Climate change and a growing world population are expected to lead to water scarcity and food shortage in the near future. There is an urgent need to increase yield, water usage efficiency and stress tolerance of food crops. We propose to achieve this through controlled manipulation of plant sensitivity to the 'stress' hormone abscisic acid (ABA). The project builds on our recent discovery of a novel gene from Arabidopsis thaliana, which we called 'Histone de-acetylation complex 1' (HDC1). We found that over-expression of HDC1 led to decreased ABA-sensitivity of germinating seeds and to enhanced growth of mature plants, while deletion of HDC1 had the opposite effects. Thus HDC1 can be used as an adjustable 'hormostat'. This property makes HDC1 an attractive target for crop improvement. For example, in a drought-prone rain-fed field increasing ABA-sensitivity will aid plant recovery after dehydration whereas in an irrigated field decreasing ABA-sensitivity could be a means to sustain biomass production with reduced water input. The question is then; how does HDC1 change ABA-sensitivity? Ancestral precursors of HDC1 in yeast are members of large multi-protein complexes that biochemically modify (de-acetylate) histone proteins that are associated with DNA (chromatin). Histone de-acetylation (HD) determines the overall structure of the DNA which in turn exerts a hyper-level of control over gene activity. Our current hypothesis is that HDC1 'titrates' the stability of a chromatin complex thereby modifying accessibility of the DNA to ABA-dependent regulators and hence ABA-sensitivity. This is an exciting concept because it means that via HDC1 one could gain control over a whole suite of stress responses without the need to tinker with the underlying complex signalling network. However, to exploit the opportunities presented by HDC1 for crop improvement we need to understand exactly how HDC1 operates at the molecular level. For example, the composition of HD complexes and the precise functions of proteins therein are completely unknown in plants. The aim of this project is to investigate the molecular function of HDC1 in the model plant Arabidopsis. This research will run in parallel to a crop development programme carried out by the Industrial Partner. Reciprocal information flow between the two research programmes will ensure that fundamental discoveries made in the model species can immediately be translated into crop improvement. The work programme has three parts. In the first work package we will use an antibody against HDC1 to identify 'by association' other members of the HDC1-complex in plant protein extracts. We will obtain mutant lines for some of the identified associates and cross them with the HDC1mutant lines. This work will lead to a first understanding of HD complexes in plants, and to the identification of proteins that limit or enhance HDC1 function within the complex. The second work package addresses the question whether HDC1 itself is regulated and how. In particular, we will investigate whether HDC1 is a target for 'hijacking' of the ABA pathways by other hormones ('cross talk') or by pathogens. For this purpose we will measure HDC1 protein levels in plant extracts treated with hormones and pathogen elicitors. In the third work package we will investigate which genes cause the effects of HDC1 on seed germination and growth - the 'targets' of HDC1. In the first instance we will identify all genes that are differentially expressed in wildtype and HDC1 mutant plants using gene chips. To identify the DNA regions that are directly targeted by HDc1 we will pull-down HDC1-associated chromatin with the HDC1-antibody. Finally, we will measure acetylation levels of the chromatin with antibodies that recognize acetylated histone tails. The combined outcomes from this work will greatly enhance our understanding of gene regulation in plants and directly contribute to improving yield and water usage efficiency in crops.