Genetic Analysis And QTL Mapping Of Hypocotyl Length Of Cucumber Seedlings Under High Temperature Stress(Cucumis Sativus.L)

Cucumber(Cucumis sativus L.)is an important vegetable crop,which creates great economic benefits for the annual production and supply of vegetables in China.Native in the warm region,cucumber is not adapted to high temperature.In the high temperature season of southern China,high temperature stress has become one of the main factors limiting the high quality and yield of the cucumber production.The hypocotyl length is one of the most important agronomic character in cucumber 's seedling stage.Hypocotyl elongation in different degrees will lead to the reduction of cucumber yield by more than 50%in the early stage.The breeding of new cucumber varieties whose hypocotyl is not easy to elongate under high temperature is an effective way to solve these kinds of problems.In this study,'GY2'(with less hypocotyl elongation at high temperature)and ' Dabai '(with large hypocotyl elongation at high temperature)were used as parents to build F1 and F2 segregation generations.The genetic characteristics of hypocotyl length of cucumber at high temperature were studied by using the combined genetic model analysis of major gene and polygene,high throughput genome resequencing technique and BSA biological information analysis method.The QTL,which controls the length of cucumber hypocotyl under high temperature,was mapped and analyzed.The gene function was annotated,and the relative expression of candidate genes under different high temperature stress time was measured by qRT-PCR in order to verify the function of candidate genes.The results of our study are shown as follows:1.The inheritance of hypocotyl length traits in cucumber at high temperature was according to the E-0 model,which means the hypocotyl length under high temperature was controlled by two pairs of additive-dominant-epistatic major genes and additive-dominant-epistatic polygenes.The inheritance of traits in F2 segregation generation was mainly controlled by major genes,and was greatly affected by environmental conditions.2.Based on the results of BSA analysis,QTL named HL1.1 was found on Chr.1,which controlled the hypocotyl length of cucumber at high temperature.Then the genetic population was expanded for fine mapping,the HL1.1 was reduced to~414 kb region.25 genes including 19 functional genes in the interval were annotated.3.The relative expression levels of 19 candidate genes obtained by fine mapping were determined by qRT-PCR at 0 h,6 h,12 h and 24 h after high temperature treatment of'GY2' and'Dabai'.The results showed that the expression of 10 genes(CsalG495280.1,CsalG496290.1,CsalG495290.1,CsalIG497310.1,CsalG499320.1,CsalG499330.1,CsalG501840.1,CsalG502350.1,CsalG502360.1 and CsalG505920.1)was up-regulated in the treatment group(compared with CK)between the two parents.It may play an important role in regulating the growth and development of hypocotyl under high temperature.The expression of CsalG505920.1 and CsalG497310.1 genes in 'GY2' was significantly higher than that in the control(not significant in 'Dabai1).Therefore,it was speculated that these two genes might be the key genes to regulate the hypocotyl elongation of cucumber under high temperature.