「植物看得見你」公開課筆記／3.5 Plant Communication: Parasitic Plant

Some experiments carried out in the laboratory of Dr Consuela De Moraes at Penn State University, among the many projects in her lab, she studies a plant that's called Dodder / Cuscuta.

Dodder is a parasitic plant, it can't do it's own photosynthesis, and it can't live unless it attaches itself to a neighboring plant and sucks off the nutrients from that plant.

One of the questions Dr Moraes is asking is how does the parasitic plant find its prey? What you see here on the right are young Cuscuta plants that are germinating, they're sort of dancing in a movement, going in circles.

But what starts as what looks like random circles starts to become directional, as they somehow find their way to the tomato plant, and then encircle the stem of the tomato plant, attaching themselves, then start sucking off the nutrients which allows them to continue to grow.

Dr Moraes constructed the following apparatus to test her hypothesis that the Cuscuta found the tomato by smelling it.

In one box, she put a tomato plant. In the other box, she put the Dodder. These two boxes are only connected by a tube that allowed the air to transfer from one box to the other. Every time she did this, the Cuscuta grew towards the air pipe coming from the tomato.

If she did this where she put an empty pot in the other box, Cuscuta would not grow in that direction. So it wasn't that the Cuscuta was growing towards the smell of soil, or to the pot.

In order to further test her hypothesis that it's the smell coming off the tomato, she made what we might call a tomato extract. She took leaves and extracted chemicals from it, sort of making a tomato perfume. Once again, the Cuscuta grew towards the direction of the tomato perfume.

She then repeated this experiment with wheat and wheat seedlings. And surprisingly, when you put the Cuscuta in a box that's connected to the wheat seedlings, the Cuscuta will actually grow in an opposite direction of the wheat. In other words, the Cuscuta thinks the weed is disgusting.

I you would give the Cuscuta the choice between a wheat smell and a tomato smell, it will always go towards the tomato smell.

What she did was put the Cuscuta in the middle of a paper disk, if we have moist soil towards the bottom, the Cuscuta sometimes grew towards the moist soil, and sometimes it grew away from the moist soil in a random motion.

But if she put the tomato towards the bottom, and nothing towards the top, in most cases, the Cuscuta actually did grow towards the tomato. It's not a 100%, but in most cases, very statistically significant, the Cuscuta grow towards the tomato.

The scientists analyzed seven different chemicals, three of them was enough to cause the Dodder to grow towards that chemical. One of them being a chemical called Beta-myrcene. When you look at the extract from wheat seedlings, from their leaves, they also found about seven chemicals, and one of them was also beta-myrcene, the same chemical that we find in tomato that attracts the Cuscuta. So, why didn't the wheat attract the Dodder?

One of the reasons is, it only has been beta-myrcene and not the other ones also. So, we can sort of see here we're talking about the bouquet, the general smell. Wheat has a chemical that wasn't found in tomato, that's called (Z)-3-hexanyl acetate. This is the volatile chemical that the Cuscuta despises.