A comparative framework for trichome and trichome-derived epidermal appendage development across model glycophytes and halophytes.

KEY MESSAGE: This review compares trichome and trichome-derived epidermal appendage development across model glycophytes and halophytes, revealing that conserved regulatory modules are independently rewired into lineage specific circuits to produce diverse epidermal structures. In halophytes, salt glands and epidermal bladder cells further integrate these developmental programs with ion transport and stress physiology, offering entry points for engineering crop stress resilience. Trichome-derived epidermal structures have arisen repeatedly across angiosperms, yet the mechanisms by which conserved epidermal regulators are redeployed into lineage-specific developmental circuits remain incompletely understood. Here, we present a comparative synthesis of trichome and trichome-derived epidermal appendage development across representative glycophytes (Arabidopsis thaliana, Oryza sativa, Solanum lycopersicum, and Gossypium hirsutum) and extend this framework to halophytic species with specialized salt-handling structures. These systems reveal how MYB-bHLH-WD40, WOX-AP2/ERF-auxin, HD-ZIP IV-bHLH-JA, and expanded MYB/HD-ZIP networks have been independently rewired to produce unicellular hairs, secretory glands, and highly elongated fibers. In halophytes, these developmental modules are further integrated with ion transport, vesicle trafficking, and osmotic regulation pathways, enabling the emergence of salt glands and epidermal bladder cells with specialized physiological functions. By synthesizing these trajectories, we propose a unifying regulatory model for epidermal evolution. We additionally highlight potential entry points for engineering trichome traits, improved metabolite production, and synthetic salt-handling epidermal structures for crop stress resilience.