How do plants steer toward the light? They have eyes the size of molecules.
What are eyes, if not light collectors that send signals? Then plants have eyes, too. Phytochromes are proteins that are sensitive to light in plants and some bacteria. When one of these molecules gets hit by a beam of light, it undergoes a gymnastic change in shape that switches on a whole cascade of downstream effects, including steering the stem or leaf toward the source of light. That’s why the beanstalks kids grow in a box grow toward the hole where the light is.
Phytochromes are molecular machines that switch on other machines. A tiny change in the phytochrome’s shape, amounting to a few Angstrom units, can be amplified into leaf motion that is orders of magnitude larger—something like a child flipping a switch that launches a rocket.
More about this process was published this month in Nature, “Signal amplification and transduction in phytochrome photosensors.” A lay-level summary is provided by the University of Gothenburg, where some members of the research team hail from. In “Light-Sensitive Eyes in Plants,” the press release describes this wonder of nature:
Most plants try to avoid the shade and grow towards the light, which enables them, among other things, to consume more carbon dioxide through photosynthesis. Proteins known as “phytochromes” control this process. The phytochromes in the plants are thus changed through the light radiation, and signals are passed onwards to the cells.
Phytochromes have, as do most other proteins, a three-dimensional molecular structure. Light is absorbed by the phytochromes and the structure of the protein changes.…
“We already knew that some form of structural change was taking place, since the light signals must be transferred onwards to the cell. What we didn’t know, however, was how the structure changed, and this is what we have revealed. Nearly the complete molecule is rebuilt,” says Sebastian Westenhoff.
Westenhoff described these phytochromes in engineering terms: “Proteins are the factories and machines of life, and their structures change when they carry out their specific tasks.”
The paper in Nature said nothing about evolution other than to point out that a tongue-shaped part of the phytochrome molecule that provides leverage for the switch is “evolutionarily conserved” — i.e., unevolved. Indeed, “The photosensory core and the key amino acid sequence in the tongue region … are highly conserved over the whole phytochrome superfamily.”
As usual, evolutionary theory was useless to this advance in our understanding of plants at the cellular level— except, of course, to show that evolution contributed nothing. The machine was there at the start in bacteria and has remained unevolved ever since.
Think of what’s involved here. The phytochrome is composed of hundreds of amino acids precisely arranged such that the protein can switch conformations between two very different states: “nearly the complete molecule is rebuilt.” That conformational change, though, would be useless without other machines knowing how to respond to the change. The paper says,
In plant phytochromes, two additional PAS domains are included in the C-terminal regulatory region and a more complex pattern of functions has to be controlled, such as serine/threonine kinase activity, and affinity to interaction partners. In all cases, the output activity is probably controlled by a structural change in the photosensory core.
All the downstream effects of this initial switching must cooperate for the bacterium or plant to benefit from the light. This is not the work of a blind, unguided process, but one that sees a distant goal and organizes components, as in a factory, to get a job done.
It’s the engineering design approach, not evolution, that is “shedding light” on the workings of nature.