Elucidating the mechanism of organ size control by KLU-dependent intercellular signalling

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Understanding how the size of organisms and their organs is determined is an important goal of basic biology, which will allow the rational manipulation of growth and size in economically relevant species. Although several genes have been identified that influence organ size, the fundamental problem of how a growing organ can measure its size is still unresolved. We have recently shown that the cells at the margins of an organ play a particularly important part in determining its size. These marginal cells produce a small molecule acting as a mobile growth regulator that can move into the organ and maintain cell proliferation. For purely geometric reasons the margin of the organ grows more slowly than the overall area, suggesting that the growth regulator is diluted as the organ increases in size. This offers a simple means for measuring organ size via the concentration of this growth regulator. In this view, cell proliferation arrests, once the concentration of the growth regulator falls below a critical value when the organ reaches a certain size. This model is similar to current ideas about how the size of animal organs, for example fly wings, is controlled, suggesting that ultimately plants and animals use the same principle to measure organ size. The production of this presumed signal requires the activity of the KLUH (KLU) gene, which is only active at the margins of the organs and provides an excellent point of entry for further studying the control of plant organ growth. The aim of this proposal is to gain a more detailed understanding of how the KLU-dependent growth regulator controls organ size. To this end, we will focus on four questions: 1. How mobile is the KLU-dependent growth regulator? 2. Is there an assembly line of proteins to make the active growth regulator? 3. Which small molecule(s) are modified by the KLU protein? 4. Which other genes are necessary to generate or perceive the growth regulator?

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