Identification Of Carotenoids、Plastid And Transcriptome Analysis In The Red Carrot

The carrot is an important vegetable crop in the Umbelliferae family that uses root as their edible organ.It is rich in a variety of carotenoids,and that it makes their roots show different colors.Among them,the red carrot variety is one of the main cultivation types,and the content of lycopene in the root can be as high as 1000 μg/g or more(dry weight).However,the molecular mechanism and cytological characteristics of lycopene formation in the red carrot root are still unclear.In this study,the main carotenoids of carrots(red phloem and red xylem,red phloem and yellow xylem,orange and yellow)at different developmental stages were analyzed qualitatively and quantitatively by high performance liquid chromatography(HPLC).And transmission electron microscopy(TEM)was used to observe the chromoplast types in carrot roots of different colors and the changes of chromoplast at the red carrot in different developmental stages.And 65 d roots of the three carrot varieties with different colors(red,orange and yellow)were selected for transcriptome analysis.The main results are:(1)Using HPLC to deter mine the changes in the main carotenoid contents during root development of four carrot varieties.The results showed that the yellow carrot root contains lutein as the main carotenoid;the orange carrot root contained lower lutein content and higher α-and β-carotene contents;the red carrot root also contained lutein,lycopene,α-and β-carotene,the content of lycopene was significantly higher than the other three carotene,45 d-65 d were the key periods of the red carrot color change and large accumulation of lycopene.(2)Observation of the chromoplast in carrot roots by TEM found that the white carrot root only contained lipid bodies,the yellow carrot root mainly contained lipid bodies and starch grains,and the red carrot root contained lipid bodies,starch grains and plastids.The plastids were in two forms: crystalloid and plastoglobule.With the increase of lycopene accumulation in the red carrot root,the number and volume of crystalloid chromoplast increased,while the plastoglobule gradually decreased.(3)Transcriptome sequencing of 9 samples of 65 d roots of red,orange and yellow carrot varieties identified 791 genes specifically expressed in red carrot roots,of which only 4 genes of carotenoid metabolism pathway(Dc DXS1、Dc CRTISO、Dc LYCE and Dc CCD1)and 3 transcription factors(Dc NAC2、Dc PIF1 and Dc HY5)are specifically expressed in the roots of red carrots.In the follow-up study,it was found that the key periods of the accumulation of lycopene in the red carrot were from 45 d to 65 d.Therefore,it was important for Dc DXS1、Dc CRTISO、Dc LYCE、Dc CCD1、Dc NAC2、Dc PIF1、Dc HY5 and the upstream and downstream genes of lycopene metabolism(Dc GGPS1,Dc PSY1,Dc PSY2,Dc PDS,Dc ZDS1 and Dc LYCB)at different stages of the root expression levels were analyzed.The results showed that the lycopene upstream genes Dc PSY1 and Dc CRTISO increased significantly in the red carrot root at45 d and 65 d,while the downstream genes Dc LYCB at 45 d and Dc LYCE at 65 d,the expression levels were significantly reduced,while the expression level of transcription factor Dc PIF1 was also significantly reduced at 45 d and 65 d.It was speculated that these five genes jointly regulated the accumulation of lycopene in the red carrot root.