The plant growth hormone gibberellin (GA) plays a key role in seed germination, leaf expansion, stem elongation and flower and fruit development. Altered GA production or responses in cereal crops, e.g., the dwarf cultivars of wheat and rice, greatly increased the grain yields during the "Green Revolution" in the 1960s-1970s. These examples illustrate the importance of this hormone in regulating plant development and in agriculture. GA-promoted plant development is modulated by DELLA proteins, which are master growth repressors that integrate multiple cellular pathways in response to environmental cues (such as light conditions, pathogens, cold and drought stresses). Although DELLA plays such a central role in regulating diverse plant growth processes, very little is known about how DELLA controls multiple pathways and how DELLA activity is dynamically regulated. This project will focus on these two important aspects and elucidate the molecular mechanisms involved. This research will facilitate more effective strategies to manipulate plant growth and stress responses for agricultural improvements since DELLAs are highly conserved in plants. This research project will also have a broad impact in training young scientists, including a postdoctoral fellow, a graduate student and four undergraduate students.
Recent studies indicate that DELLAs function as central regulators of multiple signaling pathways by direct protein-protein interactions with key transcription factors (TFs). DELLAs do not contain a DNA-binding domain, and are likely to repress expression of target genes by sequestering their interacting TFs. DELLAs can also activate transcription by binding to specific TFs. However, the molecular mechanism of how DELLAs regulate target chromatin and gene expression remains unclear. Preliminary results from the principal investigator's laboratory showed that two missense mutations in a specific subdomain of RGA (an Arabidopsis thaliana DELLA) did not affect interaction with TFs. Instead, they were impaired in binding histone and an epigenetic regulator. These results raise an exciting possibility that histone binding may enhance DELLA association with the target chromatin, and DELLA may recruit epigenetic regulators to induce chromatin structural changes. This working model will be tested in Specific Aim 1. The roles of different subdomains in RGA will be defined by analyzing the effects of additional missense rga alleles on binding to TFs, histone, and epigenetic regulators using yeast two-hybrid and pulldown assays, and association with target chromatin by ChIP-qPCR. DELLA activity needs to be strictly modulated in response to internal and external cues, and it is known that GA induces DELLA degradation. Recent studies also indicated that DELLA activity is dynamically modulated by post-translational modifications. Preliminary data indicate that DELLA phosphorylation is altered under GA-deficient conditions, although the role of this modification remains unclear. Specific Aim 2 will identify phosphosites in RGA by MS analysis, and characterize functions of potential protein kinases for these modifications by genetic and biochemical analyses. This research project will shed new light on the molecular mechanism of how DELLAs regulate target chromatin and gene expression, and elucidate the role of phosphorylation in fine-tuning DELLA activity.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
