Regulation of Auxin Fluxes Required for Phototropic Growth

Plants depend on sunlight for photosynthesis and adapt their growth to optimise light capture. Phototropism, the reorientation of growth towards light, represents one of these important adaptive responses. Modern-day studies of phototropism began with experiments in monocotyledonous grasses by Charles Darwin and led ultimately to the discovery of the plant growth hormone auxin, establishing the concept that light perception at the shoot apex triggers differential bending in the tissues below. In the past two decades, molecular-genetic analysis in the model flowering plant Arabidopsis thaliana has identified the principle photoreceptor for phototropism, phot1, as well as the major auxin transporters. Despite extensive efforts, however, how the photoreceptor regulates auxin transport so as to establish differential growth is poorly understood, as is whether this process is conserved between monocots and dicots. Therefore, the key aim of this proposal is to identify and characterise the signalling events associated with establishing auxin fluxes required for phototropic growth. By adopting a revised approach to study phototropism in Arabidopsis in the absence of developmental events that are typically associated with seedling photomorphogenesis, we have shown that the proximity of light perception and differential growth is conserved between monocots and dicots: in both plant types, differential growth is a consequence of lateral auxin movements across the shoot apex. Moreover, we have identified two auxin transporter proteins, PIN-FORMED (PIN3) and ATP-BINDING CASSETTE B19 (ABCB19), that contribute to these movements, the latter of which is directly regulated by phot1 and serves to prime lateral auxin fluxes in the shoot apex. These findings uncover new mechanistic information of the events coupling photoreception and auxin signalling, two processes that are critical for shaping plant growth. Hence, this proposal is focused on further characterising the molecular processes involved in establishing phototropic growth. Given the fundamental importance of both light and auxin in controlling plant development, these insights should ultimately provide new strategies to manipulate plant growth for agronomic gain.

It is well accepted that lateral redistribution of the phytohormone auxin underlies the bending of plant organs towards light. In monocots, photoreception occurs at the shoot tip above the region of differential growth. Despite more than a century of research, it is still unresolved how light regulates auxin distribution and where this occurs in dicots. We have established a system in Arabidopsis thaliana to study hypocotyl phototropism in the absence of developmental events associated with seedling photomorphogenesis. By doing so, we have found that auxin redistribution to the epidermal sites of action occurs at and above the hypocotyl apex, not at the elongation zone. Within this region, we have identified the auxin efflux transporter ATP-BINDING CASSETTE B19 (ABCB19) as a substrate target for the photoreceptor kinase PHOTOTROPIN 1 (phot1). Phosphorylation of ABCB19 by phot1 inhibits its efflux activity, thereby increasing auxin levels in and above the hypocotyl apex to halt vertical growth and prime lateral fluxes that are subsequently channeled to the elongation zone by PIN-FORMED 3 (PIN3). Together, these findings provide new insights into the roles of ABCB19 and PIN3 in establishing phototropic curvatures and demonstrate that the proximity of light perception and differential phototropic growth is conserved in angiosperms. The experiments outlined in this proposal are therefore aimed at further understanding how phot1 regulates auxin transport through ABCB19 and PIN3. In addition, cell biological and proteomic strategies will be employed to identify the role of additional targets of phot1 action that contribute to auxin movements required for phototropism in dicots and monocots. Together, this work will provide significant advances in our mechanistic understanding of how light and auxin signals are coordinated to shape the developmental plasticity of plants.

Beneficiaries
Beneficiaries of the research include: academic scientists interested in the effects of light and hormone signalling on plant development, synthetic biologists interested in the design of artificial photoreceptor systems, commercial organisations interested in developing new strategies to improve crop production, companies interested in the design of specialised light facilities for the growth of glasshouse crops, individuals (text books for teaching), and organisations involved in science communication to schools and to the wider public (e.g. Glasgow Science Centre). Phototropism research will also benefit the general public who can relate to the work of Charles Darwin and its impact on science and evolution.

Benefits
The impact of the research is derived from its relevance to understanding how light and hormone signalling is co-coordinately regulated to control plant growth, as well as its potential relevance to crop improvement for agronomic gain. Light and auxin are both critical for plant growth and development. Light signalling mediated by phototropin blue light receptors is of significant importance, as they function to regulate numerous adaptive responses (e.g. phototropism, solar tracking, leaf expansion, chloroplast positioning, regulation of water loss) that serve to optimise photosynthetic efficiency to promote growth. Phototropism, for instance, imparts adaptive significance under field conditions and enhances fitness under drought through increases in root growth efficiency close to the soil surface. Thus, research on phototropin function has potential to generate novel strategies to increase the efficiency of photosynthesis that will potentially provide solutions for the food, energy, and environmental challenges of the future. Phototropin signalling pathways are also known to prime plant defence responses and potentiate pathogen resistance. In addition, functional knowledge of phototropin function will facilitate advances in synthetic biology aimed at regulating enzymatic activities by light. Understanding how phototropins regulate auxin transport is also highly relevant to manipulating plant stature, a major objective in the production of agricultural and horticultural crops. For many species, height control is essential to optimise their establishment and promote efficient handling. Reduced levels of the auxin transporter ABCB19 causes dwarfing in maize and sorgham and, as a consequence, improves crop yield. A greater understanding of auxin transport regulation therefore has considerable potential to create new avenues for crop improvement that could benefit farmers, consumers and the environment and contribute to the economic competitiveness of the UK. Such studies will also have impacts on medical research. Related proteins in humans are responsible for drug resistance in tumours. Understanding ABCB function in plants and how these proteins are regulated biochemically could therefore have implications for drug resistance, cancer research and human health. The staff assigned to the project will obtain knowledge and expertise that can be applied in related research fields or more widely in the commercial or public sectors.

Activities
The proposed project will be continually managed by the PI to engage potential beneficiaries. The PI will publish the research in high-impact scientific journals, write reviews and book chapters and inform the University Media Relations Office of research highlights. Discussions with relevant commercial organisations will be initiated when appropriate to promote exploitation. The PI will communicate the research to school and university students via visits and University open days, initiate discussions with the Glasgow Science Centre, and present lectures at national and international conferences, as well as within Universities throughout the UK. The PI's web site will be routinely updated to communicate the research to the general public.