Researchment On The Mechanism Of Light On Carotenoids In Citrus Fruits

As one of the four major fruits over the world,citrus fruit is loved by consumers not only for its rich color and fruit shape,more importantly,depending on its rich nutrients and unique health benefits to provide us with daily needs.Among them,carotenoids are not only the main color determinator of citrus fruits,but also important nutritional and functional components.Therefore,it is of great theoretical and practical value to investigate the regulation of carotenoid metabolism in fruits.Light treatment is an environmentally-friendly physical method that will not do harm to the fruit,and the treatment condition is easy to be controlled,and this method has great potential to improve fruit quality.Grapefruit and sweet oranges are rich in carotenoids in citrus,and thus can be good model materials for studying carotenoid metabolism in fruit.In the present study,carotenoid composition and content of grapefruit treated with different light transmission and postharvest sweet orange fruits treated with different LED(Light Emitting Diode)were identified and analyzed during fruit development and irradiation period by High Performance Liquid Chromatography(HPLC),respectively.And transcriptome sequencing(RNA-sequencing)was conducted by Illumina.In addition,we used WGCNA,GO,KEGG and other methods to identify candidate genes related to light-induced carotenoid metabolism.Based on the correlation analysis between gene expression and carotenoid accumulation,a network of light signal to regulate carotenoid metabolism in citrus fruits was preliminary proposed.Among them,a genome wide analysis of one type of the important hub genes,phytochrome-interacting factors(PIFs),were performed to characterize their functions.Also,cis-acting elements of promoter regions in these genes were predicted.The main results are as follows:1.During fruit development,dark shade(DS),red light(RL),blue light(BL),and white light(WL)transmission treatments had significant effects on carotenoid accumulation,and the accumulation responsed differently to each light treatment.The effect of each treatment on carotenoid accumulation was not significant before turning stage,but the red light and blue light transmission treatments significantly promoted the carotenoid accumulation after the turning stage during fruit ripening(p <0.05).2.When sweet orange fruit were exposed to LEDs(dark shade-DS,red light-RL,blue light-BL,and white light-WL)for six consecutive days,carotenoids accumulation had different responses to these light treatments.Among them,compared with the control,RL and DS promoted the accumulation of carotenoids during treatment period,while BL treatment reduced the carotenoid content,but the difference is not significant.WL treatment caused a significant decrease in carotenoid content in sweet orange fruit(p <0.05).3.Gene co-expression network analysis(WGCNA)results showed that the transcription factor families related to carotenoid metabolism mainly included AP2-EREBP,C3 H,HSF,MYB,NAC,WRKY,and PIFs which is a subfamily of b HLH.In RL and BL treatments,these transcription factors regulate the accumulation of carotenoids in grapefruit by modulating directly the gene expression of carotenoids metabolic related enzyme,including IPI,Z-ISO,ZDS,ZEP,NXS,CCS,CYP97 C and NCED3.4.GO classification and KEGG enrichment indicated that transcription factor families such as NAC,MYB,AP2-EREBP,and b HLH involved in the regulation of carotenoid metabolism in postharvest fruit.When fruits treated with RL and DS,and the expression levels of structural genes of ZDS,CRTISO,ZEP,and CCD1 were up-regulated,and CCD1,CCD4,and NCED3 were significantly down-regulated.These genes may be important candidate genes for carotenoid metabolism of light modulation in citrus fruit.5.A total of nine PIFs family gene members were identified by genome wide analysis of sweet orange.And analysis of cis-acting elements showed that each member of PIFs contained a large number of light-responsive cis-acting elements.Based on bioinformatic analysis and recent findings,we speculated that PIF3 and PIF4 may play important roles in mediating light signals to regulate carotenoid metabolism in citrus fruit.