Why don’t plants need sunscreen?

Plants need to be in sunlight to convert carbon dioxide into hydrocarbons. Along with the blue and red photons they need, there are also bullets of UV-B light. UV-B can damage biological tissue and DNA. While the ozone layer absorbs most UV-B, enough gets to the Earth’s surface to cause a problem. The first part of a plant that UV-B strikes is the cuticle, a film that covers the epidermis. While there’s been a lot of work on how the cuticle reduces water loss, protects against pathogens, or supports a plant, few people have studied how it interacts with UV-B. Ana González Moreno and colleagues have published a study in Nature Communications, showing how phenolic compounds in the cuticle can protect a plant from UV-B.

Phenols, or phenolics, are a particular kind of organic compound that take carbon’s ability to bond with itself to an extreme. Carbon can form long chains called polymers, and a spine of carbon atoms can hold various chemical groups to form complex molecules. Phenols are what happens when this spine curves back on itself to form a ring, with one part of the ring holding a hydroxyl group (-OH).

González Moreno and colleagues note that while there are a lot of phenolic acids in the cuticle, Coumaric, Caffeic, and Ferulic acids have an acyl group that can either be trans or cis in formation. This labelling means that the acids have the same atoms, but they differ slightly in how they’re arranged.

The scientists used transient absorption spectroscopy to see how these phenolic compounds interacted with UV-B. They hit an epidermis with UV-B photons and examined what light the epidermis gave off in response. The team found that something odd happened when UV-B hit the phenolics in the trans configuration.

“The cinnamic acids present in cuticles have an aromatic molecular structure conjugated with a double bond that absorbs radiation, specially of the UV-B spectrum. The molecule absorbs energy and spins instantly”, said co-author Eva Domínguez in a press release.

When UV-B hits the molecule, it takes the energy to snap into a new configuration. This change is extremely rapid. The measurements show that it takes less than a millionth of a second. If this were all, then plants would need a constant supply of phenolics as shields against new UV-B photons. However, the new configuration is not stable, and this instability helps the plant.

The reconfigured molecule emits the energy it gained from the UV light as it moves back into its initial state. When this happens, the energy is released at a longer wavelength, effectively converting the UV light into heat that causes much less damage. Once the molecule has reset, it’s ready to absorb another UV photon.

González Moreno and colleagues write that these phenolic acids turn up in cuticles across the plant kingdom, with p-coumaric acid being very common. They also note that palaeontologists have identified p-coumaric acid in fossil plant cuticles.

What makes compounds so useful as shields is their selectivity. They react to UV light, but not visible light. That means that the red and blue photons that plants need for photosynthesis can pass through.

READ THE ARTICLE

González Moreno, A., de Cózar, A., Prieto, P., Domínguez, E. and Heredia, A. (2022) “Radiationless mechanism of UV deactivation by cuticle phenolics in plants,” Nature communications, 13(1), https://doi.org/10.1038/s41467-022-29460-9