Preliminary Genetic Studies On Resistance To Powdery Mildew In Cucumber

Cucumber is grown throughout the world and is the fourth most important vegetable crop after tomato, cabbage and onion. It has also become an indispensable asset to both farmers and traders in China because it is a major source of income to them. However, cucumber cultivation in China is beset with many problems mainly biotic and abiotic stresses. Powdery mildew is a biotic stress and one of the most serious diseases in cucumber which occurred frequently in greenhouses and open fields. It would not only reduce the photosynthetic efficiency of plants, but also impact the quality and the yield of fruits. Therefore, powdery mildew resistance breeding of cucumber has become one of the main objectives at present. In order to obtain the genetics and molecular data concerning powdery mildew resistance of cucumber, we carried out this study focused on the following objectives:1. To develop SCAR markers linked to powdery mildew resistance for selection of the near-isogenic lines in cucumber.2. To map quantitative trait loci (QTL) associated with powdery mildew resistance in cucumber in a set of188F2lines derived from the cross between two cucumber lines JIN5-508(resistant) and D8(susceptible).3. To study the expression changes of WRKY and MYB genes during infection with powdery mildew in two cucumber lines JIN5-508(resistant) and D8(susceptible).The main results are as follows:1.170F2plants from a cross of the cultivar D8(susceptible) and JIN5-508(resistant) were generated to screen the powdery mildew resistance of cucumber. A combination of bulked segregant analysis (BSA) and ISSR methodology was employed to identify markers linked to powdery mildew resistance in cucumber. One ISSR marker UBC809was identified as being tightly linked to the powdery mildew resistance. Based on the sequence information of the coloned flanking sequence of marker UBC809, one SCAR marker was developed and designated as S2F+S2R. Then, the SCAR marker was used to screen14resistant backcross lines which were produced by six generations of backcrosses between D8and JIN5-508. Specific band amplified by S2F+S2R was found in each individual of the line97*92-2, and the disease index of this line showed the lowest value among all14lines. Therefore,97*92-2could be identified as the near-isogenic line with powdery mildew resistance, and could be useful for the development of cucumber cultivars with powdery mildew resistance and facilitate the cloning of genes responsible for powdery mildew resistance.2. Based on a population of188F2families derived from a cross of the cultivar D8(susceptible to powdery mildew) and JIN5-508(resistant to powdery mildew) of cucumber, a genetic linkage map was constructed and QTLs related to the powdery mildew was mapped, In this map, a total of65markers including ISSR and SRAP markers were clustered into7major linkage groups, spanning a total of831.6cM, with an average distance of12.7cM. The minimum distance of markers intervals is4.8cM, and the maximum is22.3cM. The association of genetic markers and cucumber powdery mildew resistance was investigated. By using of composite interval-mapping approach,2QTLs associated with powdery mildew resistance were found, which located on the third linkage group, and they explained7.6%and13.5%of the trait variation, respectively. The results would be useful for the marker assisted selection for the cucumber with high powdery mildew resistance.3. The expression change pattern of three transcription genes (WRKY30, WRKY6and MYB) in two cucumber lines with contrast powdery mildew resistance (resistant line ’JIN5-508’ and susceptible line ’D8’) were investigated during the infection process with powdery mildew by using real-time quantitative PCR (RT-PCR). Gene expression analysis during different time points revealed that the response of WRKY30, WRKY6and MYB in the susceptible line ’D8’ was earlier than it in the resistant line ’JIN5-508’. The expression in the line D8increased rapidly at time points8h,8h and4h hours for the three genes respectively, while in the resistance line JIN5-508it increased rapidly at time points16h,72h and16h respectively. This result suggests that the susceptible line D8responeded as early as possible in order to maintain growth and enhance powdery mildew resistance by regulating related genes expression.