Defining the role of SUMO in regulating chloroplast biogenesis and functions

到2050年，全球人口将超过90亿，这将对粮食安全构成重大挑战，并对自然资源造成越来越大的压力。因此，现在比以往任何时候都更需要提高作物产量，同时适应次优生长条件。为了满足这些需求，必须发展我们主要作物的改良品种。通过对模式植物塔勒水芹的研究，已经确定了“SUMO”（代表“小泛素样修饰剂”）对不同非生物胁迫（包括高盐、高温、冷冻、干旱、氧化和重金属胁迫）的蛋白质修饰程度增加;并且这种“SUMO化”是植物胁迫响应的重要方面。先前的研究在塔勒中发现了一千多个SUMO靶点，其中大部分位于一个称为细胞核的中央细胞结构中。然而，我们最新的研究结果表明，SUMO也作用于植物细胞的不同部分，称为叶绿体。在这个项目中，我们将定义SUMO如何以及为什么作用于叶绿体，并在这样做的过程中了解它如何被用来提供更有弹性的作物。叶绿体是定义植物的细胞成分（或细胞器）。它们含有绿色色素叶绿素，是光合作用的场所-光合作用是利用阳光能量为细胞活动和植物生长提供动力的过程。由于光合作用是能量输入到生命世界的唯一重要机制，叶绿体不仅对植物而且对地球上的所有生命都非常重要。此外，叶绿体在植物对胁迫的反应中起着关键作用，因此它们是作物改良的理想目标。叶绿体由数千种不同的蛋白质组成，其中大多数由细胞核中的基因编码，因此，在细胞器外的细胞基质中被称为细胞质。由于叶绿体被双膜“信封”包围，需要复杂的机器将这些蛋白质导入细胞器;这包括两种膜中的分子机器，称为TOC（“叶绿体外膜的转位子”）和TIC。我们最近在这一领域取得了一些重大突破：我们发现TOC机器的组成蛋白质被一种名为“CHLORAD”（代表“叶绿体相关蛋白质降解”）的新调控过程分解。在CHLORAD中，不需要的TOC蛋白质被称为泛素的蛋白质修饰剂标记，其靶向它们以进行去除和分解。因此，CHLORAD调节其他蛋白质进入细胞器，这反过来又影响细胞器的发育和运作。值得注意的是，改变CHLORAD的活性可以使植物对逆境更有耐受力。现在，我们有新的结果表明，SUMO系统也作用于叶绿体。我们认为，这种SUMO化使TOC机制不稳定，从而调节蛋白质的输入。在本项目中，我们将详细研究SUMO依赖的叶绿体调控机制。我们将定义作用于TOC蛋白的SUMO通路，并阐明其生理意义。此外，受我们最新数据的启发，SUMO实际上作用于大量的叶绿体蛋白质，我们将系统地鉴定SUMO化的叶绿体蛋白质的全部范围，并研究这种SUMO化的影响。最后，我们将研究SUMO和CHLORAD系统在TOC调节中是否存在串扰，我们的实验将为SUMO依赖的叶绿体功能控制机制和意义以及植物发育提供前所未有的新视角。这些知识对于开发具有改善的叶绿体性能和抗逆性的作物将是非常宝贵的。

The human population is set to exceed 9bn by 2050, presenting significant challenges to food security and placing ever increasing pressure on natural resources. Thus, the need for increased crop yields with resilience to sub-optimal growing conditions is stronger than ever. To meet these demands it will be essential to develop improved varieties of our staple crops. Through research on the model plant thale cress, it is well established that the extent of protein modification by "SUMO" (which stands for "small ubiquitin-like modifier") increases in response to different abiotic stresses, including high salinity, high temperature, freezing, drought, and oxidative and heavy metal stresses; and that such "SUMOylation" is a vital aspect of plant stress responses. Previous studies identified over a thousand SUMO targets in thale cress, most of which are located in a central cellular structure called the nucleus. However, our latest results show that SUMO also acts on different parts of the plant cell called chloroplasts. In this project, we will define how and why SUMO acts on chloroplasts, and in so doing understand how it may be used to deliver more resilient crops.Chloroplasts are the cellular constituents (or organelles) that define plants. They contain the green pigment chlorophyll, and are the site of photosynthesis - the process which harnesses sunlight energy to power the activities of the cell and the growth of the plant. As photosynthesis is the only significant mechanism of energy-input into the living world, chloroplasts are of huge importance, not just to plants but to all life on Earth. Moreover, chloroplasts have critical roles in plant responses to stress, and so they are ideal targets for crop improvement.Chloroplasts are composed of thousands of different proteins, most of which are encoded by genes in the cell nucleus and, therefore, are made outside of the organelle in the cellular matrix known as the cytosol. As chloroplasts are surrounded by a double-membrane "envelope", sophisticated machinery is needed to import these proteins into the organelle; this comprises molecular machines in both membranes, called TOC (for "Translocon at the Outer membrane of Chloroplasts") and TIC. Each machine is composed of several proteins that work cooperatively to drive the import process.We recently made some significant breakthroughs in this area: We discovered that the constituent proteins of the TOC machinery are broken-down by a novel regulatory process named "CHLORAD" (which stands for "chloroplast-associated protein degradation"). In CHLORAD, unwanted TOC proteins are tagged with a protein modifier named ubiquitin, which targets them for removal and break-down. Thus, CHLORAD regulates the import of other proteins into the organelle, which in turn influences the development and operation of the organelle. Significantly, modifying CHLORAD activity makes plants more tolerant of stress.Now, we have new results revealing that the SUMO system also acts on chloroplasts. We believe that such SUMOylation destabilizes the TOC machinery to regulate protein import. In this project, we will study the mechanisms of SUMO-dependent chloroplast regulation in detail. We will define the SUMO pathway that acts on TOC proteins, and elucidate its physiological significance. Furthermore, inspired by our latest data suggesting that SUMO actually acts on a large number of chloroplast proteins, we will systematically identify the full range of SUMOylated chloroplast proteins, and study the effects of such SUMOylation. Lastly, we will investigate whether there is crosstalk between the SUMO and CHLORAD systems in TOC regulation.Together, our experiments will shed unprecedented new light on the mechanisms and significance of SUMO-dependent control of chloroplast functions and, in turn, plant development. This knowledge will be invaluable for the development of crops with improved chloroplast performance and stress resilience.