SEUSS and SEUSS-LIKE 2 coordinate auxin distribution and KNOXI activity during embryogenesis

In Arabidopsis, SEUSS (SEU) and SEUSS-LIKE 2 (SLK2) are components of the LEUNIG (LUG) repressor complex that coordinates various aspects of post-embryonic development. The complex also plays a critical role during embryogenesis, as seu slk2 double mutants have small, narrow cotyledons and lack a shoot apical meristem (SAM). Here we show that seu slk2 double mutant embryos exhibit delayed cotyledon outgrowth and that this is associated with altered PIN-FORMED1 (PIN1) expression and localisation during the early stages of embryogenesis. These observations suggest that SEU and SLK2 promote the transition to bilateral symmetry by modulating auxin distribution in the embryonic shoot. This study also shows that loss of SAM formation in seu slk2 mutants is associated with reduced expression of the class I KNOX (KNOXI) genes SHOOTMERISTEMLESS (STM), BREVIPEDICELLUS and KNAT2. Furthermore, elevating STM expression in seu slk2 mutant embryos was sufficient to restore SAM formation but not post-embryonic activity, while both SAM formation and activity were rescued when SLK2 expression was restored in either the cotyledons or boundary regions. These results demonstrate that SEU and SLK2 function redundantly to promote embryonic shoot development and likely act through a non-cell autonomous pathway to promote KNOXI activity.