Tangled in transcription The web of transcription factors regulating tomato type VI glandular trichome development and specialized metabolsim

Plant trichomes are specialized epidermal structures that mediate ecological interactions through morphological and metabolic diversity. In tomato (Solanum lycopersicum), type VI glandular trichomes consist of four glandular cells that represent the main site of volatile terpenes biosynthesis. The regulatory network coordinating the development and metabolism of this trichome type involves transcription factors such as SlMYC1 and SlWO, and jasmonic acid signaling. However, the transcriptional network upstream and downstream of SlMYC1 controlling trichome development and metabolism remains not fully known.
This thesis addresses these gaps by exploring the transcriptional network, with a particular focus on SlMYC1 as a master regulator. SlZHD18 was identified as a putative upstream regulator of SlMYC1, capable of binding its promoter. Functional analyses using virus-induced gene silencing and CRISPR-Cas9 mutants demonstrated that SlZHD18 positively regulates type VI trichome density and modulates volatile sesquiterpene composition. SlMYC1 binds its own promoter, suggesting autoregulatory feedback. Transcriptomic analyses of SlMYC1 overexpression and loss-of-function lines identified three transcription factors, SlbHLH114, SlMYB14, and SlWRKY1. Their functional characterization via knockout mutants showed that SlbHLH114 and SlWRKY1 promote trichome density while negatively affecting specific terpene synthase genes.
Genetic mapping of glandless, a type VI trichome mutant, identified SlHDZ38 as essential for type VI trichome formation and glandular cell differentiation. Knockout of SlHDZ38 resulted in absence of type VI glandular trichomes and broad downregulation of trichome-associated transcription factors and terpene biosynthetic genes.
Collectively, these findings expands our understanding of the intricated transcription factor network coordinating developmental and metabolic programs in tomato type VI glandular trichomes.