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Brassinosteroids are important plant hormones influencing, among other processes, chloroplast development, the electron transport chain during light reactions of photosynthesis, and the Calvin-Benson cycle. Medium-chain-length polyprenols built of 9-11 isoprenoid units (C45-C55 carbons) are a class of isoprenoid compounds present in abundance in thylakoid membranes. They are synthetized in chloroplast by CPT7 gene from Calvin cycle derived precursors on MEP (methylerythritol 4-phosphate) isoprenoid biosynthesis pathway. C45-C55 polyprenols affect thylakoid membrane ultra-structure and hence influence photosynthetic apparatus performance in plants such as Arabidopsis and tomato. So far nothing is known about the hormonal or environmental regulation of CPT7 gene expression. The aim of our study was to find out if medium-chain-length polyprenol biosynthesis in plants may be regulated by hormonal cues.We found that the CPT7 gene in Arabidopsis has a BZR1 binding element (brassinosteroid dependent) in its promoter. Brassinosteroid signaling mutants in Arabidopsis accumulate a lower amount of medium-chain-length C45-C55 polyprenols than control plants. At the same time carotenoid and chlorophyll content is increased, and the amount of PsbD1A protein coming from photosystem II does not undergo a significant change. On contrary, treatment of WT plants with epi-brassinolide increases C45-C55 polyprenols content. We also report decreased transcription of MEP enzymes (besides C45-C55 polyprenols, precursors of numerous isoprenoids, e.g. phytol, carotenoids are derived from this pathway) and genes encoding biosynthesis of medium-chain-length polyprenol enzymes in brassinosteroid perception mutant bri1-116. Taken together, we document that brassinosteroids affect biosynthetic pathway of C45-C55 polyprenols., Competing Interests: Declaration of competing interest Authors declare no conflict of interests., (Copyright © 2023 The Authors. Published by Elsevier GmbH.. All rights reserved.)