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The beauty of plants lies in their ability to form complex, yet precise and reproducible, geometrical forms. This ability reaches its peak in the mechanism of flower formation. For example, the head of a sunflower consists of many hundred individual, tiny flowers generated in spirals encroaching from the circumference of an expanding apical structure (capitulum) to form a gigantic but geometrically perfectly patterned flower (Fig. 1). In this issue, Dosio et al. (pp. 711–722) provide a quantitative analysis of the process of capitulum growth, and the gradual but controlled process by which elements within the capitulum become destined to form individual flowers. These data provide a solid foundation for the modelling of flower formation, and for linking the observed parameters of floral morphogenesis with the wealth of molecular data on organogenesis in plants. The combination of careful quantitative analysis to create models of plant morphogenesis with molecular tools to test those models is an emerging and exciting area of research in plant developmental biology.
‘… can the recently published models developed in Arabidopsis be extended to a field of tissue which is orders of magnitude larger in size?’
Auxin flux and organ formation
With respect to the underlying principles of organ-pattern formation in plants, the most significant insight has come from recent data on the analysis of auxin flux and distribution during leaf initiation. Auxin is a small indole-related compound, which has long been recognized as a key plant growth regulator. It has also long been known that the flux of auxin through tissue can be mediated via a specific transport process, the polar auxin-transport system (Rubery & Sheldrake, 1974). A flood of significant papers over the past few years have identified key components of this system that mark the cells through which auxin flux occurs, enable the direction of auxin flux to be interpreted and, via molecular genetic manipulation, enable auxin flux within the plant to be modified (Benkováet al., 2003; Friml et al., 2003; Blilou et al., 2005). One important conclusion from this work is that, in the shoot apical meristem, the position of the presumptive leaf initiation is marked by a local accumulation of auxin as a result of directed auxin flux within the tissue (Reinhardt et al., 2003). Moreover, it appears that the system transporting auxin is itself responsive to auxin (Paciorek et al., 2005), creating a dynamic system in which fluxes of auxin are integrated across a growing sheet of tissue to create patterns of auxin that dictate the pattern of organogenesis. As organogenesis itself disrupts or alters the pattern of auxin flux, an iterative system is set in place. Recent papers have described mathematical models that can mimic this iterative process and predict the outcome of alteration in particular components of the transport process (Barbier de Reuille et al., 2006; Jönsson et al., 2006; Smith et al., 2006). Although there are some differences in the modelling techniques used and the interpretations made, a key experimental approach has been to obtain in vivo quantitative analysis of growth in the shoot apical meristem, as also exemplified by other recent publications (Kwiatkowska & Dumais, 2003; Reddy et al., 2004; Reddy & Meyerowitz, 2005).
The models that have been developed can be used to describe alternative patterns of leaf formation (phyllotaxis), but as yet have not been extended to patterns of floral initiation. Does a similar system underpin floral patterning? It seems likely, yet there are distinct differences between, for example, leaf initiation in an Arabidopsis shoot apical meristem and floral patterning on a sunflower capitulum. One of the most obvious is scale – can the recently published models developed in Arabidopsis be extended to a field of tissue that is orders of magnitude larger in size? In floral oraganogenesis, it is common for several (indeed, many) organs to be initiated at different points around the circumference of a meristem essentially simultaneously, in contrast to the generally sequential formation of leaf primordia. Can patterns of auxin flux respond in an appropriate frame of time and space to allow this to happen in such large fields of tissue? Or is the apparently large responding field of a sunflower capitulum in reality made up of many much smaller interacting subdomains? The quantitative data provided by Dosio et al. provide important physiological parameters for the modellers to work with. An interesting conclusion of the three recent phyllotaxis modelling papers is that they all predict or require that there is ‘something special’ about the central region of the meristem, so that auxin either does not act there to induce organogenesis, or is somehow excluded. The sunflower data are consistent with a similar phenomenon, but show clearly that this special feature of the central region can gradually be lost, so that eventually all tissue is consumed in flower formation. The situation in the capitulum may be linked with the spatial and temporal control of factors specific to flower formation, but it might also provide an insight into what is so special about the central zone of the meristem. Irrespective of this particular point, the conservation of molecular machinery involved in auxin transport should allow future investigation of whether the polar auxin-transport system underlies the patterning process so carefully analysed in the present study in the sunflower meristem, and whether the emerging models of phyllotaxis are applicable to the process of floral patterning.