Auxin, chief architect of the shoot apex

Abstract

Plants have a unique capacity for continuous postembryonic development linked to the existence of permanent stem cell niches, located in specialized tissues called meristems. The activity of the shoot apical meristem (SAM), which is located at the tip of stems and branches, allows for the continuous production of all aerial organs that will develop as lateral shoots, leaves, or flowers. As it defines the number, type, and position of lateral primordia, the SAM is at the basis of plant architecture and its activity can be modulated by both internal and environmental cues. Successive initiations of new organ primordia occur in the meristem following very precise spatiotemporal patterns, called phyllotaxis. The maintenance of the meristem over time is thus expected to require precise spatiotemporal control of cell fate to allow for the continuous emergence of new primordia at precise positions and the maintenance of the stem cell niche. Signaling initiated by the plant hormone auxin plays a central role in the control of cell identities during organogenesis and in the dynamics of phyllotaxis. We first describe the structure and function of the SAM, focusing on the model species Arabidopsis thaliana. We then discuss the central role played by auxin in the coordination of cellular behaviors and cell identities in the SAM, and thus in providing the primary instructions for phyllotaxis and for elaborating the shoot architecture. Finally, we illustrate how the gene network downstream of auxin and mechanical properties of tissues participate in controlling morphogenesis and phyllotaxis dynamics.