| Powdery mildew can occur in cucumber(Cucu,mis sativus L.)C|iltivation areas all over the world.The photosynthesis of leaves was weakened at the onset of disease,which affected the yield and quality of fruit.The cμltivation and utilization of disease-resistant varieties are the fundamental means to reduce the harm of the disease.In the early stage of this study,high powdery mildew resistant inbred line Jin5-508(long vine)and susceptible inbred line D8(short vine)were crossed,and D8 was used as recurrent parent for 12 generations.Short vine plants were selected for each backcross population according to field phenotype.Molecular marker was used to assist in screening resistant plants,and a chromosome fragment substitution line SSL508-27 was obtained,which is highly resistant to powdery mildew,has a background of D8.Genome-wide re-sequencing of D8 and SSL508-27 showed that SSL508-27 had a chromosome substitution fraglent on Chr5,which came from Jin5-508,and the length is 6.81Mb.In view of the similar genetic background between SSL508-27 and D8,the difference between them is the root cause of the high resistance of SSL508-27 to powdery mildew.Therefore,SSL508-27 contains the major-effect QTL P,rm5.1 related to powdery mildew resistance.In order to shorten the interval and obtain powdery mildew resistance genes,we used SSL508-27 and D8 as parents to construct secondary F2 isolation popjilation,developed fragments of SNP-dCAPs on both sides with resequencing information,and screened recombinant plants.To exclude the influence of environmental factors on quantitative traits,F23 falilies were prepared for phenotypic identification.Molecjilar markers(SNP and InDel)showing polymorphism among parents were used to identify the genotype of the recombinant strain and to fine mapping the Pm5.1 by combining phenotype.qRT-PCR was used to analyze the differential expression of genes in final region at different time of inocμlation and among different resistant and susceptible strains.The main results are as follows:1.According to the re-sequencing information,SNP and InDel markers were developed in the disease-resistant region.Recombinant F2(29 plants)were genotyped,and Pm5.1 was located to 98.4kb by combining the phenotypes of F:3.The two flanking makers were InDe101 and SNP I6,996,485 respectively.It contained seven candidate gcncs:Csa5AM484660.1,Csa5M4846S0.1,Csa5M485180.1,Csa5M4H5690.1 whose functions were unknown,Csa5M484650.1 encoded histone deacetylase complex,Csa5M484670.1 coding class.Anchor-like repeat family protein,Csa5M485190.1 coding disease-resistant protein.2.qRT-PCR was used to detect the relative expression trend of seven candidate genes in different time periods after mocμlation with powdery mildew.In SSL508-27,the relative expression of Csa5M485190.1 began to increase after inoculation with powdery mildew,and reached the highest level at 48 h.The expression in D8 was low and unchanged before and after inoculation with powdery mildew.In the other six candidate genes,the expression was irregular,and it was higher in most resistant strains than in most susceptible strains.The highest relative expression in donor parents Jin5-508.On this basis,Csa5M485190.1might be the key candidate genes for PMR in cucumber.3.By comparing the sequences of the key candidate gene Csa5M485190.1 among parents,it was found that there was a non-synonymous mutation site at 16,957,504 which caused asparagine to threonine.This non-synonymous mutation site may be one of the main reasons for the high resistance of SSL508-27 to powdery mildew.The discovery of powdery mildew resistance genes in cucumber will lay an important theoretical foundation for understanding the molecμlar genetic mechanism of powdery mildew resistance in cucumber,carrying out powdery mildew resistance molecular narker-assisted breeding,cloning of powdery mildew resistance genes and breeding of new powdery mildew resistance varieties. |
