Primary Studies On Molecular Mechanism Of Inducing Resistance Of Attenuated Strains Of Cladosporium Cucumerinum In Cucumber

Resistance induced by biological factors is very important to a host plant. The study of resistance mechanism induced by fungi and bacteria lagged behind that of virus. Cucumber scab, caused by Cladosporium cucumerinum, is one of the most important diseases in protected cucumber in North China. In this study, the attenuated strains of C. cucumerinum, selected by high temperature treatment, were used to study the mechanisms of low-pathogenicity and induced resistance in cucumber. Genomic changes and differential expression at different time of C. cucumerinum attenuated strains were studied in cucumber by use of AFLP system. The results are:1. The attenuated strains of C. cucumerinum had stable low-pathogenicity by phenotype identification and significantly enhanced resistance to cucumber scab, which controlled the disease up to 77.01%.2. Spores of the attenuated strains, 2R and 3R of C. cucumerinum, were smaller than the wild ones. The spores were regular, elliptical or globose, and those of the wild ones varied from elliptical, globose, ovoid, to erose.3. The variation of C. cucumerinum attenuated strains was detected efficiently in the DNA level by AFLP technique. The genome changed significantly after treatment at 50℃. 2910 and 2216 fragments were collected after amplification of 2R/2Q and 3R/3Q (2Q and 3Q are wild strains) with 256 pair of primers, respectively. The genetic similarity of 2R/2Q and 3R/3Q were 24.81% and 43.68%.4. The PCR system for cucumber cDNA-AFLP was screened and optimized. The time sequence expression of resistance-related genes induced by the attenuated strains was analyzed in cucumber. Differential expression fragments appeared during 9 time points after treatment. A series of regulation initiated by the tested strains began at 3hr or earlier after the treatment and reached peak at 6hr, the change wasn't distinct between 9hr and 24hr, and the number of fragments significantly reduced after 24hr.5. Four specific DNA sequences verified by northern blot were acquired. Blast analysis showed that the 4 fragments had high homogeneity to those resistance-related encoding genes, including putative ATP synthase gamma chain of Pyrus communis, resistance gene of rye R1-5, salicylic acid Glucosyltransferase from Arabidopsis thaliana, phospholipase C.In conclusion, the results above provide a solid base for the advanced study on genetic characteristics, low-pathogenicity mechanism, induced resistance mechanism, and resistance-related gene clones of attenuated strains of C. cucumerinum. This study showed the potential to develop fungi and bacteria into plant vaccine against pathogens.