In higher plants, organogenesis occurs continuously from self-renewing apical meristems. Arabidopsis thaliana plants with loss-of-function mutations in the CLAVATA (CLV1, 2, and 3) genes have enlarged meristems and generate extra floral organs. Genetic analysis indicates that CLV1, which encodes a receptor kinase, acts with CLV3 to control the balance between meristem cell proliferation and differentiation. CLV3 encodes a small, predicted extracellular protein. CLV3 acts nonautonomously in meristems and is expressed at the meristem surface overlying the CLV1 domain. These proteins may act as a ligand-receptor pair in a signal transduction pathway, coordinating growth between adjacent meristematic regions.

The fate of stem cells in plant meristems is governed by directional signaling systems that are regulated by negative feedback. In Arabidopsis thaliana, the CLAVATA (CLV) genes encode the essential components of a negative, stem cell-restricting pathway. We used transgenic plants overexpressing CLV3 to show that meristem cell accumulation and fate depends directly on the level of CLV3 activity and that CLV3 signaling occurs exclusively through a CLV1/CLV2 receptor kinase complex. We also demonstrate that the CLV pathway acts by repressing the activity of the transcription factor WUSCHEL, an element of the positive, stem cell-promoting pathway.

Continuous growth and development of plants is controlled by meristems that harbour stem cell pools. Division of stem cells and differentiation of their progeny are coordinated by intercellular signaling. In Arabidopsis, stem cells in shoot and floral meristems secrete CLAVATA3, a member of the CLE protein family that activates the CLV1/CLV2 receptor complex in underlying cells to restrict the size of the stem cell population. We found that CLE40 encodes a potentially secreted protein that is distantly related to CLV3. While CLV3 transcripts are confined to stem cells of the shoot system, CLE40 is expressed at low levels in all tissues, including roots. Misexpression and promoter swap experiments show that CLE40 can fully substitute for CLV3 to activate CLV signalling in the shoot, indicating that CLV3 and CLE40 are functionally equivalent proteins that differ mainly in their expression patterns. Analysis of cle40 mutants shows that wild-type expression levels of CLE40 are insufficient to contribute to CLV signalling. High level expression of CLV3 or CLE40 results in a premature loss of root meristem activity, indicating that activation of a CLV-like signaling pathway may restrict cell fate also in roots. The cellular organization of cle40 root meristems is normal, but mutant roots grow in a strongly waving pattern, suggesting a role for CLE40 in a signaling pathway that controls movement of the root tip.

The ability of meristems to continuously produce new organs depends on the activity of their stem cell populations, which are located at the meristem tip. In Arabidopsis, the size of the stem cell domain is regulated by two antagonistic activities. The WUS (WUSCHEL) gene, encoding a homeodomain protein, promotes the formation and maintenance of stem cells. These stem cells express CLV3 (CLAVATA3), and signaling of CLV3 through the CLV1/CLV2 receptor complex restricts WUS activity. Homeostasis of the stem cell population may be achieved through feedback regulation, whereby changes in stem cell number result in corresponding changes in CLV3 expression levels, and adjustment of WUS expression via the CLV signal transduction pathway. We have analyzed whether expression of CLV3 is controlled by the activity of WUS or another homeobox gene, STM (SHOOT MERISTEMLESS), which is required for stem cell maintenance. We found that expression of CLV3 depends on WUS function only in the embryonic shoot meristem. At later developmental stages, WUS promotes the level of CLV3 expression, together with STM. Within a meristem, competence to respond to WUS activity by expressing CLV3 is restricted to the meristem apex.The shoot apical meristem (SAM) of higher plants is formed during embryogenesis and gives rise to leaves and stem after germination (Steeves and Sussex, 1989). The side branches of angiosperms originate from axillary meristems that arise in the axils of leaves, whereas flowers are formed from secondary meristems that are initiated at the flanks of the SAM. The cells in these three types of meristems are arranged in three clonal layers (L1, L2, and L3). All organs and also floral meristems are produced at the flanks of the meristem dome in the peripheral zone. Loss of cells from the meristem during organ formation has to be compensated by divisions of cells in the central zone that act as pluripotent stem cells. When these stem cells divide, their daughter cells are displaced to the periphery, where they will be incorporated into organ primordia and eventually differentiate. Therefore, the ability of meristems to continuously produce new organs depends on the activity of their stem cell populations. Stem cells can be initiated repeatedly during plant development, whenever a new axillary or floral meristem is formed. The stem cells of floral meristems are not permanent, but lose their undifferentiated state when the inner set of floral organs is produced. Thus, stem cell identity may represent a flexible state that is subject to both positive and negative regulation.A current model proposes that the size of the stem cell population in meristems is controlled by a negative feedback regulation between two pathways that promote or restrict stem cell number (Waites and Simon, 2000). The WUS (WUSCHEL) gene of Arabidopsis, encoding a nuclear-localized homeodomain protein, is expressed underneath the stem cell domain of shoot and floral meristems. In wus mutants, the cells in the central zone differentiate prematurely, indicati...

The development of higher plants depends on the activity of a shoot apical meristem. Organs are formed on the flanks of the meristem, while pluripotent stem cells are found in a separate domain in the meristem centre. Further domains are distinguished by the expression patterns of genes that control the development of the shoot meristem. Although most plant cells are immobile, their relative position within a meristem, and therefore also their function, can change after cell divisions. To maintain an active shoot meristem throughout plant life, the cells in the meristem need constantly to assess their position, transmit this information to others, and readjust their gene expression profiles and their fate. Some of the genes that permit intercellular communication have been isolated. They enable the flow of information in and between meristem regions via ligands and receptor proteins to transcription factors, that ultimately control the fate of cells in the centre of the meristem.