![]() Thus, DELLA also functions as a transactivator in planta. The transactivation activity of mutated SLR1s observed in yeast corresponds well to the severity of dwarfism in rice plants overexpressing mutated SLR1, indicating that SLR1 suppresses plant growth through its transactivation activity. ( 10) have demonstrated that the N-terminal DELLA/TVHYNP motif of the rice DELLA, SLR1, possesses transactivation activity. More recent work has demonstrated that DELLA interacts with the core subunit of chromatin remodeling factor SWI/SNF to increase the transcription of DELLA target genes ( 9). ( 8) have used transcriptome analyses in transgenic plants expressing gain-of-function versions of DELLA to identify DELLA target genes. However, how DELLA regulates downstream gene expression in GA signaling has remained unclear. DELLA is polyubiquitinated by E3 ubiquitin-ligase SCF SLY1/GID2 and finally is degraded through the 26S proteasome. By binding to active GAs, GID1 acquires the ability to interact with DELLA, allowing further interaction with an F box protein, SLEEPY1/GID2. Recent intensive studies revealed how GA is perceived by the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (GID1) and how the perceived GA signal is transmitted to DELLA. GRAS-domain transcription factors have diverse functions in growth and development. DELLA proteins are characterized by a DELLA/TVHYNP motif at the N terminus and a GRAS domain at the C terminus, placing DELLAs within the GRAS family of transcriptional regulators. Among these, DELLA protein is a key player in the regulation of GA responses. ![]() Several protein factors involved in GA signaling have been identified. Gibberellins (GAs) are diterpene phytohormones that regulate many cellular and developmental events such as cell elongation, leaf expansion, flowering, pollen maturation, and the transition from vegetative growth to flowering ( 1– 4). These results suggest that the coregulators DELLA and SC元, using IDDs as transcriptional scaffolds for DNA binding, antagonistically regulate the expression of their downstream targets to control the GA signaling pathway. In this context, DELLA and SC元 interact competitively with IDD proteins to regulate downstream gene expression. In addition, we found that SC元 protein also interacts with IDDs, resulting in the suppression of its target gene expression. Phenotypic analysis of transgenic plants expressing AtIDD3 (one of the 16 IDDs in the Arabidopsis genome) fused with the plant-specific repression domain (SRDX) supported the possibility that AtIDD3 is positively involved in GA signaling. Transient assays using Arabidopsis protoplasts demonstrated that a luciferase reporter controlled by the SC元 promoter was additively transactivated by REPRESSOR of ga1-3 (RGA) and IDDs. Here, using yeast hybrid screenings, we identified five members of INDETERMINATE DOMAIN (IDD) protein family which bind physically to both DELLA and the promoter sequence of the GA-positive regulator SCARECROW-LIKE 3 ( SC元), which previously was characterized as a DELLA direct target gene. However, because DELLA lacks a DNA-binding domain, intermediate protein(s) mediating the DELLA/DNA interaction are believed to be necessary for activating DELLA target genes. Although how DELLA regulates downstream gene expression remains unclear, DELLA has been proposed to function as a transcriptional activator. DELLA protein is a key negative regulator of gibberellin (GA) signaling.
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