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New fluorescent auxin probes visualize tissue-specific and subcellular distributions of auxin in Arabidopsis.

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 visualize auxin localization 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 characterization of auxin-related biological activity, distribution and stability of the presented compounds in Arabidopsis thaliana. Despite partial metabolization 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 characterized the subcellular localization of the fluorescent auxin analogues as being present in the endoplasmic reticulum and endosomes. Our work provides powerful tools to visualize auxin distribution within different plant tissues at cellular or subcellular levels and in response to internal and environmental stimuli during plant development.