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

植物利用阳光进行光合作用的能力是支持地球上生命的一个重要过程。光合作用是地球最重要的新能源来源，因此是我们考虑未来能源需求的核心。我们的大部分食物也来自植物，直接来自蔬菜，谷物等，或间接作为动物性食物的来源。在植物（和藻类）中，光合作用发生在称为叶绿体的细胞器中。叶绿体中含有的2000-3000种蛋白质中的大多数是由存在于另一个称为细胞核的细胞器中的DNA合成的，尽管叶绿体可以制造大约80种蛋白质。当一个新的叶绿体形成时（叶绿体生物发生），细胞核所起的重要作用意味着两个细胞器需要交流。我们知道很多关于细胞核如何向叶绿体发送信息，但叶绿体与细胞核通信的机制仍然知之甚少，尽管在这一领域进行了30多年的研究。在这个建议中，我们已经采取了大量的已发表的信息，并使用它来开发一个模型的叶绿体到细胞核的通信。该模型提出了两条途径：一条是促进途径，叶绿体向细胞核发出信号，表明一切正常;另一条是抑制途径，当出现问题时，抑制途径就会被激活。具体地说，在合成绿色、光合色素叶绿素的过程中中间产物的积累，当这些色素被光激发时，激活了抑制途径。然后抑制途径减少叶绿素的产生量。这种抑制途径是重要的，因为大多数叶绿体是在幼苗发育早期产生的，过多的叶绿素中间体对幼苗是致命的，因为在光照下它们是光毒性的。我们将测试血红素是否涉及通过使植物含有过量的血红素生物合成酶亚铁螯合酶或血红素降解酶血红素加氧酶。我们将通过在一系列不同条件下测量特定核基因表达的测定来确定这些植物是否仍然能够在叶绿体和细胞核之间进行通信。我们将测试该模型的第二种方法是检查一种重要蛋白质在叶绿体与细胞核通讯中的作用，称为GUN 1。这是一种叶绿体蛋白，已被其他人提出在叶绿体和细胞核之间的信号通路中起重要作用。在我们的模型中，我们提出这是不正确的，相反，GUN 1蛋白在叶绿体生物发生本身中起作用，影响叶绿体信号的产生。我们将通过仔细观察缺乏GUN 1的突变体中叶绿体生物发生的过程以及相反含有过量GUN 1蛋白的植物中叶绿体生物发生的过程来测试是否是这种情况。我们将确定这些植物是否缺乏制造维持核基因表达所需的叶绿体启动信号的能力。最后，我们将研究抑制途径。在先前的研究中，我们分离出不能使用抑制途径来减少核基因表达的突变体。在这个提议中，我们将确定哪些基因在这些突变体中受到影响，并使用这些信息来更好地了解这一途径可能如何工作。从叶绿体到细胞核的信号涉及对变化的环境（如寒冷和干旱）的各种反应。我们的研究结果可能对理解植物如何与不断变化的环境相互作用具有重要意义，这些信息可能对未来生产更好的粮食和能源作物很重要。

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.