A Novel Dwarfing Mutation in a Green Revolution Gene from Brassica rapa  

Abstract

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...

Shoot circumnutation and winding movements require gravisensing cells

“…The LHR I-VHIID-LHRII region may function as a DNA-binding domain, analogous to the basic leucine zipper protein-DNA interaction (30), with the LHRs mediating protein-protein interactions and the VHIID motif mediating protein-DNA interactions. Recently, Muangprom et al (31) reported that conserved amino acid sequences near the VHIID motif play an important role in plant size determination; a novel Q-to-R mutation near the VHIID motif in the REPRESSOR OF ga1-3 (RGA) homologue of Brassica rapa results in a gibberellininsensitive dwarf. Moreover, the same mutation in Arabidopsis RGA also confers a dwarf phenotype in transgenic Arabidopsis.…”

Genetic Characterization and Functional Analysis of the GID1 Gibberellin Receptors inArabidopsis

“…By contrast, the DELLA domain is only present in the DELLA proteins and is essential for their degradation in response to GA, although it is not required for F-box protein binding. A number of DELLA gain-of-function mutations have been described; all except one are located in the DELLA regulatory domain (Peng et al, 1997(Peng et al, , 1999Boss and Thomas, 2002;Chandler et al, 2002;Muangprom et al, 2005). Deletions or specific missense mutations of the conserved motifs (DELLA and/or VHYNP) within the DELLA domain render the mutant proteins resistant to GA-induced degradation, leading to a GA-insensitive dwarf phenotype Gubler et al, 2002;Itoh et al, 2002).…”

“…However, the precise role of the DELLA domain in GA-mediated degradation is not known. In Brassica rapa, a novel GRAS domain mutation affects the ability of the encoded DELLA protein to interact with the F-box component of the SCF E3 Ub ligase, resulting in its accumulation and insensitivity to GA (Muangprom et al, 2005).…”

EMS mutagenesis and characterization of Brassica rapa mutants

“…Historically, dwarf genes have been utilized in plant breeding as represented by the success of the "Green Revolution" in rice and wheat (Hedden 2003;Khush 2001). Furthermore, GA 1 -and GA 3 -deficient dwarf mutant rosette (ros; Rood et al 1989) and the semidominant GA-insensitive dwarf mutant caused by a gain-of-function mutation in the C-terminal GAI, RGA, SCARECROW (GRAS) domain of a REPRESSOR OF GA1-3 (RGA) homolog, Brrga1-d (Muangprom et al 2005), have been reported in B. rapa, and are expected to reduce lodging problems of oilseed Brassica cultivars without affecting the harvest.…”