Understanding how DELLA-mediated transcription controls GA-responsive growth and development in Arabidopsis

To survive in their surroundings, it is essential that plants are capable of adapting their growth to suit the environmental conditions to which they are exposed. Plants have evolved many complex pathways that are necessary for recognising, relaying and responding to these environmental signals. The plant hormone gibberellin (GA) functions in one such pathway, controlling growth and development in response to many environmental factors, including light, temperature and stress. The study of mutant plants that are deficient in GA production has confirmed that these hormones act largely by promoting growth. In the absence of GAs, plant growth is prevented through the action of the DELLA proteins, which act as growth repressors. When plants are exposed to specific environmental conditions that favour growth, the GA pathway is triggered through the production of the hormone. The increased levels of GAs promote the rapid removal of the DELLAs, allowing increased plant growth. In recent years, there have been dramatic advances in our understanding of the components that are responsible for the production of the GA hormone, and how specific environmental signals lead to their activation. In addition, the mechanisms and components by which GAs trigger the removal of the DELLA growth repressors are now well understood. However, very little is known about the role of DELLAs as growth repressors and how their absence leads to increased plant growth. It is believed that DELLAs act by regulating the levels of multiple components that are directly responsible for controlling plant growth. However, the identity of these components, and the mechanisms by which DELLAs control their levels are poorly understood. The work outlined in this proposal aims to improve understanding of this part of the GA signalling pathway. The role of the GA pathway in controlling growth has been intensively studied in the model plant Arabidopsis thaliana. This work has demonstrated that this pathway is responsible for stimulating growth throughout the plant, including the roots and hypocotyls. Our recent studies have illustrated the importance of focussing on specific aspects of plant growth, rather than the whole plant, if we are to understand how DELLAs function and identify their downstream targets. The aim of this project is to identify the immediate DELLA targets in Arabidopsis roots and hypocotyls, to determine their role in controlling GA-dependent growth and to understand the mechanisms by which DELLAs control their levels in the plant. This will be achieved by identifying components that act in concert with DELLAs and investigating their function in the GA signalling pathway. Our ultimate aim is to understand how DELLAs control different aspects of GA-dependent growth through a myriad of downstream targets. Considering the demonstrated potential to improve agronomic traits through modification of the GA pathway, it is likely that this knowledge will ultimately lead to an improvement in these traits in major crop plants.

In recent years there have been major advances in our understanding of how the plant hormone, gibberellin (GA) is perceived and this signal transduced, leading to changes in growth and development. The DELLA proteins (DELLAs) are central to this signalling cascade, and act as repressors of GA-responsive growth. GA signalling relieves the repression exerted through DELLAs by targeting their degradation through an ubiquitin-proteasome mediated process, which is well understood. In contrast, our understanding of the role of DELLAs in repressing GA-responsive growth is very limited. Although it is thought that DELLAs act as transcriptional regulators, their mode of action is not clear and no target genes have been identified. The work outlined in this proposal is aimed at improving our understanding of the role of DELLAs and their downstream target genes in regulating GA-responsive growth in Arabidopsis. Our proposed strategy involves using a combination of microarray and chromatin immunoprecipitation experiments to identify early GA-responsive genes that are the primary targets of the DELLAs. We will focus on specific GA-responsive tissues, rather than the whole plant. To uncover the role of these DELLA target genes in regulating GA-responsive growth, we will use a reverse genetics-based strategy. Based on the probable absence of a canonical DNA-binding domain in the DELLAs, it is likely that they regulate the expression of GA-responsive genes through their interaction with other transcription factors. To identify these components we will perform yeast two-hybrid screens using a DELLA bait construct. Putative DELLA interactors will be confirmed by testing the interactions in vivo. Furthermore, their roles in GA signalling will be assessed by analysing the phenotype of knock-out mutants.