Mutations in the biosynthesis or signaling pathways of gibberellin (GA) can cause dwarfing phenotypes in plants, and the use of such mutations in plant breeding was a major factor in the success of the Green Revolution. DELLA proteins are GA signaling repressors whose functions are conserved in different plant species. Recent studies show that GA promotes stem growth by causing degradation of DELLA proteins via the ubiquitin-proteasome pathway. The most widely utilized dwarfing alleles in wheat (Triticum aestivum; e.g. Rht-B1b and Rht-D1b) encode GA-resistant forms of a DELLA protein that function as dominant and constitutively active repressors of stem growth. All of the previously identified dominant DELLA repressors from several plant species contain N-terminal mutations. Here we report on a novel dwarf mutant from Brassica rapa (Brrga1-d) that is caused by substitution of a conserved amino acid in the C-terminal domain of a DELLA protein. Brrga1-d, like N-terminal DELLA mutants, retains its repressor function and accumulates to high levels, even in the presence of GA. However, unlike wild-type and N-terminal DELLA mutants, Brrga1-d does not interact with a protein component required for degradation, suggesting that the mutated amino acid causes dwarfism by preventing an interaction needed for its degradation. This novel mutation confers nondeleterious dwarf phenotypes when transferred to Arabidopsis (Arabidopsis thaliana) and oilseed rape (Brassica napus), indicating its potential usefulness in other crop species.Dwarfism is a desirable characteristic for many agricultural plants. In grain crops, dwarfism can reduce lodging and increase harvest index, and the breeding of dwarf wheat (Triticum aestivum) and rice (Oryza sativa) cultivars was a major factor in the success of the Green Revolution (Khush, 2001). Dwarfism is often caused by mutations in genes controlling the biosynthesis or signaling pathway of the plant hormone GA (Peng et al., 1999;Hedden, 2003;Sun and Gubler, 2004). The most widely utilized semi-dwarf wheat cultivars in agriculture contain the Rht-B1b or Rht-D1b allele that encodes a mutant form of a DELLA protein, a GA signaling repressor (Peng et al., 1999;Hedden, 2003).DELLA proteins encoded by Rht and its orthologs in Arabidopsis (Arabidopsis thaliana; GAI, RGA, RGL1, and RGL2), maize (Zea mays; d8), grape (Vitis vinifera; VvGAI), barley (Hordeum vulgare; SLN1), and rice (SLR1) have conserved function as repressors of GA signaling (Sun and Gubler, 2004). Five DELLA protein genes (GAI, RGA, RGL1, RGL2, and RGL3) are present in Arabidopsis, and with the exception of RGL3, these genes have been shown to share partially overlapping roles in repressing GA-regulated plant growth and development . Genetic analysis indicates that RGA and GAI are the major repressors that modulate GA-mediated vegetative growth and floral induction; RGL2 is important in regulating seed germination; and RGL1 and RGL2, along with RGA, control flower development King et al., 2001;Lee et al., 2002;Cheng et al., 2004;Tyle...