No abstract

Little is known about the signals that govern the network of meristem and organ identity genes that control f lower development. In Arabidopsis, we can induce a heterochronic switch from f lower to shoot development, a process known as f loral meristem reversion, by manipulating photoperiod in the f loral homeotic mutant agamous and in plants heterozygous for the meristem identity gene leafy. The transformation from f lower to shoot meristem is suppressed by hy1, a mutation blocking phytochrome activity, by spindly, a mutation that activates basal gibberellin signal transduction in a hormone independent manner, or by the exogenous application of gibberellins. We propose that LFY and AG play an important role in the maintenance of f lower meristem identity and that f loral meristem reversion in heterozygous lfy and in ag f lowers is regulated by a phytochrome and gibberellin signal transduction cascade.Plant growth and morphogenesis is controlled by meristems, organized tissues containing pluripotent stem cells whose identities and activities are regulated by intrinsic and environmental signals. In Arabidopsis, the shoot apical meristem undergoes two phases, vegetative and inflorescence; both phases are characterized by reiterative and indeterminate patterns of growth and organogenesis (1). The vegetative meristem produces a compact rosette consisting of a short stem and a variable number of leaves. By contrast, the inflorescence meristem produces an elongated stem punctuated by narrow cauline leaves, lateral secondary shoots, and flowers that are derived from the flanks of the inflorescence meristem. Although closely related spatially and by cell lineage to the inflorescence meristem, the floral meristem proceeds along a determinate developmental pathway producing four compact whorls of organs (four sepals, four petals, six stamens, and two carpels). At the completion of organogenesis, the floral meristem is thought to be depleted or its activity is suppressed (2, 3). The transition from vegetative to inflorescence shoot meristem is controlled by environmental signals including photoperiod and temperature (4-6), by intrinsic growth regulators such as the gibberellins (6-8), and by a system of flowering time genes (9). The inflorescence meristem in turn produces an indeterminate number of floral meristems. Genetic and molecular studies have shown that the establishment of floral meristem identity is governed by a network of genes including APETALA1 (AP1), APETALA2 (AP2), CAULI-FLOWER (CAL), CLAVATA1 (CLV1), CLAVATA3 (CLV3), and LEAFY (LFY) (3,(10)(11)(12)(13)(14)(15)(16)(17)(18), and several models have been proposed to explain how these genes function together (17,19,20). By comparison, much less is known about the signals and genes required for the maintenance of flower meristem identity.Recently several genes have been implicated in the maintenance of flower meristem identity including LFY and AGA-MOUS (AG) (3,15,17,(21)(22)(23). The LFY gene encodes a novel polypeptide that is reported to have DNA bin...

Novel drug targets can be identified by differential analysis of RNA transcripts isolated from cancer cell lines and tissues. We have extended this approach by analyzing differences in gene expression resulting from the drug treatment of transformed and nontransformed cells. A mouse mammary epithelial cell line (C57MG), which conditionally expresses the Wnt-1 proto-oncogene, was left untreated or treated with retinoic acid in the presence or absence of Wnt-1 expression. The experiment was performed in triplicate, and RNA extracted from the four samples was analyzed by hybridization to over 12,000 unique oligonucleotide probe sets. Reproducible alterations in gene expression that occurred in response to retinoic acid, Wnt-1, or retinoic acid plus Wnt-1 relative to untreated cells were identified. Greater attention was given to genes encoding cell surface antigens that were selectively up-regulated by the combination of Wnt-1 and retinoic acid. These genes included the tumor necrosis factor family 4-1BB ligand, ephrin B1, stra6, autotaxin, and ISLR. Administration of retinoic acid to mice bearing tumors driven by activation of the Wnt-1/␤-catenin pathway resulted in increased expression of stra6 in the tumors but not in normal tissue. In principal, the therapeutic index of antibodies directed against these antigens should be enhanced by co-administration of retinoic acid.The aberrant growth and survival of cancer cells is attributed to underlying genetic defects that alter normal cellular homeostasis. In the case of colorectal cancer, inactivation of the adenomatous polyposis coli tumor suppressor occurs early in tumor progression and provides a growth advantage resulting from the inappropriate activation of genes such as cyclin D, matrilysin, and c-myc (1-3). These genes are targets of T cell factor/lymphoid enhancer factor (TCF/LEF) 1 transcription factors that are activated by their interaction with ␤-catenin, a protein that is normally down-regulated by adenomatous polyposis coli (4, 5). The up-regulation of this signaling pathway in cancer can also result from missense mutations in the ␤-catenin gene that render the ␤-catenin protein refractory to downregulation by adenomatous polyposis coli (6, 7). Mutations in ␤-catenin have been identified in a wide variety of human tumors and are particularly prevalent in human hepatocellular cancer (5). Activation of ␤-catenin signaling also occurs when the cell surface frizzled receptors are stimulated by the secreted Wnt ligands (8). Although it is not known whether the Wnt ligands themselves contribute to human cancers, early experiments have demonstrated that their overexpression in mouse mammary tissue was tumorigenic (9). Thus, Wnt signaling represents a mechanism that contributes to the progression of a high percentage of human cancers for which appropriate animal and cell culture models are available.Signals emanating from the Wnt receptors are thought to proceed via the activation of disheveled, which in turn, negatively regulates glycogen synthase kinase 3␤ (10...

We have analyzed the contributions of phytochrome and gibberellin signal transduction to the control of flower meristem identity in the Arabidopsis mutants apetala1 (ap1) and apetala2 (ap2). ap1 flowers are partially defective for the establishment of flower meristem identity and are characterized by the production of ectopic secondary or axillary flowers and by branching. Axillary flower production is also induced in ap2-1 flowers by short-day photoperiod and is suppressed by hy1, a mutation blocking phytochrome activity. The production of axillary flower by ap2-1 is also suppressed by exogenous gibberellins and by spindly (spy), a mutation that activates basal gibberellin signal transduction in hormone-independent manner. Ectopic axillary flower production and floral branching by ap1 flowers are also suppressed by spy. We conclude that gibberellins promote flower meristem identity and that the inflorescence-like traits of ap2-1 and ap1-1 flowers are due in part to SPY gene activity.

We have analyzed the contributions of phytochrome and gibberellin signal transduction to the control of flower meristem identity in the Arabidopsis mutants apetala7 (ap7) and apetala2 (ap2). ap7 flowers are partially defective for the establishment of flower meristem identity and are characterized by the production of ectopic secondary or axillary flowers and by branching. Axillary flower production is also induced in ap2-7 flowers by short-day photoperiod and is suppressed by hy7, a mutation blocking phytochrome activity. The production of axillary flowers by ap2-7 is also suppressed by exogenous gibberellins and by spindly (spy), a mutation that activates basal gibberellin signal transduction in a hormone-independent manner. Ectopic axillary flower production and floral branching by ap7 flowers are also suppressed by spyLWe conclude that gibberellins promote flower meristem identity and that the inflorescence-like traits of ap2-7 and ap7-7 flowqrs are due in part to SPY gene activity.\

The Wnt gene family encodes proteins that serve key roles in differentiation and development. Wnt proteins interact with seven transmembrane receptors of the Frizzled family and activate a signaling pathway leading to the nucleus. A primary biochemical effect of Wnt-1 signaling is the stabilization of cytoplasmic (beta)-catenin which, in association with transcription factors of the Lef/tcf family, regulates gene expression. The recent identification of a new class of secreted proteins with similarity to the extracellular, ligand-binding domain of Frizzled proteins, soluble Frizzled related proteins (sFRP), suggested that additional mechanisms could regulate Wnt signaling. Here we demonstrate that FrzA, a sFRP that is highly expressed in vascular endothelium and a variety of epithelium, specifically binds to Wnt-1 protein, but not Wnt-5a protein, and modulates Wnt-1 signaling. FrzA associated with Wnt-1 either when expressed in the same cell or when soluble FrzA was incubated with Wnt-1-expressing cells. FrzA efficiently inhibited the Wnt-1 mediated increase in cytoplasmic (beta)-catenin levels as well as the Wnt-1 induction of transcription from a Lef/tcf reporter gene. The effects of FrzA on (beta)-catenin levels could be demonstrated when co-expressed with Wnt-1 or when individual cells expressing FrzA and Wnt-1 were co-cultured. These data demonstrate the existence of a negative regulatory mechanism mediated by the selective binding of FrzA to Wnt-1 protein.