The CLE family of plant polypeptide signaling molecules.

Polypeptide ligands have long been recognized as primary signaling molecules in diverse physiological processes in animal systems. Recent studies in plants have provided major breakthroughs with the discovery that small polypeptides are also involved in many plant biological processes, indicating that the use of polypeptides as signaling molecules in cell-to-cell communication is evolutionarily conserved. The CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) proteins are currently the best understood family of small polypeptides in plants. The recent isolation of MCLV3 from Arabidopsis and TDIF from a Zinnia cell culture system indicates that biologically active CLE polypeptides are produced by post-translational proteolysis and modification, similar to peptide hormone production in animals and yeast. Here, we review exciting discoveries involving the identification of the CLE proteins and their functions in various aspects of plant development, including restriction of stem cell accumulation by CLV3 and inhibition of xylem differentiation by TDIF.

The ULTRAPETALA gene controls shoot and floral meristem size in Arabidopsis.

The regulation of proper shoot and floral meristem size during plant development is mediated by a complex interaction of stem cell promoting and restricting factors. The phenotypic effects of mutations in the ULTRAPETALA gene, which is required to control shoot and floral meristem cell accumulation in Arabidopsis thaliana, are described. ultrapetala flowers contain more floral organs and whorls than wild-type plants, phenotypes that correlate with an increase in floral meristem size preceding organ initiation. ultrapetala plants also produce more floral meristems than wild-type plants, correlating with an increase in inflorescence meristem size without visible fasciation. Expression analysis indicates that ULTRAPETALA controls meristem cell accumulation partly by limiting the domain of CLAVATA1 expression. Genetic studies show that ULTRAPETALA acts independently of ERA1, but has overlapping functions with PERIANTHIA and the CLAVATA signal transduction pathway in controlling shoot and floral meristem size and meristem determinacy. Thus ULTRAPETALA defines a novel locus that restricts meristem cell accumulation in Arabidopsis shoot and floral meristems.

Dependence of stem cell fate in Arabidopsis on a feedback loop regulated by CLV3 activity.

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.

Cell signaling within the shoot meristem.

Shoot apical meristems are self-renewing stem cell populations that generate all of the above-ground organs (i.e. stems, leaves and flowers) of higher plants. Recent studies have identified new molecular components required for proper shoot meristem activity, and they have revealed that complex, intercellular communication pathways play important roles in coordinating meristem function.

Signaling of cell fate decisions by CLAVATA3 in Arabidopsis shoot meristems.

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 long view: how genetic discoveries will aid healthcare reform.

This article addresses social and ethical priorities on the agenda for women's health research in the next century. Specifically, the relevance of genetic discoveries for healthcare reform and why both topics ought to be among these priorities are discussed. If the present gap between diagnosis and treatment of genetic diseases could be bridged, especially in such common diseases as cancer and heart disease, arguments for universal access to healthcare would be vastly strengthened. Important scientific and ethical problems are involved in bridging the gap, including volatile issues of research with human embryos and fetuses. Other issues deserve high priority on the women's health research agenda: adequate funding for biomedical research, maintaining an equitable number of women in clinical trials, and creating a knowledge base in gender-based biology. Progress in genetic research is clearly relevant to women's health for understanding and treating common diseases, as well as for reproductive decisions. Until recently, genetic services have been aimed exclusively at reproductive choices, including counseling, prenatal diagnosis, and carrier screening. However, the meaning of the term needs to be vastly broadened to include the uses of genetic information in mainstream medicine. Through genetic testing and DNA technology, it will be increasingly possible for physicians to have access to family history and a reliable profile of each patient's genetic health risks. Drugs will be designed that will be responsive to individual genetic variability and risks for disease. If scientific and ethical obstacles are overcome, there may also be effective human gene therapy to correct mutations and prevent lifelong harm. In my view, linking the agenda for women's health research with the unfinished social task of ensuring access to adequate primary and preventive care, which foreseeably will include genetic services and human gene therapy, strengthens the agenda and promotes solidarity with the needs of the majority of Americans. This link will help to prevent political and moral isolation of women's health research.

Ethical issues in reproductive genetics.

Because of technological developments in reproductive genetics, couples now have many options not available a decade ago. Some developments are controversial and deeply divide our society. This article examines some of these issues and establishes guidelines of approach.

A genetic screen for modifiers of UFO meristem activity identifies three novel FUSED FLORAL ORGANS genes required for early flower development in Arabidopsis.

In a screen to identify novel genes required for early Arabidopsis flower development, we isolated four independent mutations that enhance the Ufo phenotype toward the production of filamentous structures in place of flowers. The mutants fall into three complementation groups, which we have termed FUSED FLORAL ORGANS (FFO) loci. ffo mutants have specific defects in floral organ separation and/or positioning; thus, the FFO genes identify components of a boundary formation mechanism(s) acting between developing floral organ primordia. FFO1 and FFO3 have specific functions in cauline leaf/stem separation and in first- and third-whorl floral organ separation, with FFO3 likely acting to establish and FFO1 to maintain floral organ boundaries. FFO2 acts at early floral stages to regulate floral organ number and positioning and to control organ separation within and between whorls. Plants doubly mutant for two ffo alleles display additive phenotypes, indicating that the FFO genes may act in separate pathways. Plants doubly mutant for an ffo gene and for ufo, lfy, or clv3 reveal that the FFO genes play roles related to those of UFO and LFY in floral meristem initiation and that FFO2 and FFO3 may act to control cell proliferation late in inflorescence development.

The WIGGUM gene is required for proper regulation of floral meristem size in Arabidopsis.

The study of cell division control within developing tissues is central to understanding the processes of pattern formation. The floral meristem of angiosperms gives rise to floral organs in a particular number and pattern. Despite its critical role, little is known about how cell division is controlled in the floral meristem, and few genes involved have been identified. We describe the phenotypic effects of mutations in WIGGUM, a gene required for control of cell proliferation in the floral and apical meristem of Arabidopsis thaliana. wiggum flowers contain more organs, especially sepals and petals, than found in wild-type flowers. This organ number phenotype correlates with specific size changes in the early floral meristem, preceding organ initiation. Genetic studies suggest that WIGGUM acts on a similar process but in a separate pathway than the CLAVATA1 and CLAVATA3 genes in meristem size regulation, and reveal interactions with other genes affecting meristem structure and identity. Analysis of double mutant phenotypes also reveals a role for WIGGUM in apical meristem function. We propose that WIGGUM plays a role in restricting cell division relative to cellular differentiation in specific regions of the apical and floral meristems.

Ethical and social issues in prenatal sex selection: a survey of geneticists in 37 nations.

In a recent 37-nation survey of 2903 geneticists and genetic counselors, 29% would perform prenatal diagnosis (PND) for a couple with four girls who want a boy and would abort a female fetus. An additional 20% would offer a referral. The percentage who would perform PND in the United States (34%) was exceeded only by Israel (68%), Cuba (62%), Peru (39%), and Mexico (38%). In all, 47% had had requests for sex selection. There appears to be a trend toward honoring such requests since a similar survey in 1985. This paper discusses reasons for this trend and the ethical dilemmas of refusing patient requests in societies where individual autonomy is stressed.

Bioethics in a legal forum: confessions of an "expert" witness.

This article reflects on the author's modest experience as an expert witness in two trials: Osheroff vs. Greenspan (1983), and In the Matter of Baby K (1994). Bioethicists' expertise as scholar-teachers and consultants on particular issues merits qualification by judges as expert witnesses. The article argues that a different kind of expertise-strong moral advocacy-is required to be an effective expert witness. The major lessons of expert witnessing for the author concern the demands and strains on the bioethicist's role as scholar, teacher, and consultant. The Baby K case is analyzed in some detail, due to its importance for bioethics, ethics consultation, and the testimony of bioethicists on either side of the case. Rules of thumb are offered to guide decisions as to choices regarding expert witnessing, as well as a discussion of the interaction of law and bioethics.

A steroid-triggered switch in E74 transcription factor isoforms regulates the timing of secondary-response gene expression.

The steroid hormone 20-hydroxyecdysone (referred to here as ecdysone) directs Drosophila metamorphosis by activating a series of genetic regulatory hierarchies. ETS domain transcription factors encoded by the ecdysone-inducible E74 early gene, E74A and E74B, act at the top of these hierarchies to coordinate the induction of target genes. We have ectopically expressed these E74 isoforms to understand their regulatory functions during the onset of metamorphosis. We show that E74 can regulate its own transcription, most likely through binding sites within its gene. Ectopic expression of E74B can partially repress the E78B and DHR3 orphan receptor genes, suggesting a role for E74 in the appropriate timing of early-late gene expression. Furthermore, E74A is both necessary and sufficient for E78B induction, implicating E74A as a key regulator of E78B expression. We also show, consistent with our studies of E74 loss-of-function mutations, that E74B is a potent repressor of late gene transcription and E74A is sufficient to prematurely induce the L71-1 late gene. However, ectopic expression of both Broad-Complex and E74A activators in an E74B mutant background is not sufficient to prematurely induce all late genes, indicating that other factors contribute to this regulatory circuit. These observations demonstrate that the steroid-triggered switch in E74 transcription factor isoforms plays a central role in the proper timing of secondary-response gene expression.