Research On The Mechanism Of Green Millet Colour Of Daqinggu

Foxtail millet (Setaria italica (L.) Beauv.) is an important minor crop in Northern China. Hulled foxtail millet is rich in nutrients. Millet colour is a key visual index to evaluate foxtail millet quality. There are many kinds of colours of millet, such as yellow, white and green. Currently, the mechanism on how millet colour is generated remains unclear. In this study, Daqinggu with green millet colour was selected as research material and Jingu 21 with yellow millet colour as the control. We tried to explore the mechanism for the formation of Daqinggu millet colour from three aspects including carotenoid metabolism, chlorophyll degradation and anthocyanidin metabolism. The main results were as follows:(1) Compared with Jingu 21, the total carotenoids content of matured millets of Daqinggu was lower, only 6.25 mg/kg. Duing the millet maturation, the total carotenoids contents in Daqinggu millets were significantly higher than in Jingu 21 at the stages of S2 and S3.(2) Applying Quantitative Real-time PCR, we analysed the expression patterns of carotenoid metabolic genes in Daqinggu and Jingu 21 during millet maturation. The results showed that the relative expression levels of five genes of SiPSY3, SiPDS, SiZ-ISO, SiLCYB and SiCYP97C in Daqinggu were significantly higher than in Jingu 21, but other genes did not show significant differences.(3) With the muturation of Daqinggu millet, the total chlorophyll contents were decreasing. During the maturing stages from S2 to S5, the total chorophyll contents in Daqinggu were significantly higher than in Jingu 21.(5) The SiSGR genes were cloned from Daqinggu and Jingu 21. The bioinformatic analysis indicated the CDS sequence of SiSGR with a molecular mass of 31847.3 Da and isoelectric point of 8.83 consisted of an 861 bp ORF which encoded a 286-amino acid sequence and shared 90% and 83% identity with two monocotyledons plants, maize and rice, respectively. There were four a helices and ten β folds in secondary structure of SiSGR protein; The DNA sequence of SiSGR contained 3 exons separated by 2 introns; The promoter sequence of SiSGR contained 8 TATA-boxes, one CAT-box, 3 CAAT-boxes and other cis-acting elements related to light response, drought-inducibility and so on. Compared with Jingu 21, the DNA sequence of SiSGR gene in Daqinggu had a single base (A) deletion in the second intron region and the promoter sequence contained a AE-box related to light reponse due to one mutated nucleotide. The expression level of SiSGR in Daqinggu was significantly higher than in Jingu 21 during the millet maturation from S1 to S4.(6) The anthocyanidin content of matured Daqinggu millet was very low. There was no difference in the expression of anthocyanidin metabolic genes between in Daqinggu and Jingu 21 grains at mid-filling stage.Above all, our preliminary conclusion is that the green millet colour of Daqinggu might be formed due to the mutation of DNA or promoter sequence and the low expression level of SiSGR gene related to chlorophyll degradation in Daqinggu millet.