Without sunlight, the earth would be a lifeless ball of ice; as life on earth depends on sunlight for warmth, weather, and energy for plants. Plants in turn provide the oxygen we breathe and the food we eat. The sunlight reaching our planet consists of a range of wavelengths, from ultraviolet (UV), through visible light, to infrared. UV comprises only a small, though potentially harmful, part of the sun’s light that reaches the earth. UV light consists mainly of UV-A, for a small part of UV-B and exposure to the latter is required by humans to produce vitamin D, though prolonged exposure can lead to sunburn. In plants, UV-light is thought to have a similar dual role; it can induce oxidative damage but might also induce the production of antioxidants and signalling molecules.
In this Special Issue of Physiologia Plantarum, we focus on UV-B and learn how plants apply their own sunscreen, how plants can be helped to withstand drought after a pre-treatment of UV-B and hydrogen peroxide and how we can utilise the plants’ ability to produce certain metabolites upon UV-B exposure to enhance the nutritional value of crops. We are highlighting 5 papers thereafter.
Sunscreen in plants
Only a small part of the total solar radiant energy reaching the earth’s surface, ultraviolet (UV), can be rather harmful. As anyone who has stood outside for too long on a sunny summer’s day will discover: UV-B light can cause sunburn. Thankfully, we can protect ourselves from an excess of sunlight by retreating to the shade or applying sunscreen. Plants however are also sensitive to UV-B; an excess can cause oxidative damage (sunburn) in plants too. As plants are unable to retreat to the shade to avoid UV-B exposure, how do they cope? It turns out that they can produce their own sunscreen in the form of UV absorbing compounds, many of them flavonoids. A big proportion of these compounds accumulate in plants’ leaves as a response to UV exposure and mitigate its harmful effect. The paper by Neugart et al. (2021) shows that plants can respond quickly to fluctuating UV-B conditions and adapt the amount of ‘sunscreen’ accordingly. The paper by Nichelmann and Pescheck (2021) shows that temperature influences the accumulation of UV-absorbing compounds in Arabidopsis leaves.
Drought tolerance upon UV-B exposure
We have seen the effect UV can have on the production of sunscreen in plants, a necessary measure to avoid oxidative damage through prolonged UV exposure. Nevertheless, the fact that plants encounter UV radiation is not all bad: UV-B can act as a signal to promote tolerance to several stress conditions as many of the molecules that act as sunscreen can simultaneously act as antioxidants and signalling molecules. Antioxidants are the plants’ way to deal with potentially harmful reactive oxygen species (ROS). These ROS seem, like UV-B, to be wearing two hats and are both beneficial and harmful. Take for example hydrogen peroxide (H2O2), though being a ROS, its application can trigger the production of antioxidant enzymes, thereby reducing the accumulation of more and different kinds of ROS.
So here we have both UV and H2O2 that can trigger the production of antioxidants, can this be used? The article by Sáenz-de la O et al. (2021) explores different treatments to confer drought tolerance in Nicotiana tabacum. In the case of drought stress, the harmful effect of ROS is seen through its accumulation, leading to oxidative damage, causing a reduction in growth, reduced photosynthesis and ultimately crop losses. The authors show that, although the treatment seemed to inhibit growth in tobacco plants, a combination of UV-B and H2O2 application prior to drought treatment resulted in plants more resistant to a lack of water.
Crop enhancement
In the previous posts, we have learned that plants can produce a variety of compounds in response to UV radiation. Among these are compounds that actively absorb UV radiation as some form of sunscreen or compounds that can act as antioxidants and allow plants to deal with biotic and abiotic stresses. Interestingly, these compounds might be beneficial for the nutritional value of crops and are therefore worth a closer look. It turns out that when the application of UV-B is combined with different background lights (blue, green, red), the makeup of the sunscreen changes markedly. The article of Palma et al. (2021) shows the effect these different light-regimes have in cucumber, highlighting that these effects are highly species- and cultivar-dependent and speculating how these treatments can be used to manipulate fruit quality. In grapes, metabolites accumulated in the fruit are of big influence on the taste of wine. In the article of Del-Castillo-Alonso et al., (2021), the authors explore the possibility to manipulate grape quality, taste and size through different UV-B treatments that alter the metabolic contents and physiological characteristics of grapes.