How does a plant know where the light comes from?

The simplest questions are always the best: Everyone knows that plants grow toward the light. But how does this plant know where the light comes from? There has certainly been no agreement on this among scholars yet. Researchers from the University of Lausanne have now discovered at least one mechanism used to “see” sunlight.

They had a lucky chance. The Swiss were experimenting with garden cress, a cruciferous plant often used in plant genetic research. It is standard procedure to grow new mutants in the dark and illuminate them artificially.

“You then have a kind of bean sprout, but it is very thin, about fifteen cells in diameter,” says Ronald Perek, professor of molecular biology at Wageningen. He was not involved in this research himself, but he knows the procedure well. “Then you shine light on those strings from the side. Because they are so thin, light passes through them, so you could say that all the cells receive the same amount of light. But the plant can still determine where the light is coming from and respond accordingly. How this could be achieved was “It’s still really an open question. Although I only realized that when I was asked.”

Optical orientation

This phenomenon is called phototropism. In one of the mutants of the thall cress, the Swiss researchers noticed something: The thread was much more transparent than in the wild thall cress, which is milky white at that stage of its development. Meanwhile, the mutant did not respond to the light in the same way. Could this be related?

This has actually turned out to be the case. The white color in wild watercress results from the presence of tiny air spaces between the cells. It is only a small fraction of the size of a cell, but it performs an important function. This air refracts incoming light differently than water. The difference between the two helps determine the source of the light, the researchers wrote in the journal Sciences.

Rainbow

Compare that to how a rainbow forms, says researcher Martina Legris of the Swiss team. This so-called “refraction” of light enables the plant to identify the light source. Just as you look at a rainbow, you can be sure that the sun is behind you.

In the pellucidum mutant, the intercellular space was largely filled with fluid. Phototropism is not completely absent here, because the cell wall also plays a role. It also has a different refraction than air and liquid, so the plant was still able to determine its direction to some extent. This effect was further confirmed when the researchers saturated different species of garden cress with water. This no longer mattered for the mutant, as there was already no air between the cells. But the wild whale cress lost most of its phototropism and ended up at a level similar to the mutation.

This discovery is certainly not the final word on the “sensing ability” of plants. For example, the researchers point to a layer around air cavities filled with electrons. What exactly it is made of and what effect it has remains to be clarified. The study shows once again that there are still many questions to be answered, including regarding garden cress. I always asked them.