to grow down.
DANIELA FLOSS: And I'm a post doc in Maria Harrison's lab at Boyce Thompson Institute for plant research, an affiliate institute from Cornell. Namely, I basically showed them how to grow Medicago truncatula. That's this little plant here in this magenta box.
And in order to make a movie to show root growth, we had to use a transparent agar. Here you only can see one, but you don't see how the root growth. So what we did we kind of used this gel, which is very transparent. It contains macro and micro nutrients that the plant can grow. And you clearly can see how the root growth towards the bottom of the plate.
ITAI COHEN: My lab has been interested in the way that plants grow in rough terrain. And the basic question is how their roots overcome obstacles that they encounter as they try to grow deeper into the soil. And then the plant growing game-- going deeper-- means you get more nutrients, more water. So that's the goal.
So one of the experiments that we just reported on in PNAS had to do with what happens when roots encounter a barrier as they grow down.
MARIA HARRISON: So it's known that in some of the cells in the tip of the root there are small, dense particles full of starch that sink to the bottom of the cell. So if the root bends when the root's like this, these tiny bodies are called statoliths. And this somehow sends a signal to the cell and then to the root that it's vertically oriented and that it should keep growing down. Now, how exactly that happens it isn't known.
ITAI COHEN: But the question comes what happens when a root encounters a plane that's flat or tilted at some angle? And what my student and Chris Henley's student, Tzer Han, observed-- he was an undergraduate here at Cornell who's now in Boston, MIT. What he observed was that roots display all these different morphologies.
Sometimes they'll grow in little sort of circles. Other times they wave. And if the surface is very steep, they'll actually go straight down the surface.
And prior to his work, people were trying to understand the different root morphologies as arising from some sort of underlying genetic characteristics of the plant. But what Tzer Han was able to explain was that all of these morphologies actually arose from a search strategy. So the basic idea is this.
If a plant has these gravitation detection cells-- so this ability to tell which way is down-- if that detector is not perfect, then when a root encounters a surface that's a little bit flatter than perpendicular, then those gravitational cells can't direct the root to grow straight down. You have to search for the downward slope. And the less tilted the surface, the harder it is for those gravitational cells to detect which way is down.
And so what happens is that because of this limited fidelity of these cells to be able to tell the route which direction is straight down the slope, the root ends up creating a search strategy. And the search strategy is such that the root sort of meanders back and forth. And you're going to see this in the video that we took, which is a time lapsed video of Medicago truncatula as it's growing through a gel, a transparent gel, and then encounters a slide, a glass slide that we put into the gel, which is tilted. And as the root encounters the glass slide, what you'll see is it starts to grow. And then it starts to search for the downward direction.
And the amazing thing about this search is that when the root switches the direction, it's almost always picking the correct way down with 90% fidelity. And so the idea is that this root has developed a grow and switch strategy to determine which way is the most efficient way down the slope. And that allows it to look for the nutrients and get the water.
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Most plants grow towards the light, but roots have to grow down into the soil to find water and nutrients. How do they do this? And what happens to a growing root when it encounters a rock? In this video a group of investigators from the Physics Department and from the Boyce Thompson Institute for Plant Research show us how plants manage to grow in Ithaca's rocky soil.