Molecular Mechanism Related To The Metabolism Of Sugar, Acid And Domestication For Ziziphus Jujuba Mill

Jujube(Ziziphus jujuba Mill.) is the most important economic tree species in Rhamnaceae. With high nutritional value, especially the high sugar content, it is regarded as "woody grain". Fruit sugar and acid content affect the quality and market value. In this study, we performed the expression profiling analysis of jujube fruits based on RNA-sequencing and qRT-PCR, and investigated the abundance of sugar and acid during fruit development to clarify jujube fruit sugar and acid metabolism, and transport mechanism; At the same time, we re-sequencd 20 cultivars and 11 wild jujubes to detect the domestication process. The main results are as follows:1. Sucrose is the principal form of jujube fruit sugar, mainly accumulated from the white mature period to mature stage. We identified four key genes for jujube sucrose synthesis(ZjSPS1, ZjSPS2, ZjSS1 and ZjSS2). Invertase genes ZjvINV and ZjnINV, which showed little expression during the late fruit development stages in jujube, play important roles in determining the sucrose content. This study reveals jujube sugar accumulation mechanism, explaining the important reason for its content mainly being sucrose. This study provides production guidance for cultivation management in fruit sugar accumulation period and increasing fruit sugar content.2. Sugar transporters play important roles in the long-distance transport of soluble sugars and their allocation into source and sink cells. A total of 83 sugar transporter genes were selected, involving three sucrose transport proteins, 59 monosaccharide transporters,and 21 SWEET transporters. RNA sequencing combined with semi-quantitative PCR revealed that several sugar transporter genes expressed in a tissue specific manner, in which two sink-specific sugar transporters were identified. Gene expression analyses showed that ZjSUC2 and the sink-specific sucrose transporter ZjSWEET2 contribute to fruit sucrose accumulation. Besides, ZjSTP12, ZjSTP16, along with ZjSUC2 and ZjSWEET2, might specially expressed in jujube fruit when compared with their orthologous genes previous studied, which could contribute to the higher sugar content in jujube than most common fruits. The sugar transporters in leaves were especially highest expressed in the fruit enlargement stage or fruit rippening stages. These results enhance our understanding of the physiological functions of sugar transporters and of sugar accumulation in jujube fruit.3. This study reveals that malic acid is the principal organic acid in jujube fruit; while in wild jujube, the concent of both malic acid and citric acid is high. This study identified nine acid metabolism key genes expressed significantily higher in sour jujube than in jujube,which is the important reason for the higher acid concentration in sour jujube. Malate dehydrogenase(ZjMDH1 and ZjMDH3) play a regulatory role for accumulation of malic acid. ZjMT3 and ZjMT11 play an important role in malic acid transport. This study reveals significant reasons for the sour taste of wild jujube, and explores the key genes leading to organic acid accumulation. It provides a molecular basis in the breeding process for eliminating varieties of acid taste.4. Based on whole genome re-sequencing, 4 key genes related to acid metabolism(PEP, NADP-ME, Aco and IDH), 13 sugar transporters and 3 sugar metabolism related genes were selected. Our results revealed that the selection of sugar metabolism genes,especially sugar transporters, has an important role in the accumulation of high sugar content for jujube fruit, while the selection of acid metabolism genes contributed to the decline of organic acid accumulation in the domestication process. The domestication from wild jujube to cultivate ones is the progress for acid reducing the sugar content increasing.This finding suggested an important domestication pattern in fruit taste and also provided insights into the fruit molecular breeding and improvement.5. We investigated the abundance of AsA in the fruit development stages, the leaf and flower of Z. jujuba cv Junzao, and the mature fruit of one type of wild jujube(Z. jujuba var.spinosa Hu cv Yanchuansuanzao). And the expression patterns of genes involved in AsA biosynthesis, degradation and recycling were analyzed. The result showed that AsA biosynthesis during early fruit development(the enlargement stage) is the main reason for jujube high accumulation. The L-galactose pathway plays a predominant role in the biosynthesis of AsA during jujube fruit development, and the genes GMP1, GME1, GGP,and GaLDH involved in the determination of AsA concentration during fruit development and in different genotypes; the myo-inositol pathway along with the genes GME2 and GMP2 in the L-galactose pathway play a compensatory role in maintaining AsA accumulation during the ripening stage. These findings enhance our understanding of the molecular mechanism in regulating AsA accumulation for jujube.