A new model for chloroplast-to-nucleus communication during seedling development

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The ability of plants to use sunlight for photosynthesis is an essential process that supports life on Earth. Photosynthesis represents the most significant source of new energy to the planet and is therefore central to our considerations on future energy needs. Much of our food is also derived from plants, either directly from vegetables, cereals etc, or indirectly as a source of animal food. In plants (and algae), photosynthesis takes place in organelles called chloroplasts. Most of the 2000-3000 proteins contained in the chloroplast are synthesized from DNA present in another organelle called the nucleus, although the chloroplast can make about 80 of its own proteins. When a new chloroplast is made (chloroplast biogenesis), the important role played by the nucleus means that the two organelles need to communicate. We know quite a lot about how the nucleus sends information to chloroplasts, but the mechanisms by which chloroplasts communicate with the nucleus have remained poorly understood despite over 30 years of research in this area. In this proposal we have taken a large body of published information and used it to develop a model for chloroplast-to-nucleus communication. The principal aim of this proposal is to robustly test this model to see if it is correct.The model proposes two pathways: a promotive pathway in which chloroplasts signal to the nucleus that all is well; and a second, inhibitory pathway that is activated when things go wrong. Specifically, the accumulation of intermediates in the synthesis of the green, photosynthetic pigment, chlorophyll, activates the inhibitory pathway when these pigments are excited by light. The inhibitory pathway then reduces the amount of chlorophyll being made. Such an inhibitory pathway would be important as most chloroplasts are made during early seedling development and too many chlorophyll intermediates would be lethal to a seedling because in the light they are photo-toxic.The promotive pathway is proposed to be mediated by heme, a molecule related to chlorophyll. We will test whether heme is involved by making plants that contain excess of the heme biosynthesis enzyme ferrochelatase or the heme-degrading enzyme heme oxygenase. We will ascertain whether these plants are still able to communicate between chloroplasts and the nucleus by using assays that measure the expression of specific nuclear genes under a range of different conditions. The second way we will test the model is to examine the role of an important protein in chloroplast-to-nucleus communication called GUN1. This is a chloroplast protein that has been proposed by others to be important in the signaling pathway between chloroplasts and the nucleus. In our model we propose that this is incorrect, and that instead the GUN1 protein has a role in chloroplast biogenesis itself that affects the making of the chloroplast signal. We will test whether this is the case by looking carefully at processes involved in chloroplast biogenesis in mutants lacking GUN1 and conversely in plants that contain excess GUN1 protein. We will determine whether these plants lack the ability to make the chloroplast promotive signal required to maintain nuclear gene expression.Finally, we will investigate the inhibitory pathway. In a previous study we isolated mutants that were unable to use the inhibitory pathway to reduce nuclear gene expression. In this proposal we will determine which genes are affected in these mutants and use this information to better understand how this pathway might work.Signals from chloroplasts to the nucleus have been implicated in all sorts of responses to changing environments such as to cold and drought. Our results may have important implications for understanding how plants interact with their changing environment, information that may be important in the future for producing better food and energy crops.