A matter of time: auxin signaling dynamics and the regulation of auxin responses during plant development.

As auxin is a major regulator of plant development, studying the signaling mechanisms by which auxin influences cellular activities is of primary importance. In this review, we describe current knowledge on the different modalities of signaling, from the well-characterized canonical nuclear auxin pathway, to the more recently discovered or re-discovered non-canonical modes of auxin signaling. In particular, we discuss how both the modularity of the nuclear auxin pathway and the dynamic regulation of its core components allow specific transcriptomic responses to be triggered. We highlight the fact that the diversity of modes of auxin signaling allows for a wide range of time scales of auxin responses, from second-scale cytoplasmic responses to minute-/hour-scale modifications of gene expression. Finally, we question the extent to which the temporality of auxin signaling and responses contributes to development in both the shoot and the root meristems. We conclude by stressing the fact that future investigations should allow an integrative view to be built not only of the spatial control, but also of the temporality of auxin-mediated regulation of plant development, from the cell to the whole organism.