Induction of epidermal cell fate in Arabidopsis shoots

Abstract

Land plants have evolved a cuticle-bearing epidermis to protect themselves from environmental stress and pathogen attack. Despite its important role, little is known about the molecular mechanisms regulating shoot epidermal cell identity. In a recent study, we found that the Arabidopsis thaliana ATML1 gene is possibly a master regulator of shoot epidermal cell fate. We revealed that ATML1 has the ability to confer shoot epidermis-related traits to non-epidermal cells of the seedlings. These data are consistent with the previous loss-of-function mutant analyses, which implied a positive role of ATML1 in epidermal cell differentiation. Importantly, ectopic epidermal cells induced in ATML1-overexpressing lines provide a novel tool to assess the intrinsic properties of epidermal cells and to study epistatic interactions among genes involved in epidermal/mesophyll differentiation. Using this system, we obtained data revealing that ATML1 negatively influenced mesophyll cell fate. In addition, we provided a working model of how division planes in epidermal cells are determined.

Keywords: ATML1; Arabidopsis thaliana; HD-ZIP class IV transcription factor; epidermal cell differentiation; mesophyll cell differentiation.

Figure 1. Overexpression of ATML1 affects epidermal/mesophyll cell differentiation in leaves. (A) Transverse section of a wild-type leaf. Red arrowheads, stomata. Red arrow, trichome. (B-D) Transverse sections of mature leaves from 14-d-old ATML1-overexpressing plants. (B) Green mesophyll cells decreased in the inner tissues of the ATML1-overexpressing leaf. (C) Magnified view of the inset in B. A pair of chloroplast-containing guard cells was observed. (D) A large cell (black arrow), expressing the trichome cell marker GL2-GUS (shown in blue), was found in the inner tissues. This GL2-GUS positive cell was surrounded by small accessory cell-like cells. The section was stained with Safranin O. (E) Our results suggest that ATML1 promotes epidermal differentiation and negatively regulates mesophyll cell differentiation in the outermost cell layers of the leaves. Scale bars, 100 µm in A and B and 20 µm in C and D.