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Abstract

Even the most complex organism has its origin in a single cell. During development the descendants of this cell adopt diverse fates in response to a variety of endogenous and exogenous factors, giving rise to the overwhelming diversity of life. Plants use the phytohormone auxin as a major patterning factor. Auxin controls asymmetric growth in response to photo- and gravitropic stimuli, suppresses shoot branching and promotes lateral root initiation. Because of the stereotypic cell division pattern in early Arabidopsis thaliana embryogenesis, the role of auxin in embryonic root formation is particularly well understood (Lau et al. 2012). Auxin degrades the Aux/IAA protein BODENLOS (BDL), thereby releasing the AUXIN RESPONSE FACTOR (ARF) MONOPTEROS (MP), and MP in turn initiates primary root formation. To study the regulation of the auxin response mediated by the MP-BDL module, we employed a suppressor screen on the root initiation defect of the bdl mutant which expresses a stabilized version of the BDL inhibitor. This screen resulted in the identification of the nuclear import receptor IMPORTIN ALPHA 6 (IMPα6) as a critical determinant of auxin response. In impα6-1 bdl double mutants the primary root initiation defect of bdl is partially rescued, presumably because of a reduced nuclear uptake of bdl into the nucleus. Incorporation of delayed nuclear import into a previously established computational model auxin-modulated MP-BDL interaction revealed that such a delay can be sufficient to trigger an auxin response in bdl after a short auxin pulse (Herud et al. 2016). Traditionally auxin localization is inferred from the expression of reporter genes under the control of the auxin-inducible promoter DR5. The presence of reporter signal is usually referred to as an auxin-response maximum, and it is assumed that each reporter-signal maximum represents a local auxin maximum. Quantitative measurements indicate that this is not necessarily the case. We employed the similarities between auxin and tryptophan to develop an auxin sensor based on Förster resonance energy transfer (FRET) by semi-rational redesign of an established tryptophan sensor. This sensor enables us to visualize auxin directly and with high temporal and spatial resolution.