Functions of the Whirly 1 protein in chloroplast-nucleus crosstalk

As the world population grows and we are facing a 70% increase of food demand over the next four decades, it is clear that the need to optimize plant productivity is one of the most important issues of our time. Plants use the process called photosynthesis, to harvest light energy and convert it into chemical energy that can be used for growth and to produce storage carbohydrates, and a wide range of other compounds. The photosynthetic processes are housed in sub-cellular compartments called chloroplasts. Unlike mobile animals, sessile plants do not have the capacity to move away from adverse environmental conditions and have therefore evolved an extensive capacity to recognise and respond to environmental changes. Chloroplasts are major sensors of such changes, particularly the availability and quality of light and they this transmit information to the nucleus in order regulate gene expression, so that appropriate adjustments can be made to optimise photosynthetic processes over a wide range of conditions. Chloroplast-to-nucleus signalling pathways not only allow the continuous acclimation of photosynthesis but also the coordinated regulation of photosynthesis, growth and defence functions. While the phenomenon of chloroplast to nucleus signalling has been known for over 30 years, very little is known about the mechanisms involved or the different pathway components. During the process of photosynthesis light energy is used to split water into its component parts of hydrogen and oxygen with the release of electrons which are then passed down the photosynthetic electron transport (PET) chain to generate the power required to fix CO2. A photosynthetic component that gains electrons is described as reduced while one that loses electrons is described as oxidised and together these concepts describe the reduction/oxidation (redox) reduction/oxidation (redox) status of photosynthetic components. Current concepts suggest that monitoring of the redox status of the PET chain is an important sensing/signalling mechanism but we lack a clear understanding of how this mechanism is used to transmit information to the nucleus. Plants possess a small family of DNA binding proteins, called WHIRLY. In the model plant Arabidopsis thaliana, two WHIRLY proteins (WHIRLY1 and 3) are targeted to chloroplasts, but WHIRLY1 is also found in the nucleus. The crop plant barley also has a WHIRLY1 protein that is located in chloroplasts and the nucleus of the same cells. This project will test the hypothesis that WHIRLY1 is involved in chloroplast to nucleus signalling, and specifically that it is regulated by chloroplast redox signals, particularly the ones that are generated by photosynthetic electron transport pathways, in such a way as to alter both the location of WHIRLY1 and its DNA binding properties and thus regulate gene expression. The vision of these studies is to deliver an improved understanding of the sensor role of chloroplasts, particularly the central role of WHIRLY1 in the regulation of gene expression, together with a deeper knowledge of how this integration can be used to maximize plant productivity under optimal and stress conditions.

Chloroplasts are the major sensors of changes in environmental conditions, a function that is intrinsically linked to the regulation of photosynthesis. The photosynthetic electron transport chain uses redox signals as direct and dynamic means of regulating metabolism and gene expression both in chloroplasts and the nucleus. While literature evidence supports the concept that information is transmitted from the chloroplast to the nucleus in retrograde signalling pathways for appropriate regulation of gene expression, the mechanisms involved remain poorly characterised. WHIRLY1 belongs to a small family of single stranded DNA binding proteins. In barley, WHIRLY1 is located in chloroplasts and in the nucleus of the same cell. Moreover, WHIRLY1 is can move from the chloroplasts to the nucleus, where it enhanced the expression of pathogen response (PR) genes. In the chloroplasts WHIRLY1 is located at the boundary between the thylakoids and the nucleoids. We will test the hypothesis that redox regulation of its binding properties links the operation of photosynthetic electron transport chain to gene expression and that WHIRLY1 is a novel coordinator for the expression of photosynthetic genes in the nucleus and chloroplasts. We will therefore determine how oligomerization of the WHIRLY1 protein is regulated by the regulated functions of the photosynthetic electron transport chain. The role of chloroplast glutathione peroxidases in this regulation will also be explored. We will characterize the mechanisms by which WHIRLY1 co-ordinates photosynthetic gene expression in the chloroplast and nuclei, together with effects on leaf structure and chloroplast development and the effects of epigenetic modifications (DNA methylation and histone modifications) on these processes. These studies will be undertaken in key crop plant barley that is currently the fourth most important cereal worldwide, as well as in the model plant, Arabidopsis thaliana.

A. Science. The outcomes of the proposed research will have a direct and influential impact on a wide range of scientific investigations in plants. 1) Our discovery of novel functions of WHIRLY1 and redox-dependent chloroplast to nucleus signalling pathways in plants will increase the interest and focus of scientists studying a wide range of biological processes that may be regulated by redox processes. 2), by identifying WHIRLY1 as a new protein that influences leaf development and a mechanism that may integrate gene expression in the chloroplasts and nucleus to underpin cross-tolerance to biotic and abiotic stresses, the proposed work will promote new research aimed at generating a step-change in knowledge that will shed new light on what is currently a very poorly understood phenomenon.
B. Industry. The indication of WHIRLY1 function and the associated mechanisms of gene regulation that we aim to discover in barley have significant potential to help increase crop yield. The current project will provide the foundations and scope for preparing a new patent, for example, by identifying how WHIRLY1 controls chloroplast development and senescence and hence controls crop productivity.
C. Producers and Consumers. Although the impact of this proposed work downstream of the plant biotech sector remains uncertain and speculative until the crop assessments are completed, it is worthwhile noting that the outputs of this project are well aligned with the interests of many agri-biotech industries such as Bayer, BASF and Monsanto alliance, leading to a global impact on food security. Making crops with higher yield is a key priority for these industries, so rapid progress can be expected if their assessment is positive.
D. Researcher. This project provides outstanding opportunities for the researcher in terms of a very promising and productive project, training in biochemistry, cell biology and molecular biology in model and crop species, along with transferable skills development and working with scientists both in the University of Leeds and the James Hutton Institute in collaboration with researchers at the University of Kiel, Germany . The impacts include enhanced career opportunities, increasing the skills base of the UK, and preparation for possible career in industry.
E. James Hutton Institute (JHI). This project builds on background work done in collaboration with Dr rob Hancock at the James Hutton Institute, which has significant expertise in barley genomics and metabolite profiling as well as barley transformation. The project will have a significant impact on JHI knowledge transfer activities because of the strategic relevance of the work, and through publication in open access and high profile journals, adding to JHI scientific standing.
F. BBSRC and policy makers. The project, through its impact plan, directly supports BBSRC and BIS strategic priorities in food security and sustainability by creating new knowledge to increase crop yields.