Showing papers by "Detlef Weigel published in 1996"

Sizing Up the Floral Meristem.

Abstract: About 200 years ago, the poet and naturalist Goethe (1790) suggested that flowers were modified shoots and floral organs were modified leaves. In 1991, this assertion was spectacularly confirmed by Bowman et al., who showed that they could transform the sepals, petals, stamens, and carpels of Arabidopsis flowers into leaf-like organs simply by eliminating a set of three floral regulatory genes, AGAMOUS (AG), PISTILLATA (PI), and APETALA2 (AP2). At first it might seem surprising that flowers and shoots are homologous structures, especially when one thinks of such extremes as the tiny flowers of duckweed, barely visible to the naked eye, and the trunk of a mature tree, which can be tens or even hundreds of feet tall. It becomes a little less mysterious if one looks at the earliest stages of development, when flowers or shoots start to arise. Both types of structures are formed from collections of stem cells, termed meristems, which have an organization that is very similar for both flowers and shoots, with at least three domains that can be distinguished by histological and experimental criteria. The central zone, in which cells remain undifferentiated, serves as a stem cell pool for the renewal of the meristem. Surrounding this is the peripheral zone, where new organs are initiated. Underlying the central zone is the rib meristem, which forms the bulk of the interior tissue (Fig. 1). Starting with this common structure, shoot meristems produce leaves with associated axillary shoots and flowers, whereas flower meristems give rise to floral organs such as sepals, petals, stamens, and carpels. Apart from the differences in the types of organs they produce, shoot and flower meristems often differ in phyllotaxis, which is the pattern with which these organs are produced, and in determinacy, which is whether a meristem is consumed in the production of a terminal structure. Given the similarities between flower and shoot meristems, as well as the obvious differences in their subsequent development, one would expect to find at least two classes of genes to be active in early shoot and flower meristems: one class that controls the structural features common to both types of meristems and another class that discriminates between the two types. Both classes of genes have been identified: genes in the first class are called meristem-structure genes and members of the second class are commonly known as meristem-identity genes. Genes in the latter category in turn regulate other factors that elaborate the species-specific differences between shoot and flower meristems. We review here recent progress made in the understanding of these different classes of genes.