Molecular and genetic mechanisms of plant organ size control

尽管雏菊和玛格丽特在颜色、结构和组织上很相似，但我们如何区分它们呢？我们可以识别它们，因为雏菊总是比玛格丽特小得多。正如这个例子所说明的，一个特定物种的个体及其组成器官往往会长到令人惊讶的统一大小，而不同物种之间的器官大小存在巨大差异，无论是植物还是动物。这些观察结果表明，生物体的遗传发育程序对其器官的生长和最终大小施加了严格的控制。尽管这个问题的科学重要性和能够操纵植物器官大小的明显经济潜力，我们对植物潜在的遗传和分子机制的知识仍然非常有限。阐明这些机制以及它们在进化过程中如何被修改是我们研究的目标。我们已经分离出了许多突变体，它们形成了更大或更小的器官，这是由单个基因的缺陷或过度激活引起的。我们已经集中在遗传上容易接近的模式植物拟南芥的花器官，花器官的大小只受到环境的影响非常微弱，允许更容易和更可靠的遗传因素的识别。从我们的突变体中，我们现在已经分离出了大兄弟（BB）基因作为植物器官大小的关键决定因素。缺乏BB活性的植物由于较长的生长期而形成更大的花和更厚的茎，而随着BB活性的增加，花的大小和茎的厚度逐渐减小。BB活性和器官大小之间的紧密负相关表明BB在调节器官生长中充当中心控制元件。BB基因编码具有E3泛素连接酶功能的蛋白。E3泛素连接酶标记特定的其他细胞蛋白降解。因此，我们目前的工作假设指出，BB限制拟南芥器官的大小，通过靶向刺激生长的破坏。识别这些生长刺激物并确定它们如何起作用将是未来的主要目标，同样重要的是哪些因素将BB活性水平设定为获得正常器官大小的适当值。大小调节的一个特别迷人的方面是，它在单个细胞水平之上的器官范围内运作，并且可以补偿细胞数量或细胞大小的变化。要了解这个系统的特性，就需要分析同一器官中不同基因细胞的嵌合体。为此，我们将建立和使用普遍适用的方法来产生和分析发育器官中功能丧失和获得的克隆。这些方法应该被证明是有用的拟南芥研究社区在广大。随着我们对模式物种中大小控制机制的了解越来越多，我们将开始从其他植物（例如油菜籽）中分离同源基因，以便通过改变保守的大小控制基因的活性来开始操纵重要经济作物的器官大小，并研究进化如何改变大小控制机制。对于后者，我们将主要集中在种对荠菜rubella和C。这些在花的大小上有很大差异的植物，仍然可以产生可育的杂交种，并且很快将对它们的整个基因组进行测序，这使我们能够评估拟南芥大小调节因子的直系同源物的变化如何导致花的大小差异。总之，这些研究将为植物生物学的基本问题提供重要的新见解，具有很大的实际应用潜力。

How do we tell a daisy from a marguerite despite their similarity in colour, structure and organization? We can identify them, because daisies are always much smaller than marguerites. As this example illustrates, individuals of a given species and their constituent organs tend to grow to a surprisingly uniform size, while there are enormous differences in organ size between species, both for plants and animals. These observations indicate that the genetic developmental programme of an organism exerts a tight control over the growth and final sizes of its organs. Despite the scientific importance of this question and the obvious economic potential of being able to manipulate plant organ sizes, our knowledge about the underlying genetic and molecular mechanisms in plants remains very limited. Elucidating these mechanisms and how they have been modified during evolution is the goal of our research. We have isolated a number of mutants that form larger or smaller organs, resulting either from a defect or an overactivation of an individual gene. We have focussed on floral organs in the genetically well accessible model plant Arabidopsis thaliana, as the size of floral organs is only very weakly influenced by the environment, allowing for easier and more reliable identification of genetic factors. From our mutants, we have by now isolated the BIG BROTHER (BB) gene as a crucial determinant of plant organ size. Plants lacking BB activity form much larger flowers and thicker stems due to a longer period of growth, while flower size and stem thickness is progressively reduced, as BB activity increases. The tight inverse correlation between BB activity and organ size suggests that BB acts as a central controlling element in regulating organ growth. The BB gene encodes a protein with E3 ubiquitin-ligase function. E3 ubiquitin ligases mark specific other cellular proteins for degradation. Thus, our current working hypothesis states that BB limits the size of Arabidopsis organs by targeting stimulators of growth for destruction. Identifying these growth stimulators and determining how they act will be major future aims, as will be the question which factors set the BB activity level to the appropriate value for attaining normal organ sizes. A particularly fascinating aspect of size regulation is that it operates on an organ-wide scale above the level of the individual cells and can compensate for changes in cell numbers or cell sizes. Understanding this systems property will require the analysis of mosaics with genetically distinct cells in the same organ. To this end, we will establish and use generally applicable methods for generating and analyzing loss- and gain-of-function clones in developing organs. These methods should prove useful for the Arabidopsis research community at large. As we understand more about the mechanisms of size control in model species, we will begin to isolate the homologous genes from other plants, e.g. rapeseed, both in order to start manipulating organ sizes in economically important crops by changing the activity of conserved size control genes and to study how evolution has modified size control mechanisms. For the latter, we will mainly focus on the species pair Capsella rubella and C. grandiflora which show a large difference in flower size, can still produce fertile hybrids and will soon have their entire genome sequenced, allowing us to assess how changes in the orthologues to Arabidopsis size regulators contribute to the difference in flower size. In summary, these studies will provide important new insights into a fundamental problem of plant biology with a large potential for practical applications.