Investigating the role of gibberellin signalling in the response to drought

Lack of water is one of the most important limitations to global crop productivity and is likely to become an increasingly severe problem with the predicted global warming. Even in the UK, parts of the country may experience serious water deficits at certain times of the year. In order to attempt to maintain crop yields in the face of reduced water availability it is important to understand how plants respond to drought and how these responses affect the growth and development of both the vegetative organs that accumulate biomass and the harvested parts of the plant. Such information will guide the breeding of more drought-tolerant varieties. A common response of plants to drought, as to other stresses, is to reduce growth, often leading to poor yields. There is accumulating evidence that this growth reduction is due, at least in part, to lower production of the gibberellin growth hormones or to the ability of the plant to respond to these hormones. Furthermore, plants that have low amounts of active gibberellins are more tolerant of drought. One aim of this project is to determine precisely how drought stress modifies the concentration of gibberellins in plant tissues and/or the manner in which plants perceive the hormones and respond to them. We will focus in particular on enzymes that inactivate gibberellins and thereby lower their concentration. We will also investigate how the action of gibberellins causes plants to be more susceptible to drought. An important question is whether gibberellins act only to regulate growth, i.e. drought resistance is due to the smaller size of the plant, or it has a more direct effect on drought tolerance independent from its action to control growth. The latter scenario would suggest that it may be possible to enhance drought tolerance without compromising growth and yield. We will work with the model plant species Arabidopsis thaliana since we have accumulated substantial genetic and molecular resources with which to study gibberellin biosynthesis and action in this species. However, we believe that the information obtained will be relevant to crop species and will aid in identifying genes that would be targets in breeding for drought tolerance.

Lack of water is one of the most important limitations to global crop productivity. An understanding of the mechanisms by which plants adapt to water deficit is an important first step to identifying suitable targets when breeding to minimise the detrimental effects on yield. Adaptive responses to drought include restriction of growth, which enhances stress tolerance. The gibberellin (GA) signalling pathway has been shown to be a major target for stress responses. A common response to stress is increased expression of GA 2-oxidase (GA2ox) genes, which encode GA-inactivating enzymes. We aim to utilise the extensive genetic, mutant and gene reporter resources we have established for Arabidopsis to define the mechanisms by which GAs mediate drought responses in this species. We will determine the effect of water deficit on expression levels of potential gene targets in the GA-biosynthetic and signal transduction pathways, including all GA2ox genes, and on the accumulation of DELLA proteins. We will use gene reporter lines to determine whether water stress alters the expression domains of GA-biosynthetic genes in specific tissues of roots and shoots. We will use inducible GA-inactivation and mutants with reduced or increased GA biosynthesis or response to determine how changes in GA-signalling alters drought tolerance and the specificity of this response in terms of downstream gene regulation. We will determine whether drought modifies GA signalling through the action of AP2-type transcription factors, as is the case for cold and salt stress. GA signalling could influence drought tolerance through its effect on plant morphology and/or by a more direct mechanism. We will investigate the mechanisms involved by determining the impact of GA signalling on the profile of drought-induced gene expression. We will also compare the drought tolerance of GA-deficient plants with those in which growth is blocked down-stream of DELLA proteins or by independent pathways.

Apart from the plant science academic community, the research would benefit plant breeders, farmers and consumers. As limited water availability is likely to become an ever increasing problem for agriculture, drought tolerance will remain one of the most important targets for plant breeders. Our work aims to identify potential gene targets for use in crop improvement programmes. There appears to be considerable commonality in responses to abiotic stresses in terms of the effects on GA signalling so our work will have broad significance, although drought is its main target. We will work with Arabidopsis since we can make much faster progress with this model species than with crop species and we have already developed substantial resources that we can employ immediately. The strategic aim would be to optimise yields under water deficit by maintaining plant growth without compromising drought tolerance. This might be achieved by several different strategies, depending on the findings from the project. Genes in the GA-biosynthesis and signal transduction pathways that are sensitive to water deficit are potential targets for selection of lines that respond less strongly to drought in terms of growth reduction. As vegetative and reproductive development are under control of different paralogous GA-signalling genes, it may be possible to achieve normal seed development under conditions that impose restrictions to vegetative growth. The mildly dwarfing Rht genes of wheat, for example, enhance grain yields. The project will test the hypothesis that GA signalling can modify drought tolerance independently of its effect on growth. In this case, down-stream signalling components involved in the production of stress-induced tissue damage, through, for example, regulating levels of reactive oxygen species, would be potential targets in breeding for drought tolerance. Once identified, the importance of potential target genes will be tested by modifying their expression in Arabidopsis. Promising results would be incorporated into our wheat research programme, in which there is already a component on stress tolerance. However, we do not yet have the resources and information for wheat that is available to us for Arabidopsis. The information obtained using Arabidopsis would inform our studies with wheat and allow us to target our research more effectively.