Populations of stem cells exist in many organisms. Their most significant attribute is that they replenish themselves as well as give rise to many terminally differentiated cell types. In plants, the meristems or growing tips of roots or shoots are groups of stem cells. Shoot apical meristems in angiosperm plants are formed during embryogenesis and are comprised of a small number of undifferentiated, dividing cells. These cells will give rise to all of the above- ground portions of the plant. Thus, understanding the developmental biology of shoot apical meristems is absolutely fundamental to understanding plant growth and development. How is the shoot apical meristem initiated during embryogenesis? How are additional meristems initiated postembryonically in the axils of leaves? How is a group of undifferentiated, dividing cells maintained in the meristem? How are the positions of organs generated by the meristem determined? The long term goal of this study is to answer these questions in molecular detail. The small cruciferous plant Arabidopsis thaliana has been chosen as an experimental organism -- its short generation time and small genome size make it especially well suited for molecular genetic analysis. The focus of this project is the shoot meristemless (STM) locus. Seedlings homozygous for a loss- of-function mutation in this gene lack a shoot apical meristem but are otherwise normal. Careful morphological examination of homozygous stm-1 mutants has shown that the defect lies in their inability to initiate a shoot apical meristem during embryogenesis. Evidence that the STM gene corresponds to an Arabidopsis homolog of the maize Knotted gene is presented. This gene encodes a homeodomain containing protein and therefore probably acts as a transcriptional regulator. Although homeodomain proteins have been identified in plants previously, this is the first case in which a loss-of-function mutation has been identified in such a gene and hence its wild-type function deduced. To understand how STM promotes shoot apical meristem formation, experiments are proposed that are designed to shed light on how the STM gene is regulated and, in the long term, to identify genes that STM in turn regulates.
