Generation of reiterative growth patterns in plants

每一种开花植物都是由重复的单位组成的：叶、茎、枝、花和根轴。重复也可能发生在器官内部，如橡树叶的多个裂片。这种重复依赖于生长区域，分生组织，产生微观生长模块，这些模块具有刻板的基因活性模式。了解生长模块的起始和间隔是如何被控制的，是我们预测植物器官数量、排列和形状的基础，这些都是重要的农艺关键性状。在增长模块控制中有两个因素在起作用。一种是将生长模块的生产限制在工厂的特定区域，称为能力区。其次，生长模块之间相互作用，使新模块的产生远离以前的模块，形成间隔模式。已经提出了模块间相互作用的假设，但我们对能力区是如何控制的知之甚少。这个跨学科项目建立在拟南芥中一个基因CUC2的初步结果之上。正常情况下，CUC2活跃在叶缘的基部，那里产生锯齿状生长模块。然而，如果CUC2在整个叶片中被激活，沿着叶缘产生锯齿，形成分形图案（锯齿上的锯齿）。这些发现表明，局部CUC2活性通常限制了模块形成的能力区域，并为研究能力控制原理以及它如何与模块-模块限制相互作用提供了机会。我们假设CUC2的调控将模块限制在叶片的基部，而另一个基因KANADI将它们限制在叶缘。如果在KANADI活性降低的植物中CUC2过度活跃，我们预计整个叶片表面和边缘都会产生生长模块。我们的第一个目标是在拟南芥和大麦这两个物种上验证这一预测。通过比较这两种系统，我们的目标是建立如何在不同的分类群体中应用的一般原则，以及它们与作物的相关性。第二个目标是利用CUC2过度活跃的叶片所表现出的分形行为来测试简化环境下模块-模块相互作用的假设。通过跟踪这些植物的细胞随着叶片的发育，我们将能够建立模块-模块间距的规则，区分细胞不同末端的标记（极性标记）如何随时间变化，生长如何影响锯齿形状，细胞分裂如何被调节，以及这些特性如何在具有不同程度锯齿的叶片之间变化。第三个目标是在不同的时间和地点诱导过度活跃的CUC2，以测试哪些基因被CUC2激活，以及它们是直接激活还是通过细胞间的信号激活。我们还将确定叶片边缘的CUC2活性是否对生长的影响至关重要，并测试候选信号分子。第四个目标是确定CUC2如何与植物生长激素生长素相互作用，已知生长素会影响生长模块的形成。这将通过确定植物中过度活跃的CUC2的影响来实现，其中生长素运输受到药物或基因突变的损害。第五个目标是根据我们的实验观察开发计算模型，以澄清不同假设的预测，从而区分它们。实现这些目标将使我们更深入地了解能力是如何被控制的，以及如何与模块之间的限制相互作用，从而产生重复的生长模式，从而巩固我们预测和修改植物生长和结构的能力。

Every flowering plant is made up of repeating units: leaves, stems, branches, flowers and root axes. Repetition may also occur within organs, as with the multiple lobes of an oak leaf. Such reiterations depend on growing regions, meristems, producing microscopic growth modules, which have stereotypic patterns of gene activity. Understanding how the initiation and spacing of growth modules is controlled is fundamental for our ability to predict the number, arrangement and shape of organs on a plant, key traits of agronomic importance. Two elements are at play in growth module control. One is restriction of growth module production to defined regions of the plant, termed competence zones. Secondly, growth modules interact with each other such that new modules are produced far away from previous ones, creating a spacing pattern. Hypotheses have been proposed for module-module interaction but we know much less about how the competence zone is controlled. This interdisciplinary project builds on preliminary results with a gene in Arabidopsis, CUC2. Normally CUC2 is active at the base of the leaf margin, where serration growth modules are produced. However, if CUC2 is activated throughout the leaf, serrations are produced all along the leaf margin, creating a fractal pattern (serrations upon serrations). These findings indicate that localised CUC2 activity normally restricts the zone of competence for module formation, and provide an opportunity to study the principles of competence control and how it interacts with module-module restriction. We hypothesise that regulation of CUC2 restricts modules to basal regions of the leaf, and that another gene, KANADI, restricts them to the leaf margin. If CUC2 is overactive in a plant with reduced KANADI activity, we expect growth modules to be produced throughout the leaf surface and margin. Our first objective is to test this prediction in two species: Arabidopsis and barley. By comparing these two systems we aim to establish how generally principles apply across different taxonomic groups, and their relevance to crops. The second objective is to exploit the fractal behaviour exhibited by leaves with overactive CUC2 to test hypotheses for module-module interactions in a simplified context. By following the cells of these plants as the leaf develops, we will be able to establish rules for module-module spacing, how markers that distinguish different ends of the cell (polarity markers) change over time, how growth influences serration shape, how cell divisions are modulated, and how these properties vary between leaves with different extents of serration. The third objective is to induce over-active CUC2 at different times and places, to test ideas for which genes are activated by CUC2 and whether they are activated directly or via signalling between cells. We will also determine whether CUC2 activity in the leaf margin is critical for effects on growth, and test candidate signalling molecules. The fourth objective is to determine how CUC2 interacts with the plant growth hormone, auxin, which is known to influence growth module formation. This will be achieved by determining the effect of overactive CUC2 in plants where auxin transport is compromised by drugs or genetic mutations. The fifth objective is to develop computational models based on our experimental observations to clarify predictions of different hypotheses and thus discriminate between them. Meeting these objectives will provide a deeper mechanistic understanding of how competence is controlled and interacts with module-module restrictions to generate reiterative growth patterns, underpinning our ability to predict and modify plant growth and architecture.