Geometric-edge specification in cell growth mechanics and morphogenesis

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A fundamental challenge in biology is to explain how complex organisms develop the intricate anatomical forms observed in nature. The productivity and utility of crop plants is critically dependent on the development of particular anatomical forms that are often grossly altered from the wild state. The development of biological form requires chemical and mechanical information to be integrated at several scales of organisation: molecules assemble into larger assemblies; molecular assemblies organise internal cell structures; intracellular structures determine cellular properties; groups of cells assemble into tissues, tissues into organs, and organs into organisms. This highly complex process is nevertheless remarkably robust - petals on a symmetrical flower each have similar size and shape for example, or modern wheat varieties which grow to a remarkably uniform height. A curious feature of development is that despite variability at lower scales of organisation (e.g., cell size, shape and number) biological form is typically robust at higher scales. This is rather like a dry-stone wall having a regular height and thickness despite variability in the sizes of the stones from which it is built. A recent idea, supported by our recent work, is that the variability at subcellular scales of organisation is not simply 'noise in the system' but is an essential part of the mechanism that maintains robust reproducible form at higher scales. The overall shape of an organism is determined by the size, shape and arrangement of its component cells. Plant cells are surrounded by a rigid cell wall that resists their high internal pressure and determines their shape. Cell walls also prevent cells in plan from slipping past each other as they do during the formation of animal embryos. Consequently, the final form of the plant is determined principally by controlling the shape into which each cell grows. This requires the direction of cell growth to be controlled. Growth is driven by the cells' internal pressure, which acts equally in all directions, so the direction of growth is determined by the mechanical properties of cells' wall at the different regions of that cell. To successfully generate the final plant form, the control of cell growth must be coordinated across hundreds and thousands of cells. This requires both chemical and mechanical signalling between cells in growing organs. It also requires mechanisms that allow each individual cell to respond appropriately to these signals, adopting a shape that is appropriate to its position in the final structure. Little is known about how this happens. This research aims primarily to increase our understanding of- how growth and form are controlled at the level of individual plant cells- how this is co-ordinated between cells to achieve proper form at the multicellular level- how variability at lower scales influences the final form We will focus on an important, newly discovered, mechanism that contributes to the control of plant cell growth. We will investigate the molecular and mechanical contribution that this mechanism makes during plant development. In short, we have recently discovered an internal transport mechanism in plants that delivers material specifically to the geometric edges of cells (i.e. where two faces meet). We have shown that when this transport mechanism is disrupted, cells and tissues become disorganised. We believe that this is part of a mechanism that allows cells to adjust their own size, shape and growth rate to produce an appropriate final form. We believe this is based on the detection of, and response to, mechanical stresses in the tissue.To test our hypotheses we have assembled an interdisciplinary team of biologists, physicists and engineers to tackle this problem with a combination of computational models, genetic and biochemical analysis, plus 4D-light microscopy and mechanical measurements by dynamic atomic-force microscopy.