Specification of epidermal cell fate in plant shoots

Abstract

Land plants have evolved a single layer of epidermal cells, which are characterized by mostly anticlinal cell division patterns, formation of a waterproof coat called cuticle, and unique cell types such as stomatal guard cells and trichomes. The shoot epidermis plays important roles not only to protect plants from dehydration and pathogens but also to ensure their proper organogenesis and growth control. Extensive molecular genetic studies in Arabidopsis and maize have identified a number of genes that are required for epidermal cell differentiation. However, the mechanism that specifies shoot epidermal cell fate during plant organogenesis remains largely unknown. Particularly, little is known regarding positional information that should restrict epidermal cell fate to the outermost cell layer of the developing organs. Recent studies suggested that certain members of the HD-ZIP class IV homeobox genes are possible master regulators of shoot epidermal cell fate. Here, we summarize the roles of the regulatory genes that are involved in epidermal cell fate specification and discuss the possible mechanisms that limit the expression and/or activity of the master transcriptional regulators to the outermost cell layer in plant shoots.

Keywords: Arabidopsis thaliana; HD-ZIP class IV transcription factor; calpain-like cysteine protease; cuticle; endosperm; epidermal cell differentiation; positional signal; receptor-like kinase.

Figure 1

Regulation of ATML1 expression in the embryos and shoot apices. ATML1 promoter activity visualized using a nuclear-localized GFP reporter in the developing embryos (A–C) and the shoot apices (D,E). Two-cell stage (A), early globular stage (B), and heart stage (C) embryos are shown. GFP expression is restricted to the outermost cell layer after the 16-cell stage. Epidermis-specific expression is observed in vegetative (D) and inflorescence (E) shoot apices. (F) Genetic interactions among genes involved in epidermal cell fate specification during Arabidopsis thaliana embryogenesis. Arrows indicate positive interactions. The question mark indicates currently unknown components. For details, please see the main text. Scale bars, 20 μm in (A–C); 100 μm in (D,E).