Molecular and genetic mechanisms of plant organ size control

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.

The size of plant organs is tightly controlled by the species-specific genetic programme. However, despite its importance, very little is known about this fundamental biological problem. Thus, the aim of our research is to elucidate genetic and molecular mechanisms of plant organ size control, using Arabidopsis flowers as a model, and to understand how these mechanisms have been modified during evolution to bring about different species-specific organ sizes. We have identified the novel RING-finger E3 ubiquitin ligase BIG BROTHER (BB) as a crucial regulator of floral organ size. bb mutants form larger flowers because of a longer growth period. BB mRNA expression levels show a tight inverse correlation with final organ size, suggesting that BB is a central regulator of size. As its E3 activity is essential for in vivo function, BB appears to limit organ size by targeting key growth stimulators for degradation. We will use complementary biochemical and genetic approaches to identify substrates of BB and upstream regulators of its expression level. Understanding the organ-wide integration of growth control, which is evident from the capacity to compensate for changes in cell numbers or sizes, will necessitate the analysis of genetic mosaics. We will attempt to establish and use a novel system for targeted mitotic recombination between homologous chromosomes in Arabidopsis as a generally applicable method for generating loss-of-function clones. In a complementary approach, we will induce clones overexpressing a given size regulator and characterize any non-cell autonomous effects. Isolation of homologues to Arabidopsis size regulators from other species, e.g. Brassica and Capsella, will help us understand the evolution of size control in plants and how this affects crop plant performance. Using these approaches, I am confident that our studies will yield new insights into plant size control that will have important practical applications