Natures wonders: smart plant phototropism (Introduction)

by David Turell , Thursday, February 01, 2024, 00:02 (86 days ago) @ David Turell

They do see light by a tricky way:

https://www.quantamagazine.org/plants-find-light-using-gaps-between-their-cells-20240131/

"Plants, biologists have established, possess a powerful suite of molecular tools for measuring illumination. But in the absence of obvious physical sensing organs like lenses, how do plants work out the precise direction from which light is coming?

"Now, a team of European researchers has hit upon an answer. In a recent paper published in Science, they report that a roadside weed — Arabidopsis, a favorite of plant geneticists — uses the air spaces between its cells to scatter light, modifying the path of light passing through its tissues. In this way, the air channels create a light gradient that helps seedlings accurately determine where light is coming from.

"By taking advantage of air channels to scatter light, plants sidestep the need for discrete organs like eyes in favor of a neater trick: the ability in effect to “see” with their whole bodies.

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"...simple sensors aren’t enough on their own to give plants the ability to determine light’s direction. To best pinpoint the direction of strong illumination, a plant needs to be able to compare signals between different photoreceptors so that they can orient their growth toward the most intense light. And for that they need incoming light to fall onto their sensors in a gradient from brightest to dimmest.

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"The researchers deduced that the plant orients itself to light through a mechanism based on the phenomenon of refraction — the tendency of light to change direction as it passes through different media. Because of refraction, Legris explained, light passing through a normal Arabidopsis will scatter under the surface of the stem: Every time it moves through a plant cell, which is mostly water, and then through an air channel, it changes direction. Since some of the light is redirected in the process, the air channels establish a steep light gradient across different cells, which the plant can use to assess the light’s direction and then grow toward it.

"In contrast, when these air channels are filled with water, the scattering of light is reduced. Plant cells refract light in a similar way as a flooded channel, since they both contain water. Instead of scattering, the light passes almost straight through the cells and the flooded channels to deeper within the tissue, decreasing the light gradient and depriving the seedling of differences in light intensity.

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“'It was always baffling to us how these little, tiny — almost transparent — [embryonic plants] could detect a gradient,” Hangarter said. “We never really gave much credence to the air-space thing because we were distracted looking for molecules that were involved. You get on a certain research path, and you get blinders on.”

"The air-channel mechanism joins other ingenious devices that plants have evolved to control how light moves through them. For example, research by Hangarter helped establish that chloroplasts — the cellular organelles that perform photosynthesis — actively dance inside leaf cells to move light around. Chloroplasts can cluster greedily in the center of the cell to soak up weak light or flee to the margins to let stronger light pass deeper into plant tissues.

"For now, the new findings about air channels extend only to seedlings. While these air channels also appear in adult leaves, where they’ve been shown to play a role in light scattering and distribution, nobody’s yet tested whether they play a role in phototropism, Legris said.

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“'Many people have the feeling that plants are very passive organisms — they can’t anticipate anything; they just do what happens to them.”

"But that idea is based in our expectations of what eyes should look like. Plants, it turns out, have evolved a way of seeing with their whole bodies, one woven into the gaps between their cells. They don’t need anything so clumsy as a pair of eyes to follow the light."

Comment: phototropism is very useful, and plants have it. It is a neat trick using plant, liquid refraction variations, but a property of water is to bend light, showing another useful attribute of water in life.