BALANCING LIGHT EFFICIENTLY: THE LONG ROAD TO A SUSTAINABLE FUTURE

Photosynthetic organisms harvest light and convert energy into biomass through the process of photosynthesis. Antenna systems are responsible for light harvesting and transfer the energy towards the Reaction Centres (RCs). However, too much light can be damaging and therefore the excess energy is dissipated as heat through a process called Non-Photochemical Quenching (NPQ). NPQ is activated by decreased pH on the luminal side of the thylakoid membranes by either PSBS or LHCSR found in higher plants and green algae respectively. When the antenna systems receive too much light the singlet Chl excited states produced by photon absorption cannot be quenched by photochemistry fast enough, leading to intersystem crossing producing Chl triplet states, which react with O2 and results in the synthesis of Reactive Oxygen Species (ROS). ROS accumulation causes photoinhibition of photosynthesis, drastically limiting growth. Therefore, dissipating light when in excess and using even the last photon under low light is a difficult exercise which is essential to ensure maximal growth. However, plants are more “interested” in surviving stress and reproducing than in growing big. Therefore, they have developed a hysteretic response to light: in order to avoid damage, plants over-regulate energy dissipation thus growing less than could be afforded under farming conditions, indicating that there is large room for engineering energy dissipation and increase crop production. Furthermore, besides changes in light intensity, plants experience changes in spectral composition as well. However, PSII and PSI have slightly different absorbance spectra and for an efficient linear electron flow between the two photosystems, it is essential that the excitations between the two photosystems are balanced. This is regulated by so-called state transitions a shuttling of antenna proteins between PSII and PSI. In Chapter 1 the differences and similarities between a variety of oxygenic photosynthetic organisms is reviewed. The focus lies on the different sets of antenna systems that evolved during the evolution and how the antenna systems that we currently find in plants and green algae have such an important role in photoprotection. In Chapter 2, PSBS in A. thaliana has been replaced with LHCSR1 from the moss P. patens, an evolutionary intermediate both expressing functional PSBS and LHCSR. The complemented A. thaliana lines showed a partial recovery of NPQ. The partial recovery of NPQ was mainly caused by the reduced capacity to convert violaxanthin into zeaxanthin in A. thaliana in comparison with P. patens. In chapter 3, several different A. thaliana lines lacking one or more PSII-antenna complexes were complemented with LHCSR1 form P. patens in order to identify a possible interaction partner of LHCSR1, where Lhcb5 (CP26) has been identified as the most likely interaction partner of LHCSR1. In chapter 4, the complemented lines from Chapter 2 were grown in different fluctuating light conditions to see whether LHCSR1 could increase the biomass production. However, in all cases WT grew the same or better than the complemented lines. In specific cases the presence of LHCSR1 could partly improve growth in comparison to npq4. In Chapter 5, we looked at the locations of interaction of LHCII with PSI. This is especially interesting since LHCII is the most important protein to induce NPQ, and does perform state-transitions, a process which is essential for an even energy distribution between the two photosystems and therefore necessary to grow properly. A second LHCII-PSI interaction site has been confirmed by looking at the energy transfer in isolated stroma membranes in State I or State II of WT and a mutant devoid of the PSI antenna. We show that the presence of the PSI-antenna (Lhca1-4) increase the rate of energy transfer from LHCII to PSI by 4 times and thus are essential for a proper binding of LHCII to PSI.