Before auxin distribution matters: Prepatterning of the Arabidopsis embryo axis by MPK6-dependent AUX/IAA8 phosphorylation
In growth and development of plants, the signaling molecule auxin plays an instrumental role. Many patterning processes are controlled by auxin signaling in a self-organizing matter. After fertilization, such a patterning process needs to be started for the first time during plant life. Contrary to previous models, we show that the first differential auxin response that separates embryonic from extra-embryonic development in the two zygotic daughter cells happens independently of a detectable auxin gradient. Instead, differential phosphorylation of the transcriptional inhibitor Aux/IAA8 by MAP kinase signaling leads to differences in auxin sensitivity between the two daughter cells, initiating embryonic development in the apical cell. IAA8 phosphorylation in the basal cell prevents proteasomal degradation, suppressing canonical auxin responses, which initiates extra-embryonic development. MAP kinase signaling therefore precedes auxin gradients and prepatterns the apical-basal axis independently of an auxin gradient.