Interplay between carbon concentrating mechanism and osmoregulation / ion-regulation in Charophytes: chance or obstacle for adaptation to terrestrial conditions? (ICOR)

轮藻是一种沉水植物，属于链藻门，也包括所有陆地植物。轮藻亚科与Zygnematophyceae和鞘毛藻亚纲一起形成ZCC分支，陆地植物的共同祖先由此进化而来。轮藻是最复杂的链藻藻类，其特征被认为是陆地栖息地成功定居所必需的(例如，功能性的根状突起、配子囊被膜、皮质)。因此，它们是一个重要的垫脚石，值得详细研究，以了解陆系化的机制。无机碳(Ci)有效性的急剧下降可能是陆表化过程中最显著的挑战之一。在目前大气二氧化碳浓度较低的条件下，CO2的被动扩散不能满足复杂水生真核光自养生物的光合作用需求。因此，真核藻类通常采用复杂的碳浓缩机制(CCM)来富集Rubisco附近的二氧化碳。然而，轮藻中CCM的存在和功能尚不确定。此外，S预计pH与离子驯化和Ci获得之间存在很强的干扰。在初步实验中，我们建立了轮藻在一定条件下的培养体系。选定的物种能够在广泛的二氧化碳、盐度和pH水平下生长。对于该项目，我们将专注于Braunii轮藻，因为它可用的注释良好的基因组序列为计划中的分子工作提供了坚实的基础。对布氏梭虫基因组的搜索导致检测到许多潜在编码蛋白质的基因，这些基因参与了真核模式藻类的CCM，例如莱茵衣藻。这项提议的最初目的是揭示布氏梭虫是否进行CCM。为此，我们将比较完整的布氏隐孢子虫及其羧化酶Rubisco的二氧化碳亲和力。将该藻培养在不同的CO2和pH水平下，研究其是否能通过改变光合作用的CO2亲和力来响应不同的Ci水平。可能与查拉CCM相关的选定基因的表达将用定量聚合酶链式反应进行分析。最后，研究了不同盐度条件下培养的布氏梭子蟹Ci与离子驯化的相互作用。总的来说，我们的目标是找出轮藻是否执行CCM，哪些基因可能参与这一过程，以及它与周围水域中不同的离子关系相互作用的程度。

Charophytes are submerged macrophytes belonging to the Streptophyta, a clade which also include all land plants. Together with the Zygnematophyceae and the Coleochaetophycae, Charophyceae form the ZCC clade, from which the common ancestor of land plants evolved. Charophytes are the most complex streptophyte algae, which exhibit features thought to be mandatory for successful colonization of the terrestrial habitat (e.g. functional rhizoids, gametangia envelopes, cortication). Hence, they represent an important stepping stone worth being investigated in detail in order to understand the mechanisms of terrestrialization. The drastic decline in availability of inorganic carbon (Ci) was probably one of the most striking challenge during terrestrialization. Passive diffusion of CO2 is unable to meet the photosynthetic demand of complex aquatic eukaryotic photoautotrophs at the recent low atmospheric pCO2 conditions. Therefore, eukaryotic algae usually employ sophisticated carbon-concentrating mechanisms (CCMs) to enrich CO2 in the vicinity of Rubisco. However, the existence and function of a CCM in Charophytes is uncertain. Moreover, s strong interference between pH and ion acclimation and Ci acquisition is anticipated.In preliminary experiments we established cultivation systems for Charophytes under defined conditions. Selected species are able to grow under a wide range of pCO2, salinity and pH levels. For the project we will concentrate on Chara braunii, because its available well-annotated genome sequence provides a solid base for the planned molecular work. Searches in the C. braunii genome resulted in the detection of many genes that potentially encode proteins involved in the CCM of eukaryotic model algae such as Chlamydomonas reinhardtii.The initial aim of this proposal is to unravel whether or not C. braunii performs a CCM. For this purpose, we will compare the CO2 affinities of intact C. braunii and its carboxylating enzyme Rubisco. The alga will be cultivated under different CO2 and pH levels to investigate if it can respond to different Ci levels by changing the photosynthetic CO2 affinity. The expression of selected genes putatively involved in Chara CCM will be analyzed by qPCR. Finally, the interaction of Ci and ion acclimation will be investigated in C. braunii after cultivation at different salinities. Collectively, we aim to find out if Charophytes perform a CCM, which genes may be involved in this process, and, how far it is interacting to different ionic relations in thesurrounding waters.