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/或盐碱化条件下。因此，有些植物通过C4循环进化出生化无机碳泵，因此称为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30%左右。这是由于它们在碳吸收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只有一种光合细胞。大约35年来，所有陆生C4植物的光合作用都是通过两种光合细胞的协同作用发生的：外层的栅栏细胞，在那里大气中的二氧化碳被C4循环捕获；内层的束鞘细胞，在那里二氧化碳通过C4循环被浓缩并用于碳同化（称为“克兰兹解剖学”）。然而，我们最近通过对藜科植物的研究表明，C4光合作用可以在单个光合细胞中发挥作用。研究结果表明，二氧化碳浓缩机制的功能并不需要克兰兹解剖结构，并且在单个光合细胞中可以出现两种类型的叶绿体，每种叶绿体都具有特定的功能。在这个项目中，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分区，氧气生产和二氧化碳交换的特点。生物化学和空间区隔的信息将用于开发一个机制模型，以测试C4光合作用如何在单个光合细胞中发挥作用。由于对Kranz型C4植物发育的遗传控制知之甚少，我们还将通过突变分析来研究单基因是否控制其特殊解剖和生化区隔的发育。人们对转基因作物（如水稻）进行C4光合作用的潜力非常感兴趣。我们的项目将提出合理的基因改造策略，通过增加光合作用的能力来提高生产力。

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.