Regulation of stem initiation and its role in plant architecture

陆地植物进化的关键步骤之一是形成能够在地面上承载器官的垂直枝轴。尽管茎的形成在形成植物中起着古老而核心的作用，但茎的形成仍然是植物发育中最不被了解的过程之一。这一过程也具有实际意义：减少茎生长的突变已被广泛用于提高作物产量，但也对植物生长产生了不良的副作用，例如在种子萌发期间。为了开发更精确的遗传工具，通过改变植株高度和形状来提高植物生产力，需要更好地了解基因是如何控制茎的生长的。开发新的工具来改变植株高度的方法之一是研究基因如何在植物生命的不同阶段调节茎的生长。这种调节在具有莲座状习性的植物中明显可见，如拟南芥、甜菜、萝卜、生菜和卷心菜，在这些植物中，茎的伸长最初被抑制，但后来在开花时被激活。这种转变是由环境条件(如日长和温度)触发的，并在枝尖的特定区域启动茎的生长，称为肋区(RZ)。我们的实验室一直在研究RZ在参比植物拟南芥中的功能。我们发现，一种名为ATH1的基因在控制茎的形成过程中起着关键作用：ATH1最初在RZ中活跃，以防止茎的生长，但受到开花信号的抑制，从而启动茎。然而，目前尚不清楚开花信号如何控制ATH1以及ATH1如何抑制茎的生长。我们的初步结果表明，ATH1通过充当促进茎生长的两种激素信号的“守门人”来控制茎的启动：赤霉素和油菜素类固醇。为了验证这一观点，我们建议跟踪ATH1如何影响这些荷尔蒙参与信号传递的基因。此外，我们将测试这些基因是否在ATH1对茎生长的影响中起中介作用。我们还旨在了解开花信号如何与ATH1相连，以协调花的发育和茎的伸长。我们首先将重点放在ATH1基因内的调控序列上，这表明它与已知的开花调控因子有直接联系。我们将在这些序列中引入突变，并寻找茎生长与开花分离的植物。根据涉及的调控序列，我们将测试它们是否介导了调节开花过渡的特定调控蛋白的输入。研究ATH1调控序列的另一个动机是，它们可能在蔬菜甘蓝茎生长的现有多样性中发挥作用，并可用于在植物高度和形状上创造有用的新变异。以往的工作表明，甘蓝属作物茎高的变化与含有ATH1的基因组区域的差异有关，而且具有莲座状习性的作物(甘蓝、羽衣甘蓝)与茎伸长的作物相比，具有特定的ATH1差异。为了测试这些差异是否导致茎生长的差异，我们将培育携带ATH1的植物，这些植物来自长茎或短茎甘蓝，但在其他方面是相同的。如果Brassica ATH1的现有变异不能解释茎高的差异，我们将利用拟南芥的发现来产生新的调控突变，以使Brassica的茎生长与开花分离。总体而言，这项工作将揭示控制茎起始的遗传机制，揭示在作物育种过程中如何选择各种植物形状，并将产生控制植物高度和形状的新基因工具。

Development of a vertical shoot axis capable of bearing organs above the ground was one of the key steps in the evolution of land plants. In spite of its ancient and central role in shaping plants, formation of the stem remains one of the least understood processes in plant development. This process also has practical importance: mutations that reduce stem growth have been widely used to improve crop yield but also have undesired side effects on plant growth, for example during seed germination. A better understanding of how genes control stem growth is required to develop more precise genetic tools to increase plant productivity by modifying plant height and shape.One way to develop new tools to modify plant height is to study how genes modulate stem growth at different stages of the plant's life. This modulation is seen markedly in plants with a rosette habit, such as Arabidopsis, beet, radish, lettuce and cabbage, in which stem elongation is initially inhibited, but later activated during flowering. This transition is triggered by environmental conditions such as day length and temperature, and initiates growth of the stem in a specific region of the shoot apex, called the rib zone (RZ). Our laboratory has been studying how the RZ functions in the in the reference plant Arabidopsis. We found that a gene called ATH1 has a key role in controlling when the stem is formed: ATH1 is initially active in the RZ to prevent stem growth but is inhibited by flowering signals to initiate the stem. However, it is not known how flowering signals control ATH1 and how ATH1 inhibits stem growth. Our initial results indicate that ATH1 controls stem initiation by acting as a "gatekeeper" for two hormone signals that are known to promote stem growth: gibberellin and brassinosteroid. To test this idea, we propose to follow how ATH1 affects genes involved in signalling by these hormones. In addition, we will test whether these genes mediate the effects of ATH1 on stem growth. We also aim to understand how flowering signals connect to ATH1 to co-ordinate flower development and stem elongation. We will initially focus on regulatory sequences within the ATH1 gene, which suggest direct links to known regulators of flowering. We will introduce mutations in these sequences and look for plants in which stem growth is uncoupled from flowering. Depending on the regulatory sequences involved, we will then test whether they mediate the input of specific regulatory proteins that regulate the transition to flowering.Another motivation for studying regulatory sequences in ATH1 is that they may play a role in existing diversity in stem growth in vegetable Brassicas, and could be used to create useful, new variation in plant height and shape. Previous work showed that variation in stem height in Brassica crops is associated with differences in the genomic region containing ATH1, and that crops with a rosette habit (cabbage, kale) have specific differences in ATH1 in comparison to those with elongated stems. To test whether these differences are responsible for variation in stem growth, we will generate plants that carry ATH1 from long-or short-stemmed Brassica, but are otherwise genetically identical. If existing variation in Brassica ATH1 cannot explain differences in stem height, we will use the findings from Arabidopsis to generate novel regulatory mutations to uncouple stem growth from flowering in Brassica. Overall, this work will reveal the genetic mechanism controlling stem initiation, will shed light on how a variety of plant shapes have been selected during crop breeding, and will produce novel genetic tools to control the height and shape of plants.