Volume 27, Issue 17, Pages R882-R887 (September 2017) Phyllotaxis  Cris Kuhlemeier  Current Biology  Volume 27, Issue 17, Pages R882-R887 (September 2017) DOI: 10.1016/j.cub.2017.05.069 Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 Phyllotactic arrangements. (A) Distichous: leaves arise at divergence angles of 180° (Zea mays). (B) Decussate: each pair of opposed leaves is displaced relative to the next pair by 90° (Kalanchoë daigremontiana). (C) Unusual phyllotaxis with divergence angles varying from 30° to 60° (Costus speciosus). (D) Spiral, ∼137° divergence angle: top view of Arabidopsis thaliana vegetative meristem, leaf primordia (p1–p8) labeled with FIL-GFP transgene. (E) Higher order spiral: scanning EM image of Helianthus annuus capitulum. Images courtesy of Roman Köpfli and Peter von Ballmoos (A,B); Marc Gremillon (C); Agata Burian (D); and Siobhan Braybrook (E), Current Biology 2017 27, R882-R887DOI: (10.1016/j.cub.2017.05.069) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 2 The shoot apex. (A) Tomato shoot apex, with apical meristem highlighted in red (diameter approximately 130 μm). P1 is the youngest visible leaf primordium, P2–P8 are successively older primordia. P4–P8 were cut off to expose the meristem. Even though bulges are not visible in the leaf axils, axillary meristems are specified already in the apical meristem as groups of quiescent cells. (Scanning EM image from D. Reinhardt.) (B) Top view of Arabidopsis thaliana vegetative meristem with CLV3-expressing cells (red) and apical initials (asterisks) highlighted. The approximate outline of the central zone is indicated by a black line. (Confocal image courtesy of Agata Burian.) (C) Fate of new mutations. A mutation that arises in an apical initial (red) will propagate as an almost indefinite sector with a width of one-third to one-fourth of the circumference of the main shoot, whereas a mutation in a subapical initial (blue) will be rapidly displaced from the meristem and affect only a narrow and short section of the shoot. Due to early sequestration of axillary meristems, in a tree with 1,000 branches there will be only ∼60 cell divisions between the embryonic meristem and the meristems in the terminal branches. For details, see Burian et al. (2016). Current Biology 2017 27, R882-R887DOI: (10.1016/j.cub.2017.05.069) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 3 Auxin and phyllotaxis. (A) The NPA experiment. From left to right: control tomato shoot apical meristem; meristem incubated on medium containing the auxin transport inhibitor NPA grows vigorously, but does not initiate leaves; application of a minuscule drop of auxin (IAA, false-colored in red) to an NPA-grown meristem induces a bulge that pushes the droplet outward; the bulge develops into a recognizable tomato leaf. Modified from Reinhardt et al. (2000), www.plantcell.org Copyright American Society of Plant Biologists. (B) Coordinated subcellular PIN polarity (yellow arrows) induces an auxin convergence point. (Confocal image from Eva Pesce.) (C) An auxin–PIN positive feedback loop functions as a pattern generator. The ultimate output is organ formation with intermediate processes feeding back on the pattern generator. Inputs can, for instance, be light, metabolism, or genes that regulate auxin or PIN1 biosynthesis. (D) Computer simulation of spiral phyllotaxis. The simulation starts with a small group of cells representing a radially symmetrical embryo. Each cell produces PIN1 (red) and auxin (green). Small random fluctuations in concentration between cells are introduced to break initial symmetry. Auxin moves between cells by both diffusion and PIN1-mediated transport. Cotyledons and true leaves emerge at divergence angles that recapitulate experimentally observed values within 1 standard deviation. (For animation, see Proc. Natl. Acad. Sci. USA http://www.pnas.org/content/103/5/1301?tab=ds#M1.) From Kuhlemeier, Trends in Plant Science (2007) 12, 143–150. Current Biology 2017 27, R882-R887DOI: (10.1016/j.cub.2017.05.069) Copyright © 2017 Elsevier Ltd Terms and Conditions