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The WUSCHEL (WUS) gene is necessary for the maintenance of stem cells in the shoot apical meristem. Four recent reports show that cytokinin responsive type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) directly activate WUS expression, providing a long-awaited explanation for how cytokinin influences the maintenance of the stem cell niche.

Organogenesis entails the regulation of cell division, cell expansion, cell and tissue type differentiation, and patterning of the organ as a whole. Petals are ideally suited to dissecting these processes. Petals are dispensable for growth and reproduction, enabling varied manipulations to be carried out with ease. In Arabidopsis, petals have a simple laminar structure with a small number of cell types, facilitating the analysis of organogenesis. This review summarizes recent studies that have illuminated some of the complex interplay between the genetic pathways controlling petal specification, growth and differentiation in Arabidopsis. These advances, coupled with the advantages of using petals as a model experimental system, provide an excellent platform to investigate the underlying mechanisms driving plant organogenesis.

The genomic approach to understanding how evolution has generated the extraordinary diversity of flowers is to assemble a floral EST database for several missing-link taxa and then use gene phylogenies and expression data to identify genes that are important in flower evolution. However, such a genomic approach is likely to miss important genes that are not members of gene families that control flower development in Arabidopsis , and can overlook genes that are not expressed, or are weakly expressed, at their site of action. Therefore we propose complementary genetic approaches in which a few phylogenetically well distributed species are developed as model systems and floral differentiation among closely related species is studied using functional approaches.

The controlled ablation of cells is an effective technique with which to study and manipulate plant development. Traditional microsurgical techniques have been used to remove plant cells with varying degrees of success, but modern methods using either lasers or genetic ablation are proving to be easier to control. Lasers can be used to remove accessible single cells or small groups of cells, and genetic ablation — employing a tissue-specific promoter to control the expression of a cytotoxic gene — is effective for killing a specific set of cells. Ablation techniques have been instrumental in investigating cell—cell interactions, as well as for more applied research, such as on the control of fertility and disease resistance.