Study On The Mechanism Of 2,4-D And CPPU Induced Parthenocarpy In 'Dangshansu' Pear

Most pears are naturally self-incompatible,and the fruit set is susceptible to ad verse weather conditions resulting in fruit drop,which seriously restricted the increa se of pear yield.Parthenocarpy could produce mature seedless fruit without pollinati on or fertilization.Studying the parthenocarpy of pear is helpful to solve the proble m of poor pollination and fertilization in pear production.To explore the effect of2,4-Dichlorophenoxyacetic acid?2,4-D?and N-?2-chloro-4-pyridyl?-N'-phenylurea?CP PU?on parthenocarpy of‘Dangshansu'pear,we sprayed them at blooming stage.T he main results are as followed:1 2,4-D-induced parthecocarpy was mediated by enhancement of GA₄biosynthesis in‘Dangshansu'pear.In this study,we found exogenous 40mg L^-1 2,4-D treatment to unpollinated flowers could induce parthecocarpy in‘Dangshansu'pear.The fruit rate reached 71.11%in 2,4-D treatment group,and the mature seedless fruit could be obtained.But when 2,4-D co-treated with the gibberellin inhibitor-PAC,the young fruit fell off at 40 DAA,and could not get matured seedless fruit.Fruits in 2,4-D treatment group were significantly bigger than in pollination group at 4 DAA,and the cortex thickness was significantly higher than that of the unpollinated ovary.The cell area and cell layer of the ovary were also significantly increased after 2,4-D treatment.While PAC would weakened the effects of 2,4-D.After 2,4-D treatment,the content of IAA and GA₃ were significantly decreased,while the GA₄ content significantly increased.Compared with 2,4-D treatment group,there was no significant difference in the content of IAA and GA₃,but the content of GA₄ was significantly decreased.To support the histology and hormone data,we analyzed the relative expression levels of several genes related to cell development and hormones.Hand-pollination only significantly upregulated the cell expansion gene Pbexpansin-A10 at4 DAA,while 2,4-D treatment could promote the high expression of Pbexpansin-A10,Pbcyclin-dependent kinase B2-2 and Pbcyclin-dependent kinase B2-2-like.When 2,4-D was supplemented with PAC,however,the expression levels of these genes were significantly downregulated.2,4-D could upregulate PbGA20ox2-like and PbGA3ox-1expression levels and downregulate the expression of PbGA2ox1-like and PbGA2ox2-like to increase GA₄ content,but PAC could suppress the effect of 2,4-D.The upregulated of PbGH3.6-like and the downregulated of PbGID1B-like indicated auxin and gibberellin signaling pathway were activated.These results showed 2,4-D induces cell division and cell expansion by promoting the biosynthesis of GA₄ and eventually leads to parthenocarpy of‘Dangshan'pear.2.CPPU could induce gibberellin-independent parthenocarpy in‘Dangshansu'pear.Spraying 20 mg L^-1 CPPU treatment to unpollinated flowers could induce parthecocarpy in‘Dangshansu'pear.The fruit rate reached 93.5%,significantly higher than hand-pollination group.The fruit set rate of CPPU+PAC treatment group was 94.3%.There was no significant difference in fruit set rate between CPPU treatment and CPPU+PAC treatment,both of them could obtain matured seedless fruit.Thus,we speculated that the parthenocarpy induced by CPPU may independent of gibberellin.We found CPPU treatment could significantly promote cell division and expansion,and PAC had no effect on CPPU.Whether the content of GA₃ or GA₄ did not increase after CPPU treated,but the endogenous IAA content was significantly increased,the endogenous ABA content was significantly decreased.The quantification of hormones in CPPU+PAC treatment group was similar to CPPU treatment group.In CPPU treatment group,there were 10up-regulated genes in cell expansion family and 28 up-regulated genes in cell cycle family.In CPPU+PAC treatment group,there were 14 up-regulated genes in cell expansion family and 33 up-regulated genes in cell cycle family.So CPPU could induce cell division and expansion by promoting high expression of cell expansion and cell division family genes.Only a few gibberellin-related genes changed after CPPU treatment.There were 35auxin-related DEGs in CPPU treatment group,and 48 auxin-related DEGs in the CPPU+PAC treatment group.CPPU could significantly up-regulated the expression of the auxin synthesis gene YUCCA,which was consistent with the high level of endogenous IAA content in ovary.In addition to auxin,ABA,as an important plant hormone,also plays a key role in the growth and development of plants.The transcriptome data showed CPPU could significantly inhibit the expression of NCED,a key gene in ABA biosynthetic pathway.There were 19 and 33 ABA-related DEGs in CPPU and CPPU+PAC treatment groups respectively,indicating that ABA pathway was also activated.MADS-box family gene is a negative regulatory transcription factor and plays an important role in fruit set.There were several MADS-box genes down-regulated after treatment with CPPU and CPPU+PAC.These results suggest that CPPU may induce cell division and expansion by promoting the biosynthesis of IAA and inhibiting the biosynthesis of ABA,and at the same time inhibiting the expression of MADS-box family-related genes leading to parthenocarpy of‘Dangshansu'pear.