Molecular and Genetic Mechanisms of Plant Organ Size Control

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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, our studies will yield new insights into plant size control that will have important practical applications.

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