Mapping QTLs Of Carotenoid Contents About Carrot Root And Leaf And Expression Analysis Of Candidate Genes

Carotenoid is the nesscerary source of pro-vitamine A for human body.Carrot is one of the vegetable crops accumulated abundant carotenoid,especially orange type accumulated a large amount ofα,β-carotene.However,there has been no direct genetic evidence for the evolution of orange carrots.The carrots with various root color were used to study the mechanisms of carotenoid biosynthesis and discover the evolutionary evidences,because root colors were closely related to carotenoid components and contents.The expression levels of carotenoid biosynthesis genes were associated with carotenoid contents and components.But the genetic relationship between these genes and root color and carotenoid contents hasn’t been identified.The preliminary study showedα-carotene content in orange carrot leaves was significantly higher than that of other color types,which indicated that carotenoid biosynthesis mechanism in orange type was improved essentially.And it has not been reported whether the genetic control of carotenoid in carrot leaves was relevance to that in root.Based on whole-genome resequencing data,a high resolution genetic map was developed by using a set of backcross inbred lines（BILs）crossed by a wild species‘Songzi’（Ws）with a cultivar‘Amsterdam’（Af）.Because the root colors of BILs presented gradual diversity,they were evaluated using spectrocolorimeter and analysis the relationship with carotneoid contents.The QTLs for root color and carotenoid contents of root and leaf were detected.And the candidate genes were screened,and their potential functions have been analyzed.The results were as follows:1.A total of 3,223,651 SNPs were identified using a low-coverage resequencing technology.After filtering,a total of 13,124 recombination breakpoints for all BILs were detected with an average of119.31 breakpoints per line using a sliding-window approach.Finally,a genetic map was consisted of1,745 bins which contained 2,021,700 SNPs with an average of 1,158.57 SNP per bin.The total genetic distance of map was 323.95 cM with an average interval of 0.19 cM between bin markers.2.The parameters of spectrocolorimeter were closely related with the carotenoid contents in root.Except for h*value,the parameters were correlated with carotenoid contents of root at significant levels.The pearson correlation coefficients were 0.75,0.72,0.66 and 0.76 between a*value andα、β、ζ-carotene and total carotenoid contents,respectively.In carrot root,theα-carotene content was positively correlated withβ-carotene and total carotenoid contents at significant level（r=0.91 and 0.96,respectively）;theβ-carotene content was significantly correlated with total carotenoid content（r=0.99）.This indicated total carotenoid content of root was mainly composed ofα-carotene andβ-carotene.In carrot leaf,the lutein content was positively correlated with total carotenoid content at significant level（r=0.98）,which indicated lutein content affected total carotenoid content in leaf.There was a weak correlation between carotenoid in root and that in leaf.3.There were 2,6,3,2,4 and 2 QTLs associated with Ph-a*,Xy-a*,Ave-a*,Ave-h*,Ave-a*/b*and Ave-（a*/b*）² values respectively,with 13.38%¹8.05%,8.74%¹7.22%,10.48%¹7.84%,9.15%¹0.15%,9.30%¹8.90%and 8.83%¹3.30%phenotypic variance.There were 1,3,3,2,3and 3 QTLs associated withα、β、ζ-carotene,lutein,zeaxanthin and total carotenoid contents in root,respectively,with 12.06%,9.3%,8.85%¹1.21%¹6.23%,23.33%²4.50%,17.15%²6.34%and11.61%¹8.07%phenotypic variance.Each one QTL was associated withα、β-carotene,lutein and total carotenoid contents in leaf,respectively,with 10.64%,10.80%,16.68%and 17.53%phenotypic variance.There were 21 QTLs associated with 10 traits were located at 13.4¹4.5 cM and 24.7²6.1 cM on the 5^th chromosome and 20.7²2.0 cM on the 7^th chromosome.The Xy-a*value andβ、ζ-carotene and total carotenoid contents in root were affected by same QTLs,which suggested above four traits might be controlled by same genes.The QTLs for carotenoid contens of leaf were different from that of root,which indicated the regulation mechanism of carotenoid biosynthesis in leaves was independent of that in roots.4.The zeaxanthin epoxidase（ZEP）,9-cis epoxycarotenoid dioxygenase 1（NCED1）and ethylene-responsive transcription factor 026（ERF026）were selected from the physical positions of QTLs and supposed to be involved in the regulation of theβ-carotene,ζ-carotene,lutein,zeaxanthin and total carotenoid contents in carrot root.The expression levels of ZEP and ERF026 in Af were significantly higher than that in Ws based on RNA-seq data.Amino acid of ZEP were various between Ws and Af.The expression level of ZEP was positively correlated with theβ-carotene content at significant level,which suggested ZEP was a candidate gene that controlled theβ-carotene biosynthesis in carrot root.