[show abstract][hide abstract] ABSTRACT: The MADS-domain transcription factor APETALA1 (AP1) is a key regulator of Arabidopsis flower development. To understand the molecular mechanisms underlying AP1 function, we identified its target genes during floral initiation using a combination of gene expression profiling and genome-wide binding studies. Many of its targets encode transcriptional regulators, including known floral repressors. The latter genes are down-regulated by AP1, suggesting that it initiates floral development by abrogating the inhibitory effects of these genes. Although AP1 acts predominantly as a transcriptional repressor during the earliest stages of flower development, at more advanced stages it also activates regulatory genes required for floral organ formation, indicating a dynamic mode of action. Our results further imply that AP1 orchestrates floral initiation by integrating growth, patterning, and hormonal pathways.
[show abstract][hide abstract] ABSTRACT: To obtain detailed information about gene expression during stamen development in Arabidopsis (Arabidopsis thaliana), we compared, by microarray analysis, the gene expression profile of wild-type inflorescences to those of the floral mutants apetala3, sporocyteless/nozzle, and male sterile1 (ms1), in which different aspects of stamen formation are disrupted. These experiments led to the identification of groups of genes with predicted expression at early, intermediate, and late stages of stamen development. Validation experiments using in situ hybridization confirmed the predicted expression patterns. Additional experiments aimed at characterizing gene expression specifically during microspore formation. To this end, we compared the gene expression profiles of wild-type flowers of distinct developmental stages to those of the ms1 mutant. Computational analysis of the datasets derived from this experiment led to the identification of genes that are likely involved in the control of key developmental processes during microsporogenesis. We also identified a large number of genes whose expression is prolonged in ms1 mutant flowers compared to the wild type. This result suggests that MS1, which encodes a putative transcriptional regulator, is involved in the stage-specific repression of these genes. Lastly, we applied reverse genetics to characterize several of the genes identified in the microarray experiments and uncovered novel regulators of microsporogenesis, including the transcription factor MYB99 and a putative phosphatidylinositol 4-kinase.