Light is a vital element for plants. They require light to carry out photosynthesis and to produce their body substance from inorganic substances such as water and carbon dioxide. On the other hand, plants can also suffer from too much light. Plants have developed a number of protective mechanisms to survive light stress situations. The biologist Prof. Iwona Adamska from the University of Constance is looking closely into these protective mechanisms and has found that ELIP proteins play a key role.
Light can be both a plant's friend and its enemy. In the case of photosynthesis, light is a plant's friend. Photosynthesis takes place in the chloroplasts that are surrounded by a double membrane. Soluble stroma and thylacoid membranes are located inside the inner membrane. The stroma contains thylacoid membranes that form grana (thylacoid stacks) and unstacked stroma thylacoids. The internal space inside the thylacoid membranes is referred to as thylacoid lumen. The photosynthetic light reaction takes place in the thylacoid membranes and involves four photosynthetic complexes: the photosystem I, the photosystem II, the cytochrome b6/f complex and the enzyme ATP synthase. When a plant is exposed to far higher light intensities than the light level of photosynthesis ("light saturation point"), then the rate of photosynthesis levels off, also referred to photoinhibition or light stress.
"The light intensity that leads to the light saturation of photosynthesis varies not only between different adapted species or individuals, but can also differ between individual leaves," explains Prof. Iwona Adamska who was born in Poland and has been the head of the Department of Plant Physiology and Plant Biochemistry at the University of Constance since 2003. Adamska is working on the manipulation of the plants' light protection mechanisms to make them less sensitive to excessive light.
Her main focus of attention is the chloroplasts, the major targets of light stress. “One reason that the chloroplasts are targets of light stress is their high oxygen concentration, which is far higher than the oxygen concentration in the environment,” said Prof. Iwona Adamska. When exposed to light, photosynthesis leads to the cleavage of water, which in turn leads to the release of molecular oxygen. The reduction of the oxygen molecules can lead to extremely reactive oxygen radicals that are able to cause oxidative damage. “Unsaturated lipids of the chloroplast membranes might also trigger the development of free radicals,” said the biochemist from Constance.At normal temperatures, light stress occurs primarily at the photosystem II level. The photosystem II complex of higher plants, algae and cyanobacteria consists of several subunits comprising more than 25 different protein subunits. “The core of the photosystem II complex houses the reaction centre which consists of the two related proteins D1 and D2,” said Prof. Iwona Adamska explaining that these proteins produce all the cofactors involved in the photosynthetic electron transport. The D1 protein of the photosystem II reaction centre is the major target of oxidative damage.
In order to survive light stress, plants have developed numerous light protection strategies and repair mechanisms. Besides morphological, anatomic and physiological adaptations, different protection mechanisms also occur on the molecular level. “The major topic of our research is two of these molecular protection mechanisms,” explains Prof. Iwona Adamska. Adamska and her team of researchers are collecting findings that might help them to manipulate these protection processes in such a way as to enable plants to better tolerate excessive light. The scientists are using Arabidopsis thaliana as model plant. “Using this plant has the advantage that our findings can be transferred to the most important crops to make them more resistant to light,” said the biochemist.
Adamska’s research therefore focuses on the ELIP (early light-induced proteins) proteins that play a key role in the regulation of light stress in plants. “These proteins were discovered and described at the beginning of the 1980s. Back then, researchers found that the proteins accumulated in the thylacoid membranes very early on when pea and barley seedlings that had been grown in the dark were moved into the light,” explained Prof. Iwona Adamska. Since no ELIP proteins are found in green leaves under normal light conditions, it was initially assumed that this group of proteins only accumulated temporarily during the time the seedlings took to turn green. It was further assumed that their function was linked to the development of the chloroplasts. During her postdoctoral period in Israel, Prof. Iwona Adamska was able to show that the ELIP proteins were also induced in fully developed leaves through light stress, hinting that their function is to protect plants against excessive light.“Our topology studies suggested that the ELIP or ELIP-like proteins have between 1 and 3 transmembrane α helices, and that helix I and III are strongly conserved and related to each other,” said Adamska. Homology comparisons showed that the ELIP proteins are very similar to chlorophyll a/b-binding proteins. The chlorophyll a/b-binding proteins are the light collection complexes of photosystem I and II and are responsible for the uptake of light and the transfer of energy to the photosynthetic reaction centres. “Our investigations have shown that ELIP, which was isolated from peas, binds two pigments, namely chlorophyll a and lutein,” said the plant researcher. In addition, Adamska’s investigations into the spectral properties of the bound pigments also showed that no excitonic interactions occurred between the chlorophylls. “This means that these chlorophylls are not involved in the transfer of energy to the reaction centres, and that they must have completely different functions instead,” said Prof. Iwona Adamska.Adamska and her team therefore assume that ELIP proteins are involved in the binding of free chlorophylls under stress conditions, since such free chlorophylls are released through the degradation of damaged pigment-protein complexes and can lead to the generation of free radicals. “We assume that the ELIP proteins temporarily bind these free chlorophylls, thereby neutralising them. Since we assume that the chlorophylls that are bound to the ELIP proteins are transferred to the newly synthesised proteins, we refer to this as a ‘pigment carrier’ function,” explains the biochemist. The researchers also assume that the ELIP proteins play a role in the release of surplus energy as heat (“non-photochemical quenching”).ELIP proteins are very common and are found in all photosynthetic organisms, from cyanobacteria to higher plants. “Several ELIP-like proteins have also been identified in viruses that attack marine cyanobacteria,” said Prof. Iwona Adamska. Ten members of the ELIP family were identified in the Arabidopsis thaliana genome. They are divided into three groups: three-helix ELIPS, two-helix SEPS (stress-enhanced proteins) and one-helix OHPs (one-helix proteins) that are known as HLIPs (high light-induced proteins) in cyanobacteria. Using transgenic plants and T-DNA mutants, the researchers are currently focusing on the “pigment carrier” and the “non-photochemical quenching” functions of the ELIP proteins.
Another light protection mechanism in chloroplasts that is being investigated by Prof. Iwona Adamska and her team is the quick degradation of the D1 protein of the photosystem II reaction centre. "At elevated light intensities, the D1 protein has a lifetime of only 60 minutes. It is therefore assumed that the ability of chloroplasts to quickly degrade and newly synthesise D1 is one of the plants' most important protection mechanisms against light stress," said Adamska. The protein is therefore not only regarded as a structural protein of the reaction centre, but also as a radical catcher, which can be removed from the reaction centre and be quickly replaced due to efficient de-novo synthesis without needing to degrade the entire photosystem II complex.
"We have discovered the protease, the so-called DEG2 protease, which carries out the primary cleavage of the light-damaged D1 protein in vitro," said Adamska. According to the researchers' findings, a DEG2 protease located at the stroma side of the thylacoid membrane carries out the primary cleavage of the light-damaged D1 protein of the photosystem II reaction system. This cleavage occurs at the so-called stroma loop between two transmembrane helices, which leads to the creation of two proteolytic fragments that are cleaved into individual amino acids through secondary proteolysis. Using T-DNA insertion mutants and transgenic Arabidopsis thaliana lines, Prof. Iwona Adamska and her team are now looking for evidence that the physiological function of the DEG2 protease is what they believe it to be. "We are focusing on other physiological substrates, substrate recognition mechanisms and the formation of oligomeric complexes," added the biochemist.
Iwona Adamska studied biology at the University of Posen (Poland). After doing her diploma at the Institute of Geobotany in Posen, she continued her PhD at the Institute of Botany at the University of Hanover. After receiving her PhD in 1990, she spent two years as post-doc at the Hebrew University in Jerusalem and one year each at the Universities of Hanover and Stockholm. It was at the last two universities that she started investigating light stress and protection mechanisms in plants. In 1994, she returned to the University of Hanover. In 1996, she moved on to the University of Munich where she habilitated in botany in 1998. In the same year, Adamska was appointed associate professor at the University of Stockholm and moved to Constance in 2003 where she has been the head of the Department of Plant Physiology and Plant Biochemistry ever since.