Development and carotenoid synthesis in dark-grown carrot taproots require PHYTOCHROME RAPIDLY REGULATED1

Abstract

Light stimulates carotenoid synthesis in plants during photomorphogenesis through the expression of PHYTOENE SYNTHASE (PSY), a key gene in carotenoid biosynthesis. The orange carrot (Daucus carota) synthesizes and accumulates high amounts of carotenoids in the taproot that grows underground. Contrary to other organs, light impairs carrot taproot development and represses the expression of carotenogenic genes, such as DcPSY1 and DcPSY2, reducing carotenoid accumulation. By means of RNA-seq, in a previous analysis we observed that carrot PHYTOCHROME RAPIDLY REGULATED1 (DcPAR1) is more highly expressed in the underground grown taproot compared to those grown in light. PAR1 is a transcriptional cofactor with a negative role in shade avoidance syndrome regulation in Arabidopsis (Arabidopsis thaliana) through the dimerization with PHYTOCHROME INTERACTING FACTORs (PIFs), allowing a moderate synthesis of carotenoids. Here, we show that overexpressing AtPAR1 in carrot increases carotenoid production in taproots grown underground as well as DcPSY1 expression. The high expression of AtPAR1 and DcPAR1 led us to hypothesize a functional role of DcPAR1 that was verified through in vivo binding to AtPIF7 and overexpression in Arabidopsis, where AtPSY expression and carotenoid accumulation increased together with a photomorphogenic phenotype. Finally, DcPAR1 antisense carrot lines presented a dramatic decrease in carotenoid levels and in relative expression of key carotenogenic genes as well as impaired taproot development. These results suggest that DcPAR1 is a key factor for secondary root development and carotenoid synthesis in carrot taproot grown underground.

Generating colorful carrot germplasm through metabolic engineering of betalains pigments

“…In this study, a new pigment category of carrots (betalain group) was created through metabolic engineering, showing a unique red to red-violet color. Under the influence of tissue culture and genetic transformation, the fleshy root of transgenic carrot exhibited a certain degree of torsion [ 31 , 33 , 52 ]. No obvious unfavorable effect of betalain biosynthesis on carrot root size and yield was observed, which would be important if these betalain group carrots are used for production.…”

Analysis of Carotenoids and Gene Expression in Apple Germplasm Resources Reveals the Role of MdCRTISO and MdLCYE in the Accumulation of Carotenoids

“…Carotenoid compounds are modulated by genes that encode enzymes at the transcriptional level. As the key rate-limiting enzyme of carotenoid biosynthesis, PSY has been proven to promote the biosynthesis of carotenoids in many crops, including Arabidopsis , tomato (Solanum lycopersicum), carrot (Daucus carota), watermelon (Citrullus lanatus), and apple. − PDS in tomatoes can indirectly affect carotenoid biosynthesis by regulating the expression of carotenoid pathway genes . ZISO has been considered to play a key role in carotenoid biosynthesis in tomato and sweet orange ( Citrus × sinensis ). , LCYB is considered to be the key rate-limiting enzyme in citrus ( Citrus spp.)…”

Colors in the dark

“…In this issue of Plant Physiology , Arias et al (2022) investigated how carotenoid biosynthesis occurs and how is it regulated in dark conditions by elucidating the role of the bHLH protein PHYTOCHROME RAPIDLY REGULATED 1 (PAR1). PAR1 was previously noted to be of interest from an RNA-seq analysis where gene expression was compared between carrot roots when grown in white light versus darkness, revealing a set of dark-expressed photomorphogenesis-related genes, including PHYA (PHYTOCHROME A), PIF4 , and PAR1 ( Arias et al, 2020 ) .…”

“… Model proposed by Arias et al (2022) showing the role of the bHLH protein PAR1 in regulating carotenoid biosynthesis during darkness in carrot roots. During light conditions, PAR1 is not produced and therefore PSY (PHYTOENE SYNTHASE) gene expression and carotenoid biosynthesis is low.…”

“…During light conditions, PAR1 is not produced and therefore PSY (PHYTOENE SYNTHASE) gene expression and carotenoid biosynthesis is low. Under darkness, PAR1 expression is triggered, which causes an increase in transcripts and protein levels of PSY and therefore carotenoids accumulate (image from Arias et al, 2022 ). R/L, roots grown in light; R/D, roots grown in darkness; PHYA, PHYTOCHROME A; PHYB, PHYTOCHROME B; and HY5: LONG HYPOCOTYL.…”