The AtMYB103 gene is a member of the R2R3 MYB gene family in Arabidopsis thaliana. Using the GUS reporter gene, AtMYB103 expression was found to be restricted to the tapetum of developing anthers. Employing RT-PCR and in situ hybridisation, we now show that AtMYB103 is also expressed in trichomes. GUS expression in trichomes was obtained by incorporating the coding and 3'-untranslated regions of AtMYB103 into the promoter-GUS constructs. Sense and antisense technologies were used to downregulate AtMYB103 expression. In transgenic lines with reduced AtMYB103 transcript levels, pollen, tapetum and trichome development were altered. The majority of the pollen grains were distorted in shape and had reduced or no cytoplasmic content. Tapetal degeneration occurred early, and large opaque bodies appeared in the tapetal cytoplasm. In transgenic plants, trichomes on cauline and rosette leaves produced additional branches. These overbranched trichomes contained more nuclear DNA than the wild-type trichomes. The results indicate that AtMYB103 is required for tapetal development and microsporogenesis, and negatively regulates trichome endoreduplication linked to the trichome branching.