Plant epidermis-derived secretory structures: from glandular trichomes to secretory cavities

Trichomes develop as outward projections from the epidermal surface. By contrast, as subepidermal secretory cavities, citrus oil glands originate from epidermal cells and develop into specialized hollow structures. During the development of these epidermis-derived structures in vascular plants, transcription factors, such as the homeodomain-leucine zipper and APETALA2/Ethylene responsive factor families, regulate their initiation and morphogenesis. Subsequent biosynthesis of secondary metabolites within these secretory structures is often mediated by jasmonic acid signaling and basic helix-loop-helix proteins, particularly MYC transcription factors. Here, we compare the regulatory mechanisms governing the development of glandular trichomes and secretory cavities, along with the biosynthesis of secondary metabolites. These insights provide the basic knowledge for harnessing these secretory structures as chassis in synthetic biology applications.