Auxin: a major regulator of organogenesis.

Plant development is characterized by the continuous initiation of tissues and organs. The meristems, which are small stem cell populations, are involved in this process. The shoot apical meristem produces lateral organs at its flanks and generates the growing stem. These lateral organs are arranged in a stereotyped pattern called phyllotaxis. Organ initiation in the peripheral zone of the meristem involves accumulation of the plant hormone auxin. Auxin is transported in a polar way by influx and efflux carriers located at cell membranes. Polar localization of the PIN1 efflux carrier in meristematic cells generates auxin concentration gradients and PIN1 localization depends, in turn, on auxin gradients: this feedback loop generates a dynamic auxin distribution which controls phyllotaxis. Furthermore, PIN-dependent local auxin gradients represent a common module for organ initiation, in the shoot and in the root.

Computer simulations reveal properties of the cell-cell signaling network at the shoot apex in Arabidopsis.

The active transport of the plant hormone auxin plays a major role in the initiation of organs at the shoot apex. Polar localized membrane proteins of the PIN1 family facilitate this transport, and recent observations suggest that auxin maxima created by these proteins are at the basis of organ initiation. This hypothesis is based on the visual, qualitative characterization of the complex distribution patterns of the PIN1 protein in Arabidopsis. To take these analyses further, we investigated the properties of the patterns using computational modeling. The simulations reveal previously undescribed properties of PIN1 distribution. In particular, they suggest an important role for the meristem summit in the distribution of auxin. We confirm these predictions by further experimentation and propose a detailed model for the dynamics of auxin fluxes at the shoot apex.

A protocol to analyse cellular dynamics during plant development.

In vivo microscopy generates images that contain complex information on the dynamic behaviour of three-dimensional (3D) objects. As a result, adapted mathematical and computational tools are required to help in their interpretation. Ideally, a complete software chain to study the dynamics of a complex 3D object should include: (i) the acquisition, (ii) the preprocessing and (iii) segmentation of the images, followed by (iv) a reconstruction in time and space and (v) the final quantitative analysis. Here, we have developed such a protocol to study cell dynamics at the shoot apical meristem in Arabidopsis. The protocol uses serial optical sections made with the confocal microscope. It includes specially designed algorithms to automate the identification of cell lineage and to analyse the quantitative behaviour of the meristem surface.

Cell proliferation patterns at the shoot apical meristem.

The highly stereotypic cell proliferation patterns in many plant species suggest that the strict control of the cell cycle in time and space is an essential basis for ordered development. At the same time, conflicting evidence contradicts this view, indicating that cell division simply follows growth patterns that are dictated by the local availability of nutrients. Recent evidence shows that there is no strict hierarchical relationship between growth and proliferation. Cell expansion and proliferation, for example, are controlled by the same regulators. Cell proliferation depends on nutrient distribution but, in turn, the use of the same nutrients depends on the activity of cell cycle regulators such as E2F transcription factors.