Phytochrome A promotes carotenoid biosynthesis in the carrot taproot through regulation of DcPSY1 and DcPSY2 gene expression

Light emerged as a paramount signal influencing the autotrophic growth of plants. Phytochrome A (PHYA) is one of the five plant photoreceptors, characterized as light-labile, remaining stable in continuous far-red (FR)-rich light, playing a pivotal role in regulating seed germination, de-etiolation, chlorophyll and carotenoid synthesis, reducing hypocotyl length and contributing to shade avoidance responses. Daucus carota L. (carrot) is known for its underground carotenoid accumulation, but direct exposure of the root to white light (WL) impairs both carotenoid synthesis and development. By means of an RNA-seq analysis, light signaling genes, such as DcPHYA were found more expressed in the taproot grown underground. In this work, we present evidence on the functional characterization of DcPHYA, which exhibits 73% amino acid identity with AtPHYA in their functional domains, presents nuclear localization and a stable interaction with ELONGATED HYPOCOTYL 5 (DcHY5), which binds to G-box in promoters of DcPSY1 and DcPSY2 . Both DcPHYA and DcHY5 promote DcPSY1 and DcPSY2 expression and carotenoid synthesis in Nicotiana tabacum reporter lines and in carrot embryos. Also, N . tabacum overexpressing DcPHYA showed reduced hypocotyl length and reduced expression of the target genes NtPEPCK and NtPAR1 under FR, suggesting that DcPHYA is functional. Carrot transgenic lines with up to 50% reduced relative expression of DcPHYA have a reduction in carotenoid content between 65%-80% as well as in the relative expression of DcPSY1 and DcPSY2 in the taproot grown underground compared to wild-type plant. These results suggest that DcPHYA is functional and participates in promoting DcPSY1 and DcPSY2 expression and carotenoid synthesis in the carrot taproot, most likely through DcHY5. • DcPHYA interacts with DcHY5 in the cell nucleus under Far red light. • DcPHYA and DcHY5 promote DcPSYs expression and carotenoid synthesis in N.tabacum and carrot somatic embryos • DcPHYA promotes photomorphogenesis in N.tabacum • DcPHYA induces expression of DcPSYs in the carrot taproot grown underground. • DcPHYA is required for carotenoid synthesis in the carrot taproot grown underground.