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Languages : en
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Book Description
Carrot is one of most important vegetable crops, providing provitamin A carotenoids crucial for human nutrition and health. Carotenoid biosynthesis has been extensively studied in plants, however the mechanism underlying carotenoid accumulation is still poorly understood in carrots. Thus, gaining insight into the genetics, biochemistry and regulatory mechanisms of carotenoid accumulation would facilitate the nutritional improvement of carrot. Previously, a two-gene model, including the Y and Y2 genes, was proposed by Buishand and Gabelman to explain the variation of carotenoid accumulation in dark orange, pale orange, yellow and white carrot roots. The Y gene controls the reduction of root pigmentation, while Y2 prevents the synthesis of carotenes but not xanthophylls. This study mainly focused on identification and evaluation of the Y gene. QTL analysis was performed in the F4 mapping population 70796 with a total of 150 individuals to identify the Y gene map location. A large effect QTL with LOD score of 21.2 for total carotenoids was detected on chromosome 5. Data from the marker-trait association study using GLM (general linear model) analysis also supported the location of Y. Fine mapping was then used to identify a 90-kb region that includes 8 annotated genes. Combining this data with that from another population narrowed the candidate region to 75 kb. Comparative transcriptome analysis by RNA-seq revealed that among genes in the 75 kb region, only DCAR_032551 was differentially expressed in both populations. This gene contains a 212 nt indel in the coding region that causes a frameshift mutation in high pigmented (yy) roots, making it the most promising candidate gene. A genetic rescue test was performed to test the function of the candidate Y gene, DCAR_032551 using Agrobacterium-mediated gene transformation. Three transgenic plants were regenerated and shown to contain the integrated vector. However, after several attempts there were no transformants survived to further evaluation. Transcriptome data suggested that high carotenoid accumulation involves the overexpression of several light-induced genes operating in photosystem development and function. We hypothesized here that carotenoid accumulation is controlled at the regulatory level, and high carotenoid accumulation in carrot taproot results from the stimulation of carotenoid accumulation without light induction through the same mechanism that stimulates carotenoid accumulation in shoots in de-etiolation.
