Different auxin response machineries control distinct cell fates in the early plant embryo.

The cell types of the plant root are first specified early during embryogenesis and are maintained throughout plant life. Auxin plays an essential role in embryonic root initiation, in part through the action of the ARF5/MP transcription factor and its auxin-labile inhibitor IAA12/BDL. MP and BDL function in embryonic cells but promote auxin transport to adjacent extraembryonic suspensor cells, including the quiescent center precursor (hypophysis). Here we show that a cell-autonomous auxin response within this cell is required for root meristem initiation. ARF9 and redundant ARFs, and their inhibitor IAA10, act in suspensor cells to mediate hypophysis specification and, surprisingly, also to prevent transformation to embryo identity. ARF misexpression, and analysis of the short suspensor mutant, demonstrates that lineage-specific expression of these ARFs is required for normal embryo development. These results imply the existence of a prepattern for a cell-type-specific auxin response that underlies the auxin-dependent specification of embryonic cell types.