Regulation of Auxin Fluxes Required for Phototropic Growth

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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.