Fig. 1. Signalling processes that regulate stem cell activity and cell fate decisions at the Arabidopsis shoot apical meristem (SAM). (A) Stem cells in the outer layers of the central zone of the SAM express CLAVATA3 (CLV3, green) and their activity is promoted by a signal emanating from WUSCHEL-expressing cells (WUS, red). CLV3 signals through CLV1 (blue) to restrict the WUS expression domain. (B) Leaf cell fate at the flanks of the SAM is promoted by ASYMMETRIC LEAVES1 (orange) and is prevented by the activity of KNOTTED1-like homeodomain proteins (purple) in the SAM. Directional flux of auxin via the polar distribution of the PINFORMED1 protein (blue) determines sites of leaf initiation.