# Unraveling the Complexity of Plant Trichomes: Models, Mechanisms, and Bioengineering Strategies
## 4 Glandular Secretory Trichomes (GSTs): Natural Biofactories
### Regulatory Mechanism of GSTs Development
Given the direct SIJAZZ-SICOI interaction, the JA-induced proteasomal degradation of SIJAZZ likely facilitates trichome formation [61]. JA treatment significantly elevates trichome density (types I + II) and terpenoid synthesis. Under JA conditions, zinc finger proteins H and H-like (HL) coregulate glandular trichome formation (type I). The SIJAZZ protein physically interacts with H/HL to suppress their activity, thereby activating THM1, a trichome repressor [58]. Loss-of-function in the HD-ZIP IV transcription factor Woolly (Wo) impairs multiple epidermal trichome types (including type VI) and terpene content. Wo activates terpenoid biosynthesis by binding TPS gene promoters and recruiting the terpene regulator SIMYC1. SIJAZZ competitively binds both Wo and SIMYC1, disrupting Wo/SIMYC1 complex formation and inhibiting TPS transcription [57]. Methyl jasmonate (MeJA) treatment enlarges type IV trichome glandular cells. The trichome-enriched HD-ZIP IV factor SIHD8 interacts with SJJAZ4; SIHD8 knockdown causes trichome shortening. SIHD8 directly activates five cell-wall-loosening enzymes to regulate elongation. Thus, JA promotes trichome elongation via the SIJAZ4-SIHD8 module, which modulates cell wall remodeling [62]. SlbHLH95 acts as a negative trichome regulator (type I) by directly binding gibberelliosis genesis gem promoters (SIGA20 ox2, SIKS5) [56]. Auxin signaling also contributes to tomato trichome initiation. The auxin response factor SIARF3, crucial for auxin transduction, is essential for epidermal development: SIARF3 knockdown reduces pavement cell density and type I/V/VI trichomes [63]. Similarly, SIAA15 (another auxin response factor) participates in trichome formation; its downregulation decreases type I and VI trichome density [64]. Despite these findings, it remains unclear how genes regulate the initiation of epidermal trichomes and how they differentiate into different types of epidermal trichomes. Recent research has suggested that the Wo protein regulates epidermal cell differentiation into digitate trichomes (DTs) or peltate trichomes (PTs) by precisely controlling the dosage [65]. A marked disparity in Wo protein dosage has been observed between DTs and PTs, with DTs exhibiting substantially elevated levels compared to PTs. How does the Wo protein concentration regulate glandular secretory trichome differentiation? Two candidate regulators, SIWox3b (WOX family) and MX1 (MYB domain), have shown expression patterns strongly correlated with Wo protein abundance across tomato genotypes. In one study, in SIWox3b and MX1 mutants, DTs decreased and PTs increased, but total glandular secretory trichomes were not different from controls. ChIP-seq analysis and RNA-seq analysis of Wo, SIWox3b, and MX1 genetic plants show that the LFS gene regulates PTs formation. Moreover, knocking out the SIWox3b, MX1, and LFS genes simultaneously results in tomato epidermal trichomes remaining at the single-cell stage and unable to differentiate further. PTs or DTs are differentiated by Wo protein concentration changes. This study elaborates on the view that tomato epidermal trichome cells originate from the same cell lineage from the perspective of cell development for the first time. According to the latest research, epidermal trichome development is also closely linked to tomato reproduction. Several HD-Zip IV transcription factors are highly expressed in cells that initiate the clasp-like epidermal trichomes on tomato anthers, and they regulate the initiation and endoreduplication of these trichomes. These clasp-like epidermal trichomes contribute to tomatoes' closed anther tube development through long-term cultivation and domestication [66]. In recent years, advancing research has led to progressively deeper insights into the development and molecular regulation of glandular secretory trichomes (GSTs) in model species such as _Artemisia annua_ and tomatoes (Figure 3).