Regulation of squalene biosynthesis and plant development by ELONGATED HYPOCOTYL 5 in response to light in <i>Arabidopsis thaliana</i>

Terpenoids are a diverse group of metabolites that are crucial for plant development and used in the cosmetics and pharmacological industries. Various developmental processes and environmental factors, including light, affect terpenoid biosynthesis. However, studies on the regulatory factors involved in such regulation remain limited. Squalene synthases (SQSs), key enzymes in the terpenoid pathway, are pivotal for sterol and triterpene biosynthesis across various organisms. Here, we report that AtSQS1 expression and squalene content are higher in darkness than in light and that ELONGATED HYPOCOTYL 5 (HY5) negatively regulates AtSQS1 expression and squalene biosynthesis in Arabidopsis (Arabidopsis thaliana). Our study showed that AtSQS1 expression is unaffected in the hy5-215 mutant under light and dark conditions but is down-regulated in WT and HY5OX lines. Histochemical GUS assays and GFP expression patterns indicated that AtHY5 negatively regulates squalene biosynthesis. Yeast 1-hybrid assays, electrophoretic mobility shift assay, and chromatin immunoprecipitation experiments confirmed the physical binding of AtHY5 to the AtSQS1 promoter. We validated our results by developing AtSQS1 promoter:reporter lines in WT, hy5-215, and HY5OX backgrounds. Quantification of squalene and phytosterol content further confirmed that AtHY5 negatively regulates squalene biosynthesis in a light-dependent manner in Arabidopsis.