「植物看得見你」公開課筆記／2.4 Light-activated Switch

What we have here is actually a molecular switch. A switch that's working at the level of the protein within the cell, that lets the plant know when to flower.

The scientists finally isolated this molecule and it's called phytochrome, it is a light-activated switch. We can actually understand how light affects a plant at a molecular level.

How we imagine phytochrome to look like. It's actually a protein which has two different components, one is the protein component, the other is on the top, chromophore, the part of the protein which absorbs the light.

Phytochrome can absorb either red or far red light. It can be found in one of two confirmations in the cells. The first confirmation call Pr (phytochrome red), it absorbs red light. When it's in a second confirmation, it call Pfr (phytochrome far red), it will absorb far red light.

Pr absorbs red light, and once it absorbs the red light, it changes its conformation to become Pfr, . Molecularly, there's a slight change in the way the protein is formed. It becomes the Pfr and that is what makes it active.

If you give the flashes very quickly, you've gone up down, there's been no effect. But if you give the red light flash and then wait awhile before you give the far red, then it's too late. The signals already being transduced forward.

Absorption spectra shows what color of light phytochrome actually absorbs when the light hits it. When it's in the Pr form, we say that it's absorbing light that's primarily in the red form. It cannot absorb far red light. The slight change in its confirmation, means now in the Pfr form, it can absorb the far red light, primarily.

In the morning as the sun is rising, what we see is that the sun. which has a light wave -- a longer path through the atmosphere. There's more far red light, than there is red light.

As the sun rises, up to its zenith, the ratio of red to far red changes, that there's much more red light, than there is far red light. So for the majority of the day, a plant is seeing primarily red and not far red light.

As the sun goes down, the ratio again changes such that the longer and longer wavelengths are reaching the Earth. The ratio of red to far red becomes so low, that the plant is only seeing far red light at the end of the day, which is signalling to the plant that the night has begun.

In the experiment that we showed earlier, when we turn on the red light, it thinks that the day has begun. If we do red immediately far red, again it thinks that the night, the day has ended, and night has begun.

Plants see red and far red, not only according to the time of the day, but also where they are in your garden or in the forest.

For example, a seedling that's growing under direct light is being hit by both the red and far red light. And in the middle of the day it's much more red than it is far red.

But if the same seedling is growing under the canopy, under the shade of another plant, the type of light that's reaching it under this leaf is completely different. Because the red light is absorbed by a green leaf for photosynthesis, the energy making machinery of the plant.

The far red light isn't absorbed for photosynthesis. It goes through the leaf, and it hits the seedling growing under the canopy. So a seedling growing underneath another leaf is getting much more far red light than it is red light.

How does this affect the plant? It lets it know that it's shaded and the plant will respond by elongating in order to get out of the shade. So the red-far-red ratio not only lets a plant know if it's morning, middle of the day, or night. It lets the plant know if it's in the shade.

You've seen this probably in plants in your garden, that if they're too shaded, they become long and spindly, until they reach the sun, and then they expand their leaves.