Resolving the key photoprotective switch in photosynthetic electron transport

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We aim to understand the way in which plants are able to adapt to fluctuations in the environment by studying a specific example that has the potential to improve crop plant tolerance to stress. In the final step of photosynthetic electron transfer, the enzyme ferredoxin:NADP(H) oxidoreductase (FNR) uses photosynthetic electrons to reduce NADP+ to NADPH, which is then used in multiple reactions and is essential for C fixation. The amount of this enzyme has a strong effect (a high coefficient of control) on the entire pathway of photosynthesis (0.7 at low light and 0.94 at saturating light (1)). Interestingly, it has also been shown that the amount of FNR also strongly correlates with the ability to tolerate multiple environmental stresses in tobacco (2,3), although the reasons for this are not yet clear. One contributing factor could relate to the free radicals produced by photosynthetic electron transport (PET). We recently showed that variable FNR content and location results in disrupted free radical production, and that this could be responsible for "priming" the plant, and inducing defence mechanisms (4). Although FNR has been well studied as an enzyme, its location within chloroplasts is highly dynamic, with many interaction partners. The reason for these multiple interactions, the activity of the enzyme at these different locations and the relationship of these complexes with the rest of the PET apparatus is not understood. There are three important recent developments that will enable us to answer these important questions. Firstly, we have produced transgenic Arabidopsis plants with FNR proteins localised to different complexes within the chloroplast (5). This means we can now compare the activity of the enzyme, and its associated metabolic pathways, when it is bound to different places. Introduction of cyanobacterial FNR to higher plants has been patented as a means of improving stress tolerance in crop plants, but the interactions of this prokaryotic enzyme in higher plant chloroplasts are unknown. Our novel plants will allow us to pinpoint the interactions responsible for stress tolerance. Secondly, new equipment has been developed that will allow us to monitor the activity of the enzyme inside a living leaf (6), which is much more accurate than working with semi-purified systems, where important components or regulatory events may be lost. Thirdly, we have promising preliminary results from a microscopy approach, that will help us image where in the chloroplast membranes these events occur. This is important, as many regulatory events in chloroplasts can only be understood in the context of spatial organisation between different parts of the organelle, or are too weak to detect with standard biochemical methods.Using these tools we aim to discover how dynamic redistribution of FNR is able to regulate PET and promote stress tolerance. Plants have limited resources available to them, and must allocate these to ensure the greatest chance of survival and reproduction. Improving the efficiency of switching between protective states and assimilatory states will therefore improve the chances of the plant not only surviving stressful conditions, but conducting rapid photosynthesis afterward and achieving a high harvest index. Better understanding of this regulation may help to design or breed plants able to withstand specific stresses, or rapidly respond to the presence and absence of stresses in order to achieve survival but maintain high yields.(1) Hajirezaei MR, et al. (2002) Plant J 29(3):281-93.(2) Palatnik JF, et al. (2003) Plant J 35(3):332-41.(3) Rodriguez RE, et al. (2007) Plant Physiol 143(2):639-49.(4) Kozuleva M, et al. (2016) Plant Physiol 172: 1480-1493.(5) Twachtmann M, et al. (2012) Plant Cell 24(7):2979-91.(6) Klughammer C, et al. (2016) Photosynth Res 128(2):195-214.