The function of cytokinin in regulating flowering time

细胞分裂素对开花的促进作用在近50年前发现这种植物激素后不久就已被描述。然而，尽管在了解激素的代谢和信号传导方面取得了很大进展，但这种活性的分子机制仍然难以捉摸。本研究的目的是研究细胞分裂素调控开花时间的程度和分子途径。大部分工作将在拟南芥中进行，但也将研究其在油菜（Brassica napus L.）上的可移植性，因为油菜的开花时间是一个重要的育种指标。在第一部分中，我们将详细研究大约60个细胞分裂素代谢和信号传导基因中的大多数丧失功能对开花时间的影响，以确定功能相关基因。我们的初步工作已经确定了细胞分裂素受体AHK2和AHK3以及转录因子ARR10和ARR12在介导细胞分裂素的作用中发挥核心作用。在此基础上，我们还将进一步研究细胞分裂素在不同组织中的表达，包括茎尖分生组织、叶片、韧皮部和根。该研究包括在不同的白天长度和短暂的日班之后，对已知开花途径基因的转录水平进行比较。不同的环境因素（光、营养）对细胞分裂素调节开花时间的影响将被测试，以了解在哪种情况下它的作用是特别相关的。转录组学数据的挖掘已经产生了关于细胞分裂素在已知开花途径中的调节作用的几个假设，包括调节花抑制基因ATC， miR156， miR172以及与赤霉素和海藻糖-6-磷酸途径的相互作用，这些将通过分子和遗传方法进一步探索。这些分析以及对ARR10和ARR12下游靶点的鉴定旨在将细胞分裂素的作用与开花途径的已知组分联系起来。此外，选择了一种遗传方法来获得调节细胞分裂素作用的新成分。强烈的细胞分裂素缺乏导致短日开花不足，这可以通过从抑制筛选中分离的显性功能获得突变体来逆转。新的突变体，特别是影响开花途径应鉴定和相应的基因克隆的基于图谱的方法。最后，利用在不同组织中具有基因工程细胞分裂素状态的转基因品系，探讨细胞分裂素在调控甘蓝型油菜开花中的作用。这项研究将揭示这种重要作物的开花时间调节机制是否保守，并可能作为调节其开花行为的育种靶点。

The promotive activity of cytokinin on flowering was already described soon after the discovery of this plant hormone almost fifty years ago. However, the molecular mechanisms underlying this activity remain elusive despite great progress in understanding the metabolism and signalling of the hormone. The objective of this research proposal is to study the extent and the molecular pathways by which cytokinin regulates flowering time. Most of this work will be done in Arabidopsis thaliana but the transferability to oilseed rape (Brassica napus L.), where flowering time is an important breeding target, will be studied as well. In a first part, a detailed study of the consequences of loss-of-function of most of the ~60 cytokinin metabolism and signalling genes on flowering time will be performed to identify the functionally relevant genes. Our preliminary work has identified the cytokinin receptors AHK2 and AHK3 as well as the transcription factors ARR10 and ARR12 to play central roles in mediating cytokinin action. This analysis will be complemented by a study on the relevance of the cytokinin status of different tissues using transgenic plants with a tissue-specifically altered cytokinin status, focussing on the shoot apical meristem, the leaves, the phloem and the root. The investigation includes a comparison of the transcript levels of known flowering pathway genes under different day lengths and after a short day-to-long day shift. The influence of different environmental cues (light, nutrition) on cytokinin action in regulating flowering time will be tested to understand under which circumstances its role is particularly relevant. Mining of transcriptomic data has yielded several hypotheses about a modulatory role of cytokinin in known flowering pathways, including regulation of the flower repressor gene ATC, miR156, miR172 and interactions with the gibberellin and trehalose-6-phosphate pathways, which will be explored further by molecular and genetic approaches. These analyses and the identification of downstream targets of ARR10 and ARR12 aim to link the action of cytokinin to known components of flowering pathways. Furthermore, a genetic approach is chosen to obtain access to novel components regulating the action of cytokinin. Strong cytokinin deficiency causes lack of flowering under short days, which can be reversed by dominant gain-of-function mutants isolated from a suppressor screen. Novel mutants specifically affecting the flowering pathway should be identified and the respective genes cloned by a map-based approach. Finally, the role of cytokinin in regulating flowering in Brassica napus will be explored using transgenic lines with a genetically engineered cytokinin status in different tissues. This study should reveal whether mechanisms regulating flowering time have been conserved in this important crop plant and may be used as a breeding target to modulate its flowering behaviour.