Revisiting PIN1 polarity interpretation at the SAM based on automated multi-level reconstruction of PIN1 polarities from confocal images

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English. The shoot apical meristem (SAM) produces new aerial organs such as leaves and flowers, patterning the shoot in an arrangement called phyllotaxis. Phyllotaxis is driven by the spatiotemporal dynamics of the auxin hormone at the SAM. It has been shown that auxin accumulation emerges from auxin polar transport, and triggers organ differentiation. In particular, the PIN1 membrane protein is a polar auxin exporter necessary to a functional auxin dynamical patterning. Auxin feedbacks on PIN1 polarity that in turn transports auxin and thus changes its pattern. This feedback loop has been widely modeled to explain the emergence of phyllotactic patterns [4-7].    Most of the studies detect PIN1 polarity direction in tissues by human inspection of fluorescent images [1-3]. Based on analysis of the direction of PIN1 polarity, it was proposed that auxin accumulation results from locally converging PIN1 polarities.    PIN1 polarity quantitative extraction remains challenging in vivo because PIN1 are located at the cell membranes, separated by the cell wall whose thickness is below optical resolution. A first automated technique was proposed in [5]. The authors have shown a partial change of PIN1 polarity direction at the initiation site as observed in [2]. In a recently published work [8], an automated quantitative technique provides high resolution spatiotemporal PIN1 maps, and makes it possible to reanalyse PIN1 polarity patterns. It appears that PIN1 polarity directions globally converge toward the central zone with only limited reorientation over time and space. This work emphasizes the need to revisit the interpretation of PIN1 polarity direction as the unique factor determining auxin accumulation.    Here, we propose a new analysis of PIN1 polarity based on the above quantitative technique to understand the formation of auxin accumulation. Using a modeling approach, we study the influence of the PIN1 expression level and the polarity direction on the auxin pattern, and show that both could be regulated to achieve auxin accumulation spots.    Our approach clarifies the qDII auxin reporter and PIN1 polarity spatiotemporal observations in [8]. We explain why auxin can accumulate locally without strict PIN1 polarity directional convergence due to the effect of a gradient of the PIN1 expression level. New phyllotaxis models proposing both regulations of the PIN1 polarity direction and level of expression could lead to a better understanding of the biological observations.References[1] Reinhardt, D., Pesce, E. R., Stieger, P., Mandel, T., Baltensperger, K., Bennett, M., ... & Kuhlemeier, C. (2003). Regulation of phyllotaxis by polar auxin transport. Nature, 426(6964), 255-260.[2] Heisler, M. G., Ohno, C., Das, P., Sieber, P., Reddy, G. V., Long, J. A., & Meyerowitz, E. M. (2005). Patterns of auxin transport and gene expression during primordium development revealed by live imaging of the Arabidopsis inflorescence meristem. Current biology, 15(21), 1899-1911.[3] de Reuille, P. B., Bohn-Courseau, I., Ljung, K., Morin, H., Carraro, N., Godin, C., & Traas, J. (2006). Computer simulations reveal properties of the cell-cell signaling network at the shoot apex in Arabidopsis. Proceedings of the National Academy of Sciences, 103(5), 1627-1632.[4] Smith, R. S., Guyomarc'h, S., Mandel, T., Reinhardt, D., Kuhlemeier, C., & Prusinkiewicz, P. (2006). A plausible model of phyllotaxis. Proceedings of the National Academy of Sciences, 103(5), 1301-1306.[5] Jönsson, H., Heisler, M. G., Shapiro, B. E., Meyerowitz, E. M., & Mjolsness, E. (2006). An auxin-driven polarized transport model for phyllotaxis. Proceedings of the national academy of sciences, 103(5), 1633-1638.[6] Stoma, S., Lucas, M., Chopard, J., Schaedel, M., Traas, J., & Godin, C. (2008). Flux-based transport enhancement as a plausible unifying mechanism for auxin transport in meristem development. PLoS computational biology, 4(10), e1000207.[7] Bayer, E. M., Smith, R. S., Mandel, T., Nakayama, N., Sauer, M., Prusinkiewicz, P., & Kuhlemeier, C. (2009). Integration of transport-based models for phyllotaxis and midvein formation. Genes & development, 23(3), 373-384.[8] Galvan-Ampudia, C. S., Cerutti, G., Legrand, J., Brunoud, G., Martin-Arevalillo, R., Azais, R., ... & Vernoux, T. (2020). Temporal integration of auxin information for the regulation of patterning. Elife, 9, e55832.