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维持地球上生命的一个基本过程。光合作用是地球上最重要的新能源，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，要么直接来自蔬菜、谷类等，要么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。叶绿体中包含的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种自己的蛋白质。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沟通。关于细胞核如何向叶绿体传递信息，我们知道得很多，但尽管在这一领域进行了30多年的研究，但叶绿体与细胞核通讯的机制仍然知之甚少。在这项提案中，我们收集了大量已发表的信息，并利用这些信息开发了一个叶绿体到细胞核的通讯模型。这一建议的主要目的是强有力地测试这一模型是否正确。该模型提出了两条途径：一条是叶绿体向细胞核发出信号表示一切正常的促进途径；另一条是当出现问题时被激活的第二条抑制途径。具体地说，绿色光合色素叶绿素合成的中间产物的积累，激活了这些色素受光激发时的抑制途径。然后，抑制途径会减少所产生的叶绿素的数量。这种抑制途径很重要，因为大多数叶绿体是在幼苗发育早期产生的，太多的叶绿素中间体在光照下是光毒的，对幼苗是致命的。促进途径被认为是由与叶绿素相关的分子血红素介导的。我们将通过制造含有过量的血红素生物合成酶铁络合酶或血红素降解酶的植物来测试是否与血红素有关。我们将通过在一系列不同的条件下测量特定核基因的表达的分析来确定这些植物是否仍然能够在叶绿体和核之间进行通讯。我们将测试该模型的第二种方法是研究一种名为GUN1的重要蛋白质在叶绿体到细胞核通讯中的作用。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发挥重要作用。在我们的模型中，我们认为这是不正确的，相反，GUN1蛋白在叶绿体生物发生本身中具有影响叶绿体信号制造的作用。我们将通过仔细观察在缺乏GUN1的突变体中以及在含有过量GUN1蛋白的植物中参与叶绿体生物发生的过程来测试是否如此。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促进信号的能力。最后，我们将研究抑制途径。在之前的一项研究中，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，我们将确定这些突变体中哪些基因受到影响，并利用这些信息更好地理解这一途径如何工作。从叶绿体到细胞核的信号参与了对变化的环境的各种反应，如对寒冷和干旱的反应。我们的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，这些信息对未来生产更好的粮食和能源作物可能是重要的。

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.