Brassinosteroids (BRs) regulate the expression of numerous genes associated with plant development, and require the activity of a Ser/Thr receptor kinase to realize their effects. In animals, the transforming growth factor-beta (TGF-beta) family of peptides acts via Ser/Thr receptor kinases to have a major impact on several pathways involved in animal development and adult homeostasis. TGF-beta receptor-interacting protein (TRIP-1) was previously shown by others to be an intracellular substrate of the TGF-beta type II receptor kinase which plays an important role in TGF-beta signaling. TRIP-1 is a WD-repeat protein that also has a dual role as an essential subunit of the eukaryotic translation initiation factor eIF3 in animals, yeast and plants, thereby revealing a putative link between a developmental signaling pathway and the control of protein translation. In yeast, expression of a TRIP-1 homolog has also been closely associated with cell proliferation and progression through the cell cycle. We report here the novel observation that transcript levels of TRIP-1 homologs in plants are regulated by BR treatment under a variety of conditions, and that transgenic plants expressing antisense TRIP-1 RNA exhibit a broad range of developmental defects, including some that resemble the phenotype of BR-deficient and -insensitive mutants. This correlative evidence suggests that a WD-domain protein with reported dual functions in vertebrates and fungi might mediate some of the molecular mechanisms underlying the regulation of plant growth and development by BRs.