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Biology Solar plant power through speedy evolution In search of increased photosynthesis efficiency Photosynthesis is the driving force behind the production of plant biomass. It is the natural process by which solar light energy is harvested and stored in plants. However, many important crop species have strikingly low photosynthetic efficiencies, while there is a growing demand of food and fuel for the ever-growing human population. A team of researchers from the Institute of Biology Leiden (IBL) and the Leiden Institute of Chemistry (LIC) is on a mission to find a way to improve photosynthetic efficiency by artificially speeding up evolution. BY: Niels van Tol and Bert van der Zaal Molecular and Developmental Genetics, IBL Photosynthesis occurs in all the green parts of plants, inside the special cell organelles named chloroplasts. It is one of the most complex biochemical pathways known to man. The photosynthetic apparatus consists of four large protein complexes inside the so-called intra-cellular thylakoid membranes: Photosystem I (PSI), Photosystem II (PSII), Cytochrome b 6 f and chloroplast ATP synthase. During photos y nt he si s photons from sunlight are captured by chlorophyll molecules associated with the photosystems which excite low energy electrons towards a higher level of energy. The excited electrons are transferred along the thylakoid membranes and fuel the protein complex cytochrome b 6 f to pump protons across the membrane, driving the synthesis of the high energy compound ATP via the chloroplast ATPase. In this manner, light photons are used to generate the energy-rich compounds ATP and NADPH that are required for the fixation of CO 2 by the enzyme complex Rubisco and subsequent conversion into sugars, which determine growth of biomass. Photosynthesis is typically very inefficient and most plant species are exploiting less than one percent of the incoming solar energy. The explanation for this inefficiency can be found in restricted nutrient and water supplies, presence of pathogens and herbivores, and because only about fifty percent of the spectrum of incoming solar light can be used for photosynthesis. The latter is due to the fact that chlorophyll molecules capture ‘red’ and part of the ‘blue’ photons, but do not use the photons which we perceive as ‘green’. However, under exceptionally favorable conditions, photosynthetic efficiencies can rise up to ten percent for relatively short periods of time and in the hypothetical situation in which plants would be able to use all the photons that they harvest directly for the production of biomass, then photosynthetic efficiency could mount to an impressive thirty percent of the available solar energy. This means there is a huge potential for improving the harvest of this green energy resource if we only could make plants more efficient. ’Photosynthesis is typically very inefficient and most plant species are exploiting less than one percent of the incoming solar energy.’ 28 Origin - Universiteit Leiden
Biology Chlorophyll fluorescence (CF) imaging which can quantify the electron flow along the thylakoid membrane. The Molecular and Developmental Genetics group of the IBL, together with the Solid State NMR group of the LIC (Dr. A. Alia and Prof. Huub de Groot), recently received funding from the Foundation for Fundamental Research on Matter and the Earth and Life Sciences (FOM/ALW) both of the Netherlands Organization for Scienctific Research (NWO). They got a grant awarded within the NWO-theme “Towards BioSolar Cells” for their project “Phenotypic engineering of higher plants: Developing a new paradigm for improving photosynthetic efficiency”. The gist of the Leiden-based project is that rather than using a priori knowledge of conventional step-by-step improvement of plants used by others, it will be left to the plants themselves to make an evolutionary jump and generate phenotypes with improved photosynthetic efficiency. This will be done by drastically shaking up the usual gene expression patterns in plants, letting the plants sort out stable patterns of gene expression under conditions conducive for high photosynthetic yields, such as high light and elevated CO 2 levels. The plants with high photosynthetic efficiencies can then be selected and used to define the biological and biophysical parameters that correlate with and ultimately cause the desired properties. ’The plants with high photosynthetic efficiencies can then be selected.’ To the end of increasing the photosynthetic efficiency, we are exploiting so-called zinc finger artificial transcription factors (ZF-ATFs) in the model plants species Arabidopsis thaliana. These ZF-ATFs are small fusion proteins consisting of an array of modular zinc finger DNA binding domains fused to a domain that is able to activate transcription of genes. In our lab, a mutagenic approach was developed that is based on disturbing the transcriptome of the host organism. We are currently generating and screening a library of about 3500 plant lines by transforming plants with the gene encoding zinc fingers via Agrobacterium tumefaciensmediated transformation (AMT). We have already isolated several mutant plants with conspicuous phenotypes most likely correlated with altered photosynthetic properties. Those plant lines are either growing Origin - Universiteit Leiden 29
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