The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves

Plant trichomes are specialized unicellular structures that originate and project from above ground epidermal tissues on the surfaces of leaves, petals, stems, petioles, peduncles, and seed coats depending on species. Trichomes (also called ‘hairs’) play well-recognized roles in defense against insect herbivores, both as a physical barrier that obstructs herbivore movement and by mediating chemical defenses. By virtue of their physical properties (size, density), trichome hairs can directly operate to protect buds of plants from insect damage, reduce leaf temperature, increase light reflectance, prevent water loss, and decrease leaf abrasion. Great variety of trichomes and their accessibility makes them a useful model for studying the molecular processes of cell fate determination, cell cycle control and cellular morphogenesis. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal cells (i.e. endoreduplication cycles); (ii) transcription factors that create activator/repressor complexes with a central role in determining cell fate, initiation and differentiation of an epidermal cell in trichome; (iii) evidences that point out the interplay of the aforesaid complexes with various phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we describe trichome development in leaves, commonly subjected to environmental injury, and where most genetic regulators have been characterized.