Molecular and genetic mechanisms of plant organ size control

尽管雏菊和玛格丽特在颜色、结构和组织上都很相似，我们如何区分它们呢？我们可以认出它们，因为雏菊总是比雏菊小得多。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长得出奇地一致，而植物和动物物种之间的器官大小却存在巨大差异。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。尽管这个问题具有重要的科学意义，而且操纵植物器官大小也具有明显的经济潜力，但我们对植物潜在的遗传和分子机制的了解仍然非常有限。阐明这些机制以及它们如何在进化过程中被修改是我们研究的目标。我们已经分离出了一些突变体，它们形成了或大或小的器官，这些器官要么是由缺陷造成的，要么是由单个基因的过度激活造成的。由于花器官的大小受环境的影响非常微弱，因此我们将重点放在遗传上容易接近的模式植物拟南芥的花器官上，从而更容易和更可靠地识别遗传因素。从我们的突变体中，我们现在已经分离出大哥（BB）基因，它是植物器官大小的关键决定因素。缺乏BB活性的植株由于生长时间较长，花较大，茎粗，而随着BB活性的增加，花的大小和茎粗逐渐减小。BB活性与器官大小之间的密切负相关表明BB在调节器官生长中起着中心控制作用。BB基因编码具有E3泛素连接酶功能的蛋白。E3泛素连接酶标记其他特定的细胞蛋白进行降解。因此，我们目前的工作假设表明，BB通过靶向生长刺激物来破坏拟南芥器官的大小。确定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，同时也是哪些因素将BB活性水平设定为达到正常器官大小的适当值的问题。大小调节的一个特别吸引人的方面是，它在高于单个细胞水平的全器官范围内起作用，可以补偿细胞数量或细胞大小的变化。理解这一系统特性需要对同一器官中具有不同基因的细胞的嵌合体进行分析。为此，我们将建立和使用一般适用的方法来产生和分析发育器官中功能丧失和功能获得的克隆。这些方法将被证明对拟南芥研究界非常有用。随着我们对模式物种的大小控制机制的了解越来越多，我们将开始从其他植物（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经济上重要作物的器官大小，并研究进化如何改变大小控制机制。对于后者，我们将主要关注花大小差异较大的风蓬（Capsella rubella）和大花梗（C. grandflora）这对物种对，它们仍然可以产生可育的杂种，并将很快对它们进行全基因组测序，从而使我们能够评估拟南芥大小调节因子的同源基因的变化是如何导致花大小差异的。总之，这些研究将为植物生物学的一个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.