Respiratory Physiological Characteristics And Molecular Regulation Of Fruit Ripening For Chinese Jujube

The Chinese jujube?Ziziphus jujuba Mill.?is a traditional fruit tree originated from China.Over the past twenty years,the fresh jujube industry has greatly increased and achieved prominent economic benefits.However,the fresh fruit has a short shelf-life with serious deterioration and dehydrationduring harvested storage,by which has limited the supply of commodities and the long-term effects.For a long time,the jujube has been controversially classified as non-climacteric,and yet the molecular regulations of fruit ripening are not completely understood.Thus,studying on the respiratory physiological characteristics and molecular regulations of jujube fruit ripening would be helpful for theimprovement of strategies and technologiesfor fruit storage and transportation.This research would also provide insights into understand of the molecular mechanisms of jujube fruit quality formation and maintenance.In the present study,a widely cultivated fresh jujube cultivar,‘Dongzao',was used as the plant material for determining the dynamic changes of ethylene and abscisic acid?ABA?content during fruit development,ripening,harvested storage,and after exogenous hormone treatments.Based on the jujube genome,critical genes involved in the biosynthesis,metabolism,and signaling pathways of ethylene and ABA had been identified,and their expressions during fruit development and ripening processes were determined by qPCR.We also further performed a transcriptome sequencing for the white mature?WM?‘Dongzao'fruit which was treated with exogenous ABA or nordihydroguaiaretic acid?NDGA?at the first day after treatment to identify the associated metabolism pathways and regulatory networkswhich were consisted by hormone-related genes and transcription factors.The main conclusions were listed as following:1.The maturity of jujube fruit began from the WM stage,and fully ripened at the fully red?FR?stage.The respiration rate and ethylene production failed to perform any climacteric-like behavior during‘Dongzao'fruit normal ripening,harvested storage,and even after 100?L L^-11 exogenous ethylene treatment.Based on the jujube genome,we identified 23genes involved in ethylene biosynthesis and signaling pathway.Along with jujube fruit ripening,transcript expressions of these genes kept increasing but were inhibited by the exogenous ethylene,indicating the System-II like gene expression patterns were absence.On the other hand,the WM‘Dongzao'fruit is insensitive to 100?L L^-1 exogenous ethylene,but the 1.0?L L^-11 1-MCP significantly inhibited fruit respiration and ethylene production,which suggested there wereethylene-dependent ripening pathways in jujube fruit.This study provided molecular evidence for the non-climacteric division and also revealeda role of ethylene in jujube fruit ripening.2.Across the whole jujube genome,we identified a total of 119 APETALA2/ethylene response factor?AP2/ERF?genes which could be divided into four gene families as the AP2,RAV,ERF and Soloist according to the differential conserved domains and phylogenetic relationships.The ERF family were further divided into the ERF and DREB subfamily.Based on the published RNA-seq data,the relative expression analysis showed 64,80,and 44 genes expressed in the leaf,flower,and fruit,respectively,by which shed lights into understand and validation of gene functions in future studies.On this basis,we further identified four genes as the positive?ZjERF25 and ZjERF36?or negative?ZjERF54 and ZjDREB39?regulators during jujube fruit ripening and upon exogenous ethylene treatment,suggesting these genes would be potentially involved in ethylene-dependent ripening regulation of jujube.This study improved the understand of phylogenetic relationships and biological functions of AP2/ERF transcription factor,and also would be the basis for elucidating ethylene-dependent ripening regulation and would be helpful for identifying ethylene-related ripening pathways.3.The dynamic changes in ABA content highly increased at the WM stage and maintained a high level during ripening process,indicating a potential role of ABA in inducing fruit ripening onset and maintaining the ripening process.We identified a total of 49genes involved in ABA biosynthesis,metabolism,and signaling pathways.Their expressions underlying seven differential ripening stages of‘Dongzao'fruit were investigated by qPCR,suggesting an interaction among the expression of nine-cis-epoxycarotenoid dioxygenase?ZjNCED3?,ABA 8?-hydroxylase?ZjCYP707A2?,and beta-glucosidase?ZjBG4,ZjBG5,ZjBG8,and ZjBG9?genes;while the signaling pathway consisted by the receptor?ZjPYL8?,protein phosphatase 2C?ZjPP2C4,ZjPP2C5,ZjPP2C6,ZjPP2C7,andZjPP2C8?,and sucrose non-fermenting 1-related protein kinase 2?ZjSnRK2-2 and ZjSnRK2-5?would be important in regulating fruit sensitivity to ABA and ripening process.This study revealed the accumulation of ABA during‘Dongzao'fruit ripening process and its own regulation at gene expression levels.4.The WM‘Dongzao'fruit was sensitive to 50 mg L^-1 ABA treatment,by which could induce the rapid accumulation of endogenous ABA and promote the increase of fruit respiration and ethylene production,while the 50 mg L^-1 NDGA inhibited the same processes.The soluble sugar content kept decreased during harvested storage but showed limited response to both treatments.With the data of transcriptome sequencing of fruit treated with ABA,NDGA,and control at the 1st day after treatment,we have identified 1103 and 505differentially expressed genes upon ABA and NDGA treatment;we also found the expression of photosynthesis,secondary,lipid,cell wall,and starch and sugar metabolism pathway related genes were significantly induced,indicating the ABA could directly or indirectly induce the changes in gene expression patterns related to these metabolism processes,thereby inducing and maintaining fruit ripening.Meanwhile,the ABA showed a crosstalk with ethylene,gibberellin,auxin,and cytokinin;and also a network including several transcription factors such as the AP2/ERF,bZIP,etc.The jujube fruit ripening was regulated across these gene expressions together.This study further elucidated the ABA-associated metabolism and regulatory process,also built a potential network of jujube fruit ripening regulation.The findings would be helpful for understanding the non-climacteric fruit ripening.