| Cucumber (Cucumis sativus L., 2n=14) is one of the ten major vegetables worldwide.Yet the narrow genetic variability, as well as lacking of resistant genes, makes cucumber suffering from various plant diseases, including root-knot nematode (RKN, Meloidogyne spp.)which is the most important root disease badly affecting cucumber production. In our country,Meloidogyne incognita is the main harmful nematode specie. So far, no RKN-resistant cucumber varieties have been found. Utilization of wild resistant resources is an important way to improve the quality of cucumber. Cucumis hystrix Chakr., (2n=24) is the only wild species, harboring rich sources of useful genes including the resistant to RKN, which has been hybridized successfully with cultivated cucumber. Derived from this interspecies hybridization, a series of Cucumis sativus-hystrix introgression line (IL) were harvested. An IL, which has been identified to be resisitant to RKN, provided an excellent material for cucumber-RKN resisitant breeding. Therefore, elucidating the resistant characteristic and inheritance of resistance, as well as quantitative trait loci (QTL) mapping of resistance gene to RKN of the ILs would bring comprehensive understanding of the resistant mechanism and achieve further possibility for resistant-plant breeding.A joint analysis of four-generations from crossing the RKN resistant IL 10-1,which has been purified for two generations from the IL, and susceptible CC3 was conducted to study the inheritance of RKN resistance. Besides, QTL mapping and candidate genes predicting were conducted based on the whole genome re-sequencing data. What’s more, IL10-1 was used to investigate the defense mechanism during the early infection from histopathological and hormone level.The main results are included as follows:1. Genetic Analysis of Resistance to RKN in Cucumis sativus-hystrix introgression linePurified for two generations, the IL10-1 was used to verify the resistance to RKN.According to the root-knot index (RKI) data, IL10-1 was comfirmed again to be resistant(RKI = 2.3), which indicated that the resistance to RKN in introgression line remained stable.Using the model of the major gene plus polygenes of quantitative traits,a joint analysis of four-generations from crossing the RKN resistant IL10-1 and susceptible CC3 was conducted to study the inheritance of RKN resistance in the Pailou glasshouse and Jiangpu greenhouse. The results showed the inheritance of RKN resistance in introgression line fitted the E-1 model which implied to be controlled by two additive-dominant-epistatic major genes plus additive-dominant polygenes. These indicated that the inheritance of the resistance to RKN has relative stability and consistency in interspecific introgression line. But the tendency and genetic parameters of downy mildew were different in the places. The major gene heritability of F2 population in the Pailou was 11.53%, the polygene heritability was 12.78%. The major gene heritability of F2 population in the Jiangpu field was 9.6%, the polygene heritability was 16.37%.2. QTLs Verification and Function Prediction of RKN Resistance in Cucumis sativus-hystrix introgression line based on population genome re-sequencingBase on the re-sequencing of the 122 individual of RIL population derived from the cross of RKN resistant introgression line 10-1 and susceptible CC3, a genetic map spanning 1070.71 cM with 1048 Bin-markers was constructed and used for QTL analysis. By interval mapping analysis, three QTLs were identified on chromosome 1 (1 QTL, explained 13.36%of the variation) and chromosome 5 (2 QTLs, explained 9.07% and 9.58% of the variation,respectively). After the sequence analysis of the QTL intervals, 37 genes with nonsynonymous mutations were detected, in which three genes may involved in resistant to RKN. From the result from qRT-PCR analysis, two genes (Csa5M608240, Csa5M610420),coding the leucine-rich repeat family protein, which were up-regulated in IL10-1 while down-regulated in CC3, with significant difference between the two materials,may play a leading part in early defense to RKN. However, one gene (Csa5M623410), related to structural constituent of cell wall, which expressed in the same pattern but no difference between the two materials, may not play the main role.3. The Histopathological Characteristic to RKN Investigation in Cucumis sativus-hystrix Introgression LineThe multiple resistance indexes of IL10-1 and CC3 were recorded at different days after inoculation, respectively. The results showed from 2 days after inoculation (dai), the number and size of root knot, along with the penetration rate of the nematode of IL10-1 were significantly lower than that of CC3. At 3 dai, the observation of the acidic magenta dyeing roots showed that nematode located in different areas in the root, endothelial layer in IL10-1 while in vascular area in CC3. These indicated the nematode penetrated more slowly in IL10-1 than that in CC3. What’s more, the histological changes of the two materials were observed at early infection stage. The results demonstrated that the giant cells (GCs) have been formed at 1 dai. At 2 dai, fewer and smaller GCs were observed in IL10-1 and appeared to elongate,while more and bigger GCs in CC3. And nematode was easily to be found in CC3 roots at this time point. At 3 dai, the elongated giant cells as well as surrounding cells turned to be vacant in IL 10-1. However, the GCs in CC3 developed to enlarge and full filled with cell contents.4. The Phytohormones and Relative Genes Expression Analysis of a Cucumis sativus-hystrix introgression Line at Early Response to RKNThe resistant IL10-1 and susceptible cultivar CC3 were used to analyze the changes of phytohormone concentration and relative genes expression at early infected stage (1,2,3 dai)of Meloidogyne incognita. The water treatment was set up as the control at different time point respectively. The results showed that there was significant difference of three phytohormones contents between IL10-1 and CC3. Comparing with water treatment, much lower IAA content in IL10-1 while no change in CC3; SA were increased both in IL10-1 and CC3; ACC content were incresed in IL10-1 but reduces in CC3. These implied that the decrease of IAA along with the increase of SA and ACC might have synergic action on enhance the resistance to RKN. The plant hormone related genes showed different expression pattern. Gene CsTIR1, CsAFB2, CsPAL and CsACO were up-regulated in IL10-1 while down-regulated in CC3. Gene CsARF10a, CsARF4 and CsCTR1 were down-regulated while up-regulated in CC3. Gene CsICS and CsACO were down-regulated in both IL10-1 and CC3.The study revealed the changes of phytohormone concentration and relative genes expression during the early infection in an introgression line, suggesting that root-knot nematode resistance in IL10-1 is a highly complex and dynamic response process. |
PDF Full Text Request