Coloring Outside The Lines: Harvesting far-red photons for photosynthesis

Plants photosynthesize with light that falls within the Photosynthetically Active Radiation (PAR, 400-700 nm) and this limits the rate of photosynthesis of leaves in the shade. In an agricultural field, where crops are usually grown in dense canopies, this is a severe loss factor for crop growth. In recent years, several cyanobacteria have been discovered that can perform oxygenic photosynthesis with near infrared (NIR, 700-800 nm) light that falls outside of the range of PAR. Their capacity to do so proves that it is possible to split water with photons of lesser energy, breaking the long-held paradigm of the scientific community that the fundamental energetic limit for this process was linked to the energy of the photons on the red edge of PAR. These discoveries have therefore sparked the idea of introducing these capabilities into plants, which – as discussed – would be especially beneficial for growing crops in dense canopies. The central goal of this research was to lay the groundwork for creating plants that, like these cyanobacteria, are proficient in far red photosynthesis.
To start, we discuss which strategies several oxygenic photosynthetic organisms already use to harvest far-red light. Then, in more detail we have investigated the molecular mechanisms of light-harvesting of a particular antenna system that is capable of harvesting the far red and which is synthesized under low-light conditions during the so-called LoLiP response in some cyanobacteria. Using Cryo-electron microscopy we elucidate the unique helical shape of this antenna system, which we call far-red light allophycocyanin (FRL-AP). Using ultrafast transient absorption spectroscopy we show that excitation energy transfer between different FRL-AP subunits occurs within less than a picosecond. By using structure-based modelling of the transient absorption data it could be inferred that excitation energy diffusion between the low energy subunits should however be slow in FRL-AP, possibly making it a relatively inefficient antenna. Furthermore, it is shown that the FRL-AP far red absorption properties stem from the phycocyanobilins (PCBs) in the α-subunits that in the helical phycobilisome adopt a flattened conformation which increases their effective conjugation length, and is the result of the complexation of α-subunits with β-subunits.
Next we investigated the natural capabilities of plant light-harvesting complexes to bind and function with redshifted chlorophylls (Chls). Both LHCII, which is the major antenna complex of PSII, and Lhca4, which is an antenna system of PSI, were reconstituted with pigment mixes containing redshifted Chls. In both LHCII and Lhca4 the redshifted Chls were functionally integrated in the Light-Harvesting Complex (LHC) protein scaffolds and both complexes were properly folded. Both reconstituted LHCs displayed significantly boosted absorption in the NIR region. Ultrafast time-resolved measurements revealed that the energy transfer dynamics were still ultrafast in these complexes. Furthermore, their excited state decay kinetics were not shortened. Interestingly, in both LHCII and Lhca4 the location of the lowest energy site was preserved and filled by Chl d. In Lhca4 this led to the combined action of red-shifting through the use of inherently lower energy absorbing pigments and of red-shifting by inducing exciton-CT mixing between the pigments, resulting in the red-most absorption properties ever reported for a plant LHC. Both studies clearly indicate that it is possible to obtain fully functional plant LHCs that bind red-shifted Chls, paving the way for their introduction into plants.
Finally, using computational tools we investigated the Chl binding selectivity of LHCII for Chl a and b, using time-resolved spectroscopy we elucidated the excitation energy flows in the LHCII-CP24-CP29 PSII subcomplex and we showed the performance of our patented propagation synchronous integration scheme for streak camera’s that increases their sensitivity by a factor of ten.