Plants first made the transition from sea to land around 700 million years ago, a hell of a long time before humans walked the Earth. They learnt how to adapt to all kinds of climates and can live in the most extreme environments on Earth. There are now about 391,000 species on Earth and nearly all of them share the ability to protect themselves from being sunburnt.
One thing that the majority of people will remember from their high school science lessons is the ability of plants to use sunlight to make their own food, a process called photosynthesis. For this they use a pigment called chlorophyll, which is also what makes the leaves green. Due to this, plants need to be exposed to sunlight and thus ultraviolet (UV) radiation to survive. Yet as most us know, being exposed to UV radiation for a long time and forgetting to use sunscreen can leave our skin sunburnt. Repeated exposure to the shorter wavelengths of UV radiation (UV-B) can increase the risk of diseases like skin cancer in humans. This begs the question: how do plants absorb the sunlight they need for photosynthesis without being damaged by prolonged exposure to sunlight and harmful UV-B radiation?
In short, plants make their own version of sunscreen.
Humans have different tolerance levels of UV radiation, for example, individuals with deeply pigmented skin have protection most of the time and to a greater degree, whereas others require some exposure to UV radiation to induce their protective skin pigments. Otherwise known as developing a tan. Even with this natural protection, we have other methods of preventing damage, by moving out of the sunlight, wearing a hat, or using sunscreen. However, plants do not have this luxury.
Researchers have been investigating how plants prevent damage from prolonged exposure to sunlight for many years. It became especially relevant after the discovery of the hole in the ozone layer, which threatened to allow much more UV-B radiation to reach the Earth’s surface. The natural sunscreens the plants produce protect them from the harmful UV-B radiation, whilst still allowing them to carry out photosynthesis, working in a similar way to shop-bought sun creams. In 2011, researchers discovered a protein (UV resistance locus 8) that responds to UV light. When exposed to UV-B, the protein triggers a cellular response and the production of chemicals (sinapate esters) to produce the natural sunscreen. These chemicals are transported to the top layer of the leaves (upper epidermal layer), where they accumulate in a relatively high concentration to prevent penetration of UV-B radiation. Plants that are unable to produce the protein are not able to induce their natural sunscreens and are subsequently damaged by the UV radiation. It is believed that this is an ancient adaption and might have played an evolutionary role in the transition from sea to land.
Like with humans, plants have varying amounts of protection. Some plants, for example, those living at high altitudes or in the tropics have high levels of protection all the time, whilst others increase their protection over the day, much like tanning. Plants are not immune to damage caused by sunlight and too much exposure to strong sunlight can cause damage to the leaves or bark of many plants. Scientists have shown that on a daily basis plants are exposed to enough UV radiation to cause lasting damage to DNA and hinder plant growth. Damage can be increased when plants are dehydrated, due to the limited ability to spread the compounds used to the worst affected areas. Despite this, plants have a very effective method of protection, one that scientists are investigating for human use. Identifying specific molecules that could be added to our sunscreens to enhance their protective ability. In the years to come, sunscreens may contain some of these plant molecules.
Dean, J, C., et al (2014) Plant sunscreens in the UV-B: Ultraviolet spectroscopy of jet-cooled sinapoyl malate, sinapic acid, and sinapate ester derivatives, Journal of the American Chemical Society, 136, 14780-14795