Abstract
Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called ‘hairs’) play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. 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 pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.
Highlights
The epidermis is the superficial coating layer that wraps leaves and the primary body of the stem and it is in direct contact with the atmosphere, and is a protective barrier against abiotic and biotic factors
Xi et al [182] suggested that in Arabidopsis the PEST domain is fundamental for the functioning of the STI gene in the regulation of the process underlying the proper development of branched trichomes. This is deduced by the direct interaction of STI with the BRACHLESS TRICHOME (BLT) gene, which plays a key role in the cellular form and in the control of the endoreduplication cycle [182,183]
Important molecules involved in heterochronic processes can be identified through investigations of the activation/repression of trichome initiation
Summary
The epidermis is the superficial coating layer that wraps leaves and the primary body of the stem and it is in direct contact with the atmosphere, and is a protective barrier against abiotic and biotic factors. The MBW complex activates the transcription of genes, encoding the negative regulators (TRY/CPC) These inhibitors can move laterally in the epidermis between neighboring cells, competing with GL1 and interacting with GL3/EGL3 to establish the repressor complex GL3/EGL3-CPC/TRY-TTG1. A regulatory loop, linking to a local autonomous circuit various positive and negative regulators, controls the activation of downstream target genes and, the cell competence for trichome initiation and development (Figure 1). The MBW activator complex controlling trichome initiation positively controls the genes that act in later stages of the Arabidopsis flavonoid biosynthetic pathway, including MYB75/90/113/114, GL3/EGL3/TT8, and TTG1 [44,49,93,102,103,104]. These results suggest the probable existence of a regulation cycle concerning GL3 and UPL3 [107]
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