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 two main tasks of a meristem, self-perpetuation and organ initiation, are separated spatially. Slowly dividing cells in the meristem center act as pluripotent stem cells, and only their derivatives in the meristem periphery specify new organs. Meristem integrity and cellular proliferation are controlled in part by regulatory interactions between genes that are expressed in specific subdomains of the meristem. Using transposon-mediated activation tagging, we have identified Dornröschen ( drn-D ) mutants of Arabidopsis that prematurely arrest shoot meristem activity with the formation of radialized lateral organs. The mutated gene ( DRN/ESR1 ), which encodes an AP2/ERF protein, is expressed in a subdomain of meristem stem cells, in lateral organ anlagen, and transiently in the distal domain of organ primordia. During the development of drn-D mutants, expression of the homeobox gene SHOOTMERISTEMLESS is downregulated and later reactivated in an altered domain. In addition, we found increased expression of CLAVATA3 and WUSCHEL , two genes that antagonistically regulate stem cell fate in meristems. These findings suggest that the DRN/ESR1 gene product is involved in the regulation of gene expression patterns in meristems. Furthermore, specific misexpression of DRN in meristem stem cells affects organ polarity and outgrowth in the meristem periphery, indicating that DRN/ESR1 itself, or a process regulated by DRN/ESR1 , can act non-cell-autonomously. We elaborate on the role of DRN/ESR1 in meristem and organ development and discuss its possible role in the process of shoot regeneration.