The diterpenoid phytohormone gibberellin (GA) controls diverse developmental processes throughout the plant life cycle. DELLA proteins are master growth repressors that function immediately downstream of the GA receptor to inhibit GA signaling. By doing so, DELLAs also play pivotal roles as integrators of internal developmental signals from multiple hormone pathways and external cues. DELLAs are likely nuclear transcriptional regulators, which interact with other transcription factors to modulate expression of GA-responsive genes. DELLAs are also involved in maintaining GA homeostasis through feedback up-regulating expression of GA biosynthesis and receptor genes. However, the molecular mechanisms by which DELLAs restrict growth and development are largely unknown. This study reveals an important step of the mechanism. Previous microarray studies identified SCARECROW-LIKE 3 (SCL3) as a direct target gene of DELLA in Arabidopsis seedlings. SCL3 expression is induced by DELLA and repressed by GA. Unexpectedly, a scl3 null mutant displays reduced GA responses and elevated expression of GA biosynthesis genes during seed germination and seedling growth, indicating that SCL3 functions as a positive regulator of GA signaling. SCL3 seems to act as an attenuator of DELLA proteins. Transient expression, ChIP, and co-IP studies show that SCL3 autoregulates its own transcription by directly interacting with DELLA. Our data further show that SCL3 and DELLA antagonize each other in controlling both downstream GA responses and upstream GA biosynthetic genes. This work is beginning to shed light on how this complex regulatory network achieves GA homeostasis and controls GA-mediated growth and development in the plant.gibberellin-regulated development | hormone homeostasis | SCARECROW-LIKE 3-DELLA interaction | DELLA attenuator B ioactive gibberellins (GAs) are a class of phytohormones that plays critical roles in modulating plant growth and development in response to internal developmental programs and environmental cues (1-4). DELLA proteins are likely nuclear transcriptional regulators that function as master growth repressors by inhibiting all aspects of GA responses (1, 5, 6). Binding of GA to its receptor GA INSENSITIVE DWARF1 (GID1) enhances the GID1-DELLA interaction, which, in turn, leads to the rapid proteolysis of DELLA through the ubiquitin-proteasome pathway and allows transcriptional reprogramming of GA-responsive genes (7-11). A specific ubiquitin E3 ligase SCF SLY1/GID2 (Skp1-Cullin-Fbox protein complex) is responsible for recruiting DELLA for polyubiquitination (12)(13)(14)(15). DELLA proteins belong to a subfamily of the plant-specific GRAS family [for GA INSENSITIVE (GAI), REPRESSOR OF gal-3 (RGA), and SCARECROW (SCR)] of regulatory proteins (5, 16). In addition to the C-terminal GRAS domain that is common in all GRAS family members, DELLA protein also contains a unique DELLA domain in its N terminus that is required for GID1 binding and GA-induced degradation (7,(17)(18)(19)(20). Arabidopsis contains five DEL...

During plant development, because no cell movement takes place, control of the timing and extent of cell division and coordination of the direction and extent of cell expansion are particularly important for growth and development. The plant hormone gibberellins (GAs) play key roles in the control of these developmental processes. However, little is known about the molecular components that integrate the generic GA signaling into a specific cell/tissue to coordinate cell division and cell expansion. Here we report that SCARECROW-LIKE 3 (SCL3), a GRAS protein, acts as a positive regulator to integrate and maintain a functional GA pathway by attenuating the DELLA repressors in the root endodermis. The tissue-specific maintenance of GA signaling in the root endodermis plays distinct roles along the longitudinal root axis. While in the elongation/differentiation zone (EDZ), the endodermis-confined GA pathway by SCL3 controls primarily coordination of root cell elongation; in the meristem zone (MZ) SCL3 in conjunction with the SHORT-ROOT/SCARECROW (SHR/SCR) pathway controls GA-modulated ground tissue maturation. Our findings highlight the regulatory network of the GRAS transcription regulators (SCL3, DELLAs, and SHR/SCR) in the root endodermis, shedding light on how GA homeostasis is achieved and how the maintenance of GA signaling controls developmental processes in roots.

GRAS proteins belong to a plant-specific transcription factor family. Currently, 33 GRAS members including a putative expressed pseudogene have been identified in the Arabidopsis genome. With a reverse genetic approach, we have constructed a ''phenome-ready unimutant collection'' of the GRAS genes in Arabidopsis thaliana. Of this collection, we focused on loss-of-function mutations in 23 novel GRAS members. Under standard conditions, homozygous mutants have no obvious morphological phenotypes compared with those of wild-type plants. Expression analysis of GRAS genes using quantitative realtime RT-PCR (qRT-PCR), microarray data mining, and promoter::GUS reporter fusions revealed their tissuespecific expression patterns. Our analysis of protein-protein interaction and subcellular localization of individual GRAS members indicated their roles as transcription regulators. In our yeast two-hybrid (Y2H) assay, we confirmed the protein-protein interaction between SHORT-ROOT (SHR) and SCARECROW (SCR). Furthermore, we identified a new SHR-interacting protein, SCARECROW-LIKE23 (SCL23), which is the most closely related to SCR. Our large-scale analysis provides a comprehensive evaluation on the Arabidopsis GRAS members, and also our phenome-ready unimutant collection will be a useful resource to better understand individual GRAS proteins that play diverse roles in plant growth and development.

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