Fig. 2. The shoot meristem. Scanning electron microscopy of a wild-type shoot meristem (A) reveals little morphological differentiation, except for the distinct organ primorida (op), whereas molecular and detailed histological studies have revealed important subdomains (PZ, peripheral zone; CZ, central zone; RZ, rib zone) of the shoot meristem (D). The flower meristem gives rise to a regular number of floral organs and organ types (B), eventually forming the fruit (also called the silique; C). In plants carrying a mutation in one of the CLV genes, the shoot meristem can be massively enlarged (E), as a result of expansion of the population of stem cells (H). clv flower meristems also accumulate stem cells, resulting in the formation of additional organs of each type (F), and leading to distorted, club-shaped fruits (G). (I) A model for CLV signaling. The CLV3 protein (blue) is secreted from the stem cells and diffuses to the underlying cell layers, where CLV1 (red) is expressed. There, CLV1 binds to CLV3 and activates signaling to repress WUS expression, as well as to sequester CLV3 to prevent it from diffusing to the WUS-expressing region. CLV2 (green) is a putative co-receptor with CLV1. Scale bar in A and E: 50 µm.