Abstract
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.
Highlights
Molecular genetic studies in plants and animals have revealed that cell-type-specific transcription factors play key roles in determining cell fates through regulation of gene expression
Arabidopsis thaliana MERISTEM LAYER 1 (ATML1) belongs to the HD-ZIP class IV homeodomain protein family, and its mRNA is detected in the outermost cell layer from the early stages of development [1,4,5]
To understand the roles of ATML1 and its homologs in epidermal cell differentiation in the seedlings, I generated transgenic plants carrying the estradiol-inducible ATML1 gene fused to the SRDX repressor sequence, which can convert a transcriptional activator into a repressor (ATML1-SRDX; Figure 1A) [8,9]
Summary
Molecular genetic studies in plants and animals have revealed that cell-type-specific transcription factors play key roles in determining cell fates through regulation of gene expression. Mutations in ATML1 and its closest homolog PROTODERMAL FACTOR2 (PDF2) induced the formation of leaves lacking an epidermis, suggesting that they are redundantly required for epidermal cell differentiation in shoots [2]. Our overexpression experiments showed that ATML1 was able to confer an ectopic shoot epidermal cell fate to non-epidermal tissues of seedlings, suggesting that ATML1 functions as a master regulator of epidermal cell differentiation [3]. ATML1 homologs may positively regulate the expression of epidermis-specific genes via binding to the L1 box and thereby promote epidermal cell differentiation. The actual roles of ATML1 homologs in epidermal cell differentiation remain unclear because the effects of multiple loss-of-function mutations in the ATML1 homologs have yet to be examined
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