Meristems in the movies: live imaging as a tool for decoding intercellular signaling in shoot apical meristems.

The shoot apical meristem (SAM) is the source of all postembryonic aerial organs in plants. Because of their central importance in plant development, SAMs have been studied for more than 100 years and have also become a major focus of molecular genetic studies. These studies indicate that signaling between cells is important in the maintenance of SAM size and structure and for the positioning and development of new organs. Two signaling molecules are currently known to regulate SAMs: the secreted protein CLV3 and the classical plant hormone auxin. Studies of SAMs have been hindered by their small size and by their location within developing buds. Innovations in confocal microscopy and the development of markers for individual cells and for cell type–specific genes now allow for in vivo visualization of SAMs. These methods provide powerful tools for examining the dynamics of gene expression patterns and the instantaneous consequences of perturbation of gene function in living SAMs. In this essay, we describe the structure of SAMs, summarize experimental and molecular data for signaling events, and describe the impact of two new studies (Heisler et al., 2005; Reddy and Meyerowitz, 2005) that increase our understanding of signaling mechanisms in SAM maintenance and in organogenesis.