For research, plants are often grown under stable lighting, which does not reflect natural conditions. In a series of experiments with changing lighting conditions, which simulate the natural interaction between light and shadow, researchers from the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm (Germany) and the College of Natural Science at Michigan State University (USA) reveal the importance of two key proteins for dynamic control of photosynthesis.
Plants perform photosynthesis to grow. In this process, they use energy from sunlight, release oxygen and produce carbohydrates, which are the basic food resource for all humans and almost all animals on Earth. Under natural conditions, light availability can change rapidly in a very short time. One of the main reasons is clouds that cast light and shadow when they pass in front of the sun. Plant leaves and branches can also temporarily provide shade when moved by the wind. Plants cannot move from shade to sun when light is limited, and conversely they cannot escape from sun to shade when exposed to too much sunlight. They have to react to changing light conditions in other ways.
Just like for humans, too much sunlight is harmful to plants. In particular, a rapid change between weak and intense light is problematic. Like the retina of our eyes, plants use molecules in their leaves to capture light particles. When the light is low, these light traps are very effective at capturing as much of the dim light as possible. If the light conditions suddenly change, too much light energy can reach the plant. This energy can overload or damage the sensitive photosynthetic apparatus inside plant cells. Consequently, plants must constantly adapt their photosynthetic activity to environmental conditions in order to achieve maximum light yield on the one hand, but avoid being damaged by too much light on the other hand.
To date, plants are grown in greenhouses and laboratories almost exclusively under stable and uniform light conditions. Therefore, our understanding of how adaptation to changing light conditions works is very limited. In the worst case, this can lead to plants that grow well in laboratories and greenhouses, but suddenly perform much worse than expected when grown in the field.
Regulation of photosynthesis under changing light conditions
The researchers around Ute Armbruster from the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm and David Kramer from the College of Natural Science at Michigan State University (USA) examined the model plant Arabidopsis thaliana for their studies. Plants were grown under a variety of conditions, including static, fluctuating and natural light. The study focused on two ion transport proteins called VCCN1 and KEA3 that play a key role in dynamic adjustment of photosynthetic performance. It is known from previous studies that VCCN1 activates sun protection if the light suddenly becomes too bright. When the light intensity decreases, the second protein KEA3 quickly breaks down this sun protection so that the plant can capture more light again. However, the two proteins VCCN1 and KEA3 have never been investigated under realistic light conditions.
The researchers used an innovative new approach to measure photosynthesis in combination with a targeted use of gene knockouts — ie plants whose genes for VCCN1 and KEA3 have been switched off. They show that the activities of the proteins VCCN1 and KEA3 depend on the light conditions the plants were grown in. At the suggestion of the head of the Plant Cultivation Infrastructure Group, Dr. Karin Köhl, the researchers focused on two growth-related light factors in the analysis and were able to show that both the amount of light a plant receives , and the frequency of light oscillations has a strong influence on the function of the two ion transporters. The protective function of VCCN1 is only important in plants previously grown under low light. On the other hand, the abrogator KEA3 was even active during periods of high light when the plants were grown under conditions of elevated light intensity.
Sun protection also depends on the degree of light fluctuations the plants are exposed to. When light conditions change significantly, the plants produce the orange pigment zeaxanthin, which is also involved in sun protection. The production of this sunscreen is also suppressed by KEA3 under high light conditions. “Our study shows that we should not look separately at the effects of growth light and the rapid responses to light fluctuations,” said the study’s lead author Thekla von Bismarck, adding: “The integration of multiple time scales and metabolic levels in an increasingly complex way will be a major future challenge for crop research. This will provide key ideas for improving yields in the field.”
Materials provided by Max Planck Society. Note: Content may be edited for style and length.