Auxin responsiveness of the MONOPTEROS ‐ BODENLOS module in primary root initiation critically depends on the nuclear import kinetics of the Aux/ IAA inhibitor BODENLOS

Summary Primary root formation in early embryogenesis of Arabidopsis thaliana is initiated with the specification of a single cell called hypophysis. This initial step requires the auxin‐dependent release of the transcription factor MONOPTEROS ( MP , also known as ARF 5) from its inhibition by the Aux/ IAA protein BODENLOS ( BDL , also known as IAA 12). Auxin‐insensitive bdl mutant embryos and mp loss‐of‐function embryos fail to specify the hypophysis, giving rise to rootless seedlings. A suppressor screen of rootless bdl mutant seedlings yielded a mutation in the nuclear import receptor IMPORTIN ‐ ALPHA 6 ( IMP α 6 ) that promoted primary root formation through rescue of the embryonic hypophysis defects, without causing additional phenotypic changes. Aux/ IAA proteins are continually synthesized and degraded, which is essential for rapid transcriptional responses to changing auxin concentrations. Nuclear translocation of bdl:3× GFP was slowed down in imp α 6 mutants as measured by fluorescence recovery after photobleaching ( FRAP ) analysis, which correlated with the reduced inhibition of MP by bdl in transient expression assays in imp α 6 knock‐down protoplasts. The MP – BDL module acts like an auxin‐triggered genetic switch because MP activates its own expression as well as the expression of its inhibitor BDL . Using an established simulation model, we determined that the reduced nuclear translocation rate of BDL in imp α 6 mutant embryos rendered the auxin‐triggered switch unstable, impairing the fast response to changes in auxin concentration. Our results suggest that the instability of the inhibitor BDL necessitates a fast nuclear uptake in order to reach the critical threshold level required for auxin responsiveness of the MP – BDL module in primary root initiation.