Mechanisms Of Plant Growth Regulators Induced Parthenocarpy Of Cucumber

Cucumber(Cucumis sativus L.)is an important vegetable in China and worldwide.Parthenocarpic development of cucumber fruit has long been recognized as an important characteristic in the breeding,because the fruit set and development of cucumber fruit were often affected by environmental factors such as low temperature and light and the absence of pollinating agent in greenhouse condition.To improve the fruit growth,the application of forchlorfenuron,one kind of plant growth regulators of cytokinins,is popular in the production of cucurbita crops.In previous reports,it also indicated that auxin,cytokinin,gibberellic acid(GA),brassinosteroids,and auxin transport inhibitors,could induce the cucumber development.Exogenous application of cytokinins,N-(2-chloro-4-pyridyl)-N'-phenylurea(CPPU),auxins,naphthaleneacetic acid(NAA)and gibberellic acid 4 and 7(GA4+7)could enhance the partthenocarpy.However,the physiological and molecular mechanisms under it was not clearly understood.To understand the mechanims,the following research work was applied.(1)Investigation the effect of exogenous CPPU,NAA and GA4+7 on induction of parthenocarpic cucumber fruit and to analyze their influence on fruit quality.(2)Determination of endogenous hormones in induced parthenocarpous fruits(CPPU,NAA,GA4+7,pollination),unpollinated fruits of weak parthenocarpic lines'ZK'and unpollinated fruits of cucumber line "DDX" which with high parthenocarpic ratio.(3)Analysis of the gene expression levels of cytokinin(CTK),auxin(IAA)and gibberellin(GA)biosynthesis,degradation and signaling transduction in hormone treated,pollinated and unpollinated fruits of 'ZK' and unpollinated fruits of 'DDX'.The main research results are as follows:(1)The un-pollinated flowers of 'ZK' did not develop into fruit,whereas the fruit set ratio of pollinated,100mg/L CPPU-,NAA-and GA4+7-treated flowers were highly incresed.The weight,length and transverse diameter of CPPU-,NAA-and GA4+7-treated fruits were similar to those obtained with hand pollination at 10 day after anthesis.These results indicated that exogenous CPPU,NAA and GA4+7 could effectively increase fruit set and development of cucumber.The two days after antheis is important stage for fruit development.(2)The flesh firmness of CPPU-treated fruits at harvest was higher than other treatments.NAA treatment did not affect any parameter of morphology and nutritional characteristics at harvest and after storage.GA4+7 treatment decreased the flesh firmness,but increased total flavonoids and protein content.(3)Compared with Unpollinated fruits,CPPU treatment increased the contents of IAA and GA4 in fruits 0,1 and 2 days after anthesis.NAA treatment increased the content of GA4 in fruits and decreased the content of ABA in fruits.GA4+7 increased IAA and GA4 contents in fruits.After pollination,the contents of IAA and GA4 in fruit also increased significantly.The ovaries of strong parthenocarpic cucumber'DDX' contain high concentrations of CTK(tZ,DZ,iP)and GA4,which can stimulate cell division and elongation.After flowering,the contents of CTK(mainly tZ),GA4 and SA in'DDX'fruit increased,which stimulated fruit enlargement and induced parthenocarpy of cucumber.However,the contents of CTK and GA in ovaries of ZK' lines with weak parthenocarpy were lower,and the level of ABA was higher,which led to the low fruiting ability of ZK'lines.The results showed that the treatments of exogenous plant growth regulators and pollination mainly promoted parthenocarpy by inducing the increase of IAA and GA contents in fruits,while the differences between natural parthenocarpy line'DDX'and non-parthenocarpy line'ZK'were mainly determined by the ratios of CTK,GA and ABA.(4)Cytokinin synthesis gene CYP735A2(Csa5G644580),cytokinin degredation gene LOG2(Csa6G141420)and signal treansduction gene CsRR8/9b(Csa5G434550),CsRR8/9e(Csa6G383530)and CsRR16/17(Csa5G223020)were significantly higher in parthenocarpic fruits than in non-parthenocarpic fruits.The expression of auxin signal transduction related genes AUX22(Csa7G440550?Csa3G516530)and AUX28(Csa7G378520)in parthenocarpic fruits was significantly higher than that in non-parthenocarpic fruits;GA20ox(Csa5G172270)gene had no significant difference between parthenocarpic and non-parthenocarpic fruits.