Phyllotaxis, the regular arrangement of leaves or flowers around a plant stem, is an example of developmental pattern formation and organogenesis. Phyllotaxis is characterized by the divergence angles between the organs, the most common angle being 137°, the golden angle. This quantitative aspect makes phyllotaxis an unusual developmental problem. Models of phyllotaxis must explain its de novo establishment in the radially symmetric embryo, its stable maintenance and the transitions between patterns. I will briefly describe the experiments that demonstrated that the central regulator of phyllotaxis consists of a positive feedback loop between auxin and its transporter, the PIN1 protein. Computational modeling shows that such a regulatory loop can generate regularly spaced auxin maxima within the shoot apical meristem, which cause differential gene expression, localized growth, and organ development. We also show how the environment interacts with this regulatory system and how downstream morphogenetic processes feedback on it. In our recent work we demonstrate that phyllotactic patterning can occur in the absence of auxin transporter PIN1.

This is a joint work with Sarah Robinson, Michal Hufleit, Daniel Kierzkowski, Richard Smith and Naomi Nakayama.