Molecular Basis Of Fruit Elongation And Abscisic Acid Mediated Non-climacteric Fruit Ripening

Fruits are the products of sexual reproduction in angiosperms,which are also widely utilized by human as important food sources.The development of fruit begins with the differentiation of floral meristem and ends up with fully ripe fruit,during which the sensory and nutrients quality are formed and the final price of the commercial products are decided.Fruit shape is a very important sensory quality,which can determine the main use of a particular variety.In addition,the accumulation of pigmentation,the release of,aroma and the increase of soluble sugar content,which form the nutrients quality are all decided during the fruit ripening process.Therefore,the study on fruit shape formation and ripening mechanism not only help us gain a better understanding of plant organ formation but also promote the improvement of fruit quality.Taking wild type tomato(S.pimpinellifolium accession LA 1589),Jinqing NO.1 cucumber(Cucumis sativus L.)and Sato Nishiki sweet cherry(Prunus avium L.)as material,the study of fruit shape formation and fruit ripening mechanism are carried out at the cellular,physiological and molecular level.In terms of fruit shape study,tomato(Solanum lycopersicum)is a model plant.There are three loci controlling tomato fruit shape elongation,namely sun,ovate and fs8.1.Taking sun,ovate and fs8.1 NIL in the background of wild type tomato(S.pimpinellifolium)LA 1589 as materials,the mechanism of fruit shape regulation was explored at the cellular and transcriptome level.The results showed that sun and ovate have already significantly elongated the ovary at anthesis;sun makes an elongated ovary by uniformly increasing cell number in the proximal-distal direction;ovate elongates the ovary by specifically increasing the cell number of proximal end in the longitudinal direction and decreasing cell number in the transverse direction;while fs8.1 increased cell number both in the longitudinal and transverse direction,which makes no significant difference on ovary shape index.To further understand the regul,ation mechanism of these three fruit shape loci on fruit shape formation,transcriptome changes were analyzed in the flower buds formed at 4,6,8,10,13 and 16 days post flower initiation(dpi).The principal component analysis(PCA)shows 13 dpi may be the time when these three fruit shape loci play roles on the flower bud transcriptome.Moreover,there are more differentially expressed genes(DEG)at 16 dpi than at other developmental stages.sun NIL and ovate NIL have the most number of common DEG while ovate NIL and fs8.1 NIL have the least number of common DEG.SUN mutation has the most significant effects on flower bud transcriptome,which leads to 1349 differentially expressed genes(DEG),while OVATE and fs8.1 lead to 300 and 227 DEG,respectively.Moreover,there are more expressed genes at the later flower bud developmental stages(10,13,16 dpi)than at the earlier developmental stages(4,6,8 dpi).Last but not the least,the dynamically expressed genes affected by sun and ovate have more common functions,such as cell wall,cytoskeleton,transport,cell cycle,cell division,cell differentiation and so on.In the aspects of fruit ripening study,there are more and more studies pointed out the important role of abscisic acid(ABA)in regulating non-climacteric fruit ripening.With a short period of life cycle,clearly developmental stages and easy for genetic transformation,cucumber(Cucumis sativus)can be a very good candidate for non-climacteric fruit study.ABA application at the turning stage promotes cucumber fruit ripening by accelerating the degradation of total chlorophyll,decreasing the soluble sugar content and increasing titratable acid content,while ABA application at the immature stage has no significant effect on fruit ripening.Via homology cloning and transcriptome sequences analysis,genes involved in ABA metabolism,dynamic balance and signal transduction were detected and their expression profiling during fruit development and ripening and under stress treatment were studied.The results revealed that ABA content may be regulated by its biosynthesis(CsNCEDs),catabolism(CsCYP707A1)and reactivation genes(CsBGs)at the transcriptional level.In addition,according to the expression pattern and absolute expression level,CsNCEDl,CsCYP707A1 and CsBGl were highly expressed in pulp and may play more important roles in regulating ABA metabolism during cucumber fruit development and ripening.In the respect of ABA signal transduction,CsPYL2 and CsPP2C2 were highly expressed and both reached their peak value at 27 DAF,at which stage the ABA content was also at its highest level,which indicated that the CsPYL2 and CsPP2C2 may involve in transducing ABA signal in fruit and regulating fruit development and ripening.Since ABA signal also involves in the process of seed germination and stress response,the transcriptional regulation of ABA signal transduction during these processes were also studied.Results showed that most of the tested genes’ expression decreased during cucumber seed germination,which was in accordance with the ABA level variation.According to the absolute expression,CsPYLl,CsPYL3,CsPP2C5,CsABI1,CsSnRK2.3 and CsSnRK2.4 were highly expressed,indicating that they may play more important roles in ABA signaling during cucumber seed germination.The expression of CsPYLl,CsPYL3,CsABI1,CsSnRK2.3 and CsSnRK2.4 was sensitive to 120 mM NaC1 and 0.5 mM Cu2+treatments.Moreover,CsPYLl,CsPYL2,CsPP2C2 and CsSnRK2.2 were sensitive to drought stress in cucumber seedlings.To further study the role of ABA in regulating non-climacteric fruit ripening at the transcriptional level,ABA signal transduction core components and their expression in sweet cherry were detected and analyzed.Results showed that PaPYL2/3,PaPP2C3/4/6 and PaSnRK2.4 were highly expressed at the early stages of fruit development while PaPYL2,PaPP2C3/4,PaSnRK2.4 also had a high expression level at the onset of fruit ripening.Exogenous ABA treatment at 28 days after full bloom decreased the IAA level and promoted fruit ripening by increasing anthocyanin and soluble solids content.Most of the PaPYLs and PaSnRK2s didn’t strongly respond to ABA and IAA treatments at 28 days after full bloom,while expressions of PaPP2C3,PaPP2C5 and PaPP2C6 were significantly induced by exogenous ABA;PaPYLl and PaPP2C3 were significantly induced by exogenous IAA.