In a world that will rely increasingly on efficient plant growth for sufficient food, it is important to learn about natural mechanisms of phytohormone action. In this work, the introduction of a fluorophore to an auxin molecule represents a sensitive and non-invasive method to directly visualise auxin localisation with high spatiotemporal resolution. The state-of-the-art multidisciplinary approaches of genetic and chemical biology analysis together with live cell imaging, liquid chromatography-mass spectrometry (LC-MS) and surface plasmon resonance (SPR) methods were employed for the characterisation of auxin-related biological activity, distribution and stability of the presented compounds in Arabidopsis thaliana. Despite partial metabolisation in vivo, these fluorescent auxins display an uneven and dynamic distribution leading to the formation of fluorescence maxima in tissues known to concentrate natural auxin, such as the concave side of the apical hook. Importantly, their distribution is altered in response to different exogenous stimuli in both roots and shoots. Moreover, we characterised the subcellular localisation of the fluorescent auxin analogues as being present in the endoplasmic reticulum and endosomes. Our work provides powerful tools to visualise auxin distribution within different plant tissues at cellular or subcellular levels and in response to internal and environmental stimuli during plant development.

Centre for Plant Sciences University of Leeds Leeds LS2 9JT UK

Department of Chemical Biology Faculty of Science Palacký University Šlechtitelů 27 Olomouc CZ 78371 Czech Republic

Department of Forest Genetics and Plant Physiology Umeå Plant Science Centre Swedish University of Agricultural Sciences Umeå SE 90183 Sweden

Laboratory of Growth Regulators Faculty of Science Palacký University and Institute of Experimental Botany The Czech Academy of Sciences Šlechtitelů 27 Olomouc CZ 78371 Czech Republic

School of Life Sciences The University of Warwick Coventry CV4 7AL UK

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On the trail of auxin: Reporters and sensors

Endocytic trafficking promotes vacuolar enlargements for fast cell expansion rates in plants