C4 Photosynthetic Mechanisms: Requirements and Diversity

在陆生植物的进化过程中，大气中二氧化碳（CO2）水平下降，导致光合作用能力受到限制，特别是在高温、干旱和/或盐碱条件下。因此，一些植物通过C4循环进化出一个生化无机碳泵，所以它们被称为C4植物。虽然C4植物占陆地物种的不到10%，但由于它们在极端气候条件下的成功，估计它们占全球陆地生产力的约30%。这是由于它们在碳同化和水利用效率方面的有效性。不幸的是，大多数作物缺乏这种碳泵，这限制了它们的生产力和我们将生产扩展到不太有利的栖息地的能力。不具有C4光合作用的植物在其叶片中具有一种光合细胞类型。大约35年来的教条是，光合作用发生在所有陆生C4植物中，通过两种光合细胞类型的合作功能：外层的栅栏细胞，其中大气中的CO2被C4循环捕获，内层的维管束鞘细胞，其中CO2通过C4循环浓缩并用于碳同化（称为“Kranz解剖学”）。然而，我们最近已经表明，C4光合作用可以在一个单一的光合细胞的基础上的家庭藜科成员的研究。结果表明，Kranz解剖是不需要的CO2浓缩机制的功能，和两种类型的叶绿体，每一个专门的功能，可以发生在一个单一的光合作用细胞。在本项目中，我们将确定单细胞C4光合作用的机制和效率，考虑生物化学，光合细胞内所需酶的区室化，氧气生产和CO2交换的特性。生物化学和空间划分的信息将被用来开发一个机械模型，以测试C4光合作用如何在一个单一的光合细胞中发挥作用。由于鲜为人知的是克兰茨型C4植物的发展的遗传控制，我们也将研究，通过突变分析，是否单基因控制其专门的解剖和生化区室的发展。人们对诸如水稻等作物进行基因工程以进行C4光合作用的潜力有很大的兴趣。我们的项目将提出合理的遗传修饰策略，通过增加光合作用能力来提高生产力。

During evolution of terrestrial plants, the atmospheric levels of carbon dioxide (CO2) declined, resulting in a limitation on the capacity for photosynthesis, especially under higher temperatures, drought and/or saline conditions. Consequently, some plants evolved a biochemical inorganic carbon pump through a C4 cycle, so they are called C4 plants. While C4 plants account for less than 10% of terrestrial species, they are estimated to account for about 30% of global terrestrial productivity because of their success under extreme climatic conditions. This is due to their effectiveness in carbon assimilation and efficiency of water use. Unfortunately, most crops lack this carbon pump, which limits their productivity and our ability to extend production into less favorable habitats. Plants not possessing C4 photosynthesis have one photosynthetic cell type in their leaves. The dogma for about 35 years has been that photosynthesis occurs in all terrestrial C4 plants by the cooperative function of two photosynthetic cell types: an outer layer of palisade cells, where atmospheric CO2 is captured by the C4 cycle, and an inner layer of bundle sheath cells, where the CO2 is concentrated through the C4 cycle and used in carbon assimilation (called "Kranz anatomy"). However, we have shown recently that C4 photosynthesis can function in a single photosynthetic cell based on studies on a member of the family Chenopodiaceae. The results suggest that Kranz anatomy is not required for function of the CO2 concentrating mechanism, and that two types of chloroplasts, each with specialized functions, can occur within a single photosynthetic cell. In this project we will determine the mechanism and efficiency of single cell C4 photosynthesis considering biochemistry, compartmentation of required enzymes within the photosynthetic cell, characteristics of oxygen production and CO2 exchange. Information on biochemistry and spatial compartmentation will be used to develop a mechanistic model to test how C4 photosynthesis can function in a single photosynthetic cell. Since little is known about genetic control of development of Kranz type C4 plants, we will also study, by mutational analysis, whether single genes control development of their specialized anatomy and biochemical compartmentation. There is great interest in the potential for genetic engineering of crops, such as rice, to perform C4 photosynthesis. Our project will suggest rational strategies for genetic modifications to increase productivity via increased capacity for photosynthesis.