Unraveling plasmodesmata formation in C4 plant Gynandropsis gynandra

养活快速增长的人口需要不断提高作物产量。提高作物生产力的一个重要策略是提高植物的光合作用效率，以便植物更有效地从大气中捕获二氧化碳，并将其封存到生物质中。少数植物物种已经进行了一种比其他物种更有效的光合作用--称为C4光合作用。C4光合作用在不同的植物谱系中独立进化了60多次，涉及到在叶片的两种细胞类型：叶肉细胞和束鞘细胞之间发生的修饰生物化学。在C4植物中，二氧化碳最初固定在叶肉细胞中形成四碳酸，然后转移到维管束鞘细胞中。在那里，酸被脱羧化，二氧化碳被再生，从而形成一个泵作用，将二氧化碳浓缩在束鞘细胞中。然后，二氧化碳主要固定在这些束鞘细胞中，与非C4植物相比，那里的高二氧化碳浓度提高了高达50%的光合效率。因此，将C4光合作用转化为非C4作物(如水稻、小麦、土豆)以提高它们的产量和产量是植物生物技术的主要目标。因此，典型的C4植物具有特殊的叶解剖结构，最大限度地提高了这两种细胞类型之间的物理联系，具有许多称为胞间连丝的毛孔，调节代谢物交换。尽管细胞间的连接在促进C4新陈代谢方面至关重要，但关于叶肉细胞和束鞘细胞之间的大量胞间连丝是如何形成的，人们几乎一无所知。我的研究集中在C4植物Gynandropsis gynandra上，在那里我发现在叶片发育过程中，光是触发叶肉-束鞘界面快速和大量形成胞间连丝的线索。在我的团契项目中，我将致力于了解Gynandra叶肉-束鞘界面胞间连丝形成的细胞生物学机制及其对光合作用效率的重要性。由于G.gynandra是已知的与非C4模式植物拟南芥最接近的C4，所以我将使用关于拟南芥胞间连丝形成的最新发现作为起点。胞间连丝含有细胞骨架的成分，以及具有特殊脂类成分(脂微域)的质膜，以前在拟南芥中的工作表明，胞间连丝和脂微域都参与了胞间连丝的调节。我将在Gynandra的叶肉-束鞘界面的背景下进一步探索这一点，测试细胞骨架和脂类微域的特定化学抑制剂对胞间连丝形成的影响。我还将在Gynandra中使用基因编辑来产生三个候选基因的敲除突变，这些基因可以将胞间连丝的形成与细胞骨架和脂肪的生物合成联系起来。将评估基因敲除对叶肉-束鞘界面胞间连丝形成的影响，以及对光合作用效率的影响。最后，我的目标也是通过邻近标记来寻找与已知的胞间连丝定位蛋白相关的新的蛋白质，从而发现在Gynandra中胞间连丝形成的新候选基因。这些蛋白质的作用将在未来的独立工作中进行研究。总体而言，我的发现不仅将对C4植物中胞间连丝的形成产生新的理解，而且还将揭示支撑植物胞间连丝形成的新的细胞生物学和生化机制。这些知识可以用于全球努力，改造主要作物的C4光合作用，以提高光合作用和产量。

Feeding the rapidly growing human population requires continuous increases in crop yield. An important strategy towards higher crop productivity is to improve the efficiency of photosynthesis in plants, so that plants capture CO2 more efficiently from the atmosphere and sequester it into biomass. A small number of plant species already perform a more efficient type of photosynthesis than others - called C4 photosynthesis. C4 photosynthesis evolved independently over 60 times in different plant lineages and involves modified biochemistry that occurs between two cell types in the leaf: the mesophyll and bundle sheath cells. In C4 plants, CO2 is initially fixed in mesophyll cells into a four-carbon acid, which is then transferred into bundle sheath cells. There, the acid is decarboxylated and CO2 is regenerated, thus forming a pump action that concentrates CO2 in the bundle sheath cells. The CO2 is then fixed primarily within these bundle sheath cells, where the high CO2 concentration increases photosynthetic efficiency by up to 50% in comparison to non-C4 plants. It is therefore a major goal of plant biotechnology to engineer C4 photosynthesis into non-C4 crop species (e.g: rice, wheat, potatoes) to improve their productivity and yield.The exchange of metabolites between the mesophyll and bundle sheath cells is central to the biochemistry of C4 photosynthesis. Typical C4 plants therefore have specialised leaf anatomy that maximises physical connections between these two cell types, with numerous pores named plasmodesmata that mediate metabolite exchange. Despite the critical importance of cell-to-cell connectivity in facilitating C4 metabolism, almost nothing is known about how the large numbers of plasmodesmata form between the mesophyll and bundle sheath cells. My research is focused on this process in the C4 plant Gynandropsis gynandra, where I discovered that light is the cue that triggers the rapid and numerous formation of plasmodesmata at the mesophyll-bundle sheath interface during leaf development.In my fellowship project, I will aim to understand the cell biological mechanisms underpinning plasmodesmata formation at the mesophyll-bundle sheath interface in G. gynandra and its importance to photosynthetic efficiency. Since G. gynandra is the closest known C4 relative to the non-C4 model plant Arabidopsis thaliana, I will use the latest findings on plasmodesmata formation in Arabidopsis as a starting point. Plasmodesmata are known to contain components of the cytoskeleton, as well as plasma membrane with specialised lipid composition (lipid microdomains), and previous work in Arabidopsis suggests that both cytoskeleton and lipid microdomains are involved in plasmodesmata regulation. I will further explore this in the context of the mesophyll-bundle sheath interface in G. gynandra, testing the impact of specific chemical inhibitors of the cytoskeleton and lipid microdomains on plasmodesmata formation. I will also use gene editing in G. gynandra to generate knockout mutants in three candidate genes that could link plasmodesmata formation to the cytoskeleton and lipid biosynthesis. The effect of the gene knockouts on plasmodesmata formation at the mesophyll-bundle sheath interface, and consequences on photosynthetic efficiency, will be assessed. Finally, I also aim to discover novel candidate genes in plasmodesmata formation in G. gynandra using proximity labelling to find novel proteins associated with known plasmodesmata localised proteins. The role of these proteins will be investigated in future independent work. Overall, my findings will not only create new understanding of how plasmodesmata form in C4 plants, but reveal new cell biological and biochemical mechanisms that underpin plasmodesmata formation in plants. The knowledge can be used in the global effort to engineer C4 photosynthesis in major crops to increase photosynthesis and yield.