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那样，特定物种的个体及其组成器官的大小往往令人惊讶地统一，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。这些观察表明，生物体的遗传发育程序严格控制着其器官的生长和最终大小。尽管这个问题具有重要的科学意义，而且能够控制植物器官的大小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在进化过程中是如何被修改的是我们研究的目标。我们已经分离出了一些突变，它们形成了更大或更小的器官，这些突变要么是由于单个基因的缺陷，要么是由于单个基因的过度激活。我们把重点放在了遗传上很容易获得的模式植物拟南芥的花器官上，因为花器官的大小只受环境的非常微弱的影响，从而允许更容易和更可靠地识别遗传因素。到目前为止，我们已经从我们的突变体中分离出了决定植物器官大小的关键决定因素--老大哥(BB)基因。缺乏BB活性的植物由于生长期较长而形成更大的花和更粗的茎，而随着BB活性的增加，花的大小和茎的厚度逐渐减小。BB活性与器官大小之间的密切负相关关系表明，BB在调节器官生长方面起着中枢控制作用。BB基因编码一种具有E3泛素连接酶功能的蛋白质。E3泛素连接酶标记其他特定的细胞蛋白以供降解。因此，我们目前的工作假设是，BB通过靶向生长刺激因子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的作用方式将是未来的主要目标，还有一个问题是，哪些因素将BB活动水平设定为达到正常器官大小的适当值。大小调节的一个特别吸引人的方面是，它在器官范围内运行，高于单个细胞的水平，并可以补偿细胞数量或细胞大小的变化。要了解这一系统特性，需要对同一器官中具有不同遗传特征的细胞的马赛克进行分析。为此，我们将建立和使用普遍适用的方法来生成和分析发育中器官的功能丧失和获得克隆。事实证明，这些方法对整个拟南芥研究界都很有用。随着我们对模式物种的大小控制机制有了更多的了解，我们将开始从其他植物，如油菜中分离同源基因，以便通过改变保守的大小控制基因的活性来控制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，我们将主要关注两个物种对，它们在花大小上表现出很大的差异，仍然可以产生可育的杂交后代，并且很快就会完成它们的全基因组测序，从而使我们能够评估拟南芥大小调节器的同源基因的变化如何导致花大小的差异。总之，这些研究将为一个具有巨大实际应用潜力的植物生物学基本问题提供重要的新见解。

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.