Post-embryonic induction of ATML1-SRDX alters the morphology of seedlings

Arabidopsis thaliana MERISTEM LAYER 1 (ATML1), an HD-ZIP class IV homeobox gene, is one of the key regulators promoting epidermal cell differentiation in Arabidopsis thaliana. We recently showed that ATML1 was able to confer an ectopic shoot epidermis cell fate to non-epidermal tissues of seedlings, suggesting that ATML1 is a master regulator of epidermal cell fate. To further assess the roles of ATML1 and its homologs in epidermal cell differentiation, I generated transgenic plants expressing ATML1 fused with a transcriptional repressor sequence (ATML1-SRDX). Estradiol-induced expression of ATML1-SRDX in the seedlings decreased transcript levels of several epidermis-related genes. Moreover, these transgenic plants exhibited phenotypes such as increased permeability to a hydrophilic dye and fusion of leaves and cotyledons, which are reminiscent of epidermis and/or cuticle-deficient mutants. Epidermal cell morphology was severely affected in the strong lines: filamentous protrusions were formed on the surface of the cotyledons. Marker gene analyses showed that these protrusions did not have epidermis, mesophyll, root hair, or trichome cell identity, suggesting that post-embryonic expression of ATML1-SRDX was sufficient to alter cell identity in pre-existing protodermal cells of the cotyledons. Taken together, these results suggest that ATML1 and/or its target genes are not only necessary for the initial specification of epidermal cell fate but also may be necessary for the maintenance of epidermal cells in later stages.