Analysis Of Peel Structure And Components Related To Pigment Accumulation During Jujube Coloring

Ziziphus jujuba Mill.is originated in China which hold 98%of the world jujube production.Jujube fruit is sweet as honey,and has high value of nutriention and health maintainance.Jujube is a kind of "Chinese red" with deep cultural connotation,featuring its fruits with gorgeous red skin.Red pigment of jujube is natural pigment with absolute industrialization development advantages in China.However,the composition and formation mechanism of jujube peel pigment are still not clear,which hinders further deep development and utilisation.In the present study,‘Dongzao’was selected as materials,the change of nutritional composition in jujube fruit peel during fruit coloring was analyzed,using methods of paraffin slice,frozen slice,transcriptome and metabolome.The accumulation place of red pigments was confirmed.The composition and the pathways of biosynthesis and transcription regulation of flavonoid pigment and lignin were revealed.The candidate genes regulating the peel coloration in jujube was digged out.The purpose of this study was to provide basis for development and utilization of red pigments of jujube.The main findings are as follows:1.During the coloring process of jujube peel,the contents of main nutrients changed significantly.The contents of total flavonoids,total carotenoids,cellulose and other substances decreased gradually,while soluble sugar,malic acid,amino acids,lignin and other substances increased gradually.There was a certain correlation between peel coloring and nutrient composition.2.Jujube peel pigment is deposited in lignification peel cells.It is composed of 1 layer of keratin cells and 5 to 6 layers of epidermal cells.With the peel coloring,the number of cell layers remains unchanged,thickness decreases,cell gap increases,cell wall thickens,cell lignification,the whole cell is filled with pigmentation substances.In particular,the color around the cell wall is darker,which may be the result of the deposition of refractory lignin or the high polymer of lignin and pigment,resulting in the residue still having thick color adhesion after the extraction of jujube pigment.Lignin plays a certain role in the color of jujube peel.3.Flavonoids also play an important role in the coloring process of jujube peel.The metabolites of four materials(white,half red,half red and full red)of ’Dongzao’ jujube were analyzed by widely targeted metabolome.It was found that flavonoid biosynthesis pathway was significantly enriched.Further studies showed that the accumulation of three anthocyanins(Delphinidin,Delphinidin 3-O-glucoside,Malvidin 3-O-glucoside),three flavonols(Isorhamnetin O-hexoside,Isorhamnetin 5-O-hexoside,Isorhamnetin 3-O-glucoside)and three flavonoids(Luteolin,C-hexosyl-luteolin C-pentoside,8-C-hexosyl-luteolin O-hexoside)in jujube peel was significantly positively correlated with pigmentation.The results showed that Delphinidin,Isorhamnetin,Luteolin and their glycosides may be the main components of jujube red pigment,and the coloring of jujube peel is the result of the comprehensive accumulating of anthocyanins,flavonols,flavonoids and other flavonoids.4.The key genes and transcription factors for the synthesis of flavonoids pigment from jujube pericarp were discovered.Based on transcriptome sequencing,the differentially expressed genes between colored and uncolored pericarps were analyzed.It was found that most of the structural genes in flavonoid biosynthesis pathway were highly expressed in the white phase,and down regulated or silenced with the coloring process of jujube peel.Three UFGT structural genes(LOC107427037,LOC107406597 and LOC107427038)were significantly up-regulated with fruit coloring.It was preliminarily confirmed that UFGTs were the key genes for jujube pigment synthesis.Furthermore,two R2R3MYB transcription factors(LOC107434709,LOC107404750)and three bHLH transcription factors(LOC107407443,LOC107423582 and LOC107423649)were identified to positively regulate the coloring of jujube peel by correlation analysis of the expression models.5.The lignins type and key candidate genes of lignin synthesis were preliminarily determined in jujube peel.Comprehensive analysis of the concentration of 13 kinds of metabolic intermediates and related gene expression in the lignin biosynthesis pathway,it is revealed that the lignin is mainly based on a mixture of syringyl lignin(S-Lignin)and guaiacyl lignin(G-lignin).It was found that the expression of one F5H(LOC107424406),two CCR(LOC107420974 and LOC107412471),three POD(LOC107425304,LOC107421730,LOC107419925)were up-regulated with peel coloring,and the expression of one CAD(LOC107426959)was positively correlated with the content of p-Coumaryl alcohol.It was speculated that F5H was the key gene of S-lignin synthesis,CCR was the key gene of G-S lignins synthesis.Positive regulation of F5H and CCR can promote lignin synthesis.Two NAC transcription factors(LOC107435239 and LOC107417668)related to secondary cell wall formation and programmed cell death were screened out,and one MYB transcription factor(LOC107425254)were found to be a potential positive regulator of lignin synthesis.6.Two MYB transcription factors(LOC107415776 and LOC107404478)were extracted,which are consistent with the high similarity expression pattern of PtrMYB6 transcription factors that have been shown to positively regulate anthocyanin negatively regulate lignin synthesisIt is speculated that it can promote jujube peel coloring through positive regulation of flavonoids synthesis and negative regulation of lignin synthesis.