The Mechanism Of GA₄₊₇ Induced Parthenocarpy In 'Dangshansu' Pear

‘Dangshansu'pear was selected as material in this study,bagging flower buds before anthesis,then applying gibberellin(GA₄₊₇)to bagged flowers at anthesis to induce parthenocarpic pears.Applying morphology,histology,hormone levels and transcriptome analysis to explore the mechanism of GA₄₊₇ induced pear parthenocarpy.The main results are as follows:1.GA₄₊₇could induce pear parthenocarpy.Exogenous application of GA4+7 or a combination of GA₄₊₇ and 6-BA to unpollinated flowers at anthesis,fruit set rate could up to even higher than hand pollination?86%?,indicating exogenous application of GA₄₊₇ to unpollinated flowers could induce parthenocarpy.Application of GA₄₊₇ 75 mg L^-1 alone or GA₄₊₇ combined with 6-BA to unpollinated flowers induced parthenocarpic pears formed larger fruits,soluble solid contents and fruit shape index were increased.Moreover,Fruit firmness and fruit core diameter were significantly lower than seeded pears.GA₄₊₇ combined with 6-BA treatments showed better effects than GA₄₊₇ alone,exhibiting bigger fruits,higher soluble solid contents and lower fruit firmness.2.GA₄₊₇stimulates fruitlets growth and division,resulting parthenocarpy,moreover,GA₄₊₇ repressed the formation of absciss layer in sepal,and thus mature fruits with sepals.Hand-pollinated fruitlets formed absciss layer around the raw parts of sepals,and sepals started to drop,while GA₄₊₇ treated fruitlets did not form absciss layer,and mature fruits showed sepals persistent.Unpollinated ovaries hardly developed and became wilted at 14 days after anthesis?DAA?.Cytology observation showed that both GA₄₊₇ and pollination treated mesocarp cells expanded began at 3 DAA,while unpollinated ovaries ceased development.Unpollination exhibited wilted ovules,enlarged ventricular space,while both GA₄₊₇ and pollination promoted ovule developing and expanding.GA₄₊₇ and pollination treatments formed thicker external mesocarp with more cell layers and lager vascular bundles than unpollination.Transcriptome data analysis showed that a large amount of differentially expressed genes?DEGs?encoding cell cycle and cell expansion were modified during the process of GA₄₊₇ induced pear parthenocarpy,indicating these genes may play crucial roles in fruit set.3.GA₄₊₇changed the balance of endogenous GA,IAA and ABA.GA4+7 significantly increased GA₁₊₃ and GA₄₊₇ contents in parthenocarpic ovaries.Pollination did not change GA₁₊₃ levels,however,increased GA₄₊₇ contents,suggesting GA₄₊₇ plays major role rather than GA₁₊₃ during fruit set in pears.IAA levels did not increased after GA₄₊₇ treatments.ABA contents increased dramatically in unpollinated ovaries,while,in GA₄₊₇ and pollination treated ovaries,ABA levels were significantly lower than unpollination at same developmental stages.Transcriptome sequencing data showed that GA₄₊₇ barely induced gibberellin biosynthesis related genes expressed,while in hand pollination treatment,genes involved in gibberellin synthesis process such as CPS,KO,KAOs and GA20oxs were significantly upregulated.GA₄₊₇ significantly repressed the transcription of GID1,which is a gibberellin receptor protein,and pollination exhibited similar expression pattern.GA₄₊₇ did not promote IAA synthesis,but upregulated genes associated with auxin transportation.Genes encoding enzymes which synthesized ABA,NCED3 and NCED5 were downregulated in both GA₄₊₇ and pollination treatments,which was consistent with the results that ABA levels were lower in these two treatments than unpollination in later stages.Thus,we speculated that GA together with IAA and ABA formed a dynamic balance,which promoting fruit setting.4.GA₄₊₇induced parthenocarpy exhibited highly consistency with pollination in transcripts levels.Transcriptome sequencing results exhibited that there were 177?8.1%of the total?upregulated and 400?18.2%of the total?down-regulated genes were common to pollination and GA_4+7-treated ovaries at 3 DAA.Following fruit development,the number of common DEGs increased,with 3,029?35.6%of the total?up-regulated and 2,555?30%of the total?downregulated genes being common to the two treatments at 14 DAA.Exhibiting the highly similar developmental process during fruit setting in these two treatments.5.Photosynthesis provides necessary nutrition for fruit set,and deficient photosynthesis is a primary driver of flower,fruit and seed abortion.Our analysis showed that the DEGs involved in sucrose degradation were dramatically upregulated in parthenocarpic ovaries,as well as pollinated ovaries,suggesting that a high level of energy is demanded during fruit setting.6.Present study clarified that MADs-box family genes play roles in regulating fruit set.In our analysis,DEF was significantly repressed in parthenocarpic and pollinated ovaries,AGL9 was repressed in parthenocarpic fruits,indicating that the MADs-box family genes take part in the process of GA₄₊₇ induced pear parthenocarpy.