ATML1 activity is restricted to the outermost cells of the embryo through post-transcriptional repressions.

Cell fate determination in plants relies on positional cues. To investigate the position-dependent gene regulation in plants, we focused on shoot epidermal cell specification, which occurs only in the outermost cells. ARABIDOPSIS THALIANA MERISTEM L1 LAYER ( ATML1 ), which encodes an HD-ZIP class IV transcription factor, is a positive regulator of shoot epidermal cell identity. Despite the presence of a weak ATML1 promoter activity in the inner cells, ATML1 protein was detected mostly in the outermost cells, suggesting that ATML1 accumulation is inhibited in the inner cells. ATML1 nuclear localization was reduced in the epidermis and there was a positive, albeit weak, correlation between the amount of ATML1 in the nuclei and the expression of a direct target of ATML1 Nuclear accumulation of ATML1 was more strongly inhibited in the inner cells than in the outermost cells. Domain deletion analyses revealed that the ZLZ sequence was necessary and partially sufficient for the post-transcriptional repression of ATML1 Our results suggest that post-transcriptional repressions contribute to restrict the activity of master transcriptional regulators to specific cells to enable the position-dependent cell differentiation.