| As an economically important vegetable, cucumber (Cucumis sativus L.) is one of the most popular greenhouse plants. Fusarium wilt in cucumber, caused by Fusarium oxysporum, occurs worldwidely and can cause severe yield loss. Susceptible plants can be infected in all stages of their development. The pathogen infects the roots and then blocks the uptake of water and nutrients, killing the root and above ground tissue. Fusarium wilt is one of the three most severe diseases in cucumber production. With the scale and specialization production of cucumber, soil born diseases like Fusarium wilt are becoming more and more severe under the present production mode. So it is urgent to deal with Fusarium wilt. At present, grafting and using pestiside are main courses to control Fusarium wilt in cucumber production. But they are labour consuming or pollutive. So it is necessary to elucidate the resistance mechanism of cucumber to Fusarium wilt and the mechanism of action between cucumber and Fusarium oxysporum in developing ecologically sound strategies to overcome Fusarium wilt. In our study, a suppression subtractive hybridization (SSH) cDNA library was constructed with cucumber leaves induced by Fusarium oxysporum. And expression sequence tags (ESTs) and bioinformatics were used to analyze the expression of defence relating genes. This might set foundation for the further studies of resistance mechanisms of cucumber to Fusarium wilt and disease resistance breeding. Main conclusions of this paper are as follows:1. A suppression subtractive hybridization cDNA library was constructed with Fusarium wilt resistant cucumber cultivar (Zhongnong 13). Seedling root, picked 6, 12, 24, 36, 48 and 72 h after inoculating with Fusarium oxysporum were used as tester, and seedlings dipped with water were used as driver. And 1 400 cDNA library clones were conserved. 206 positive clones were selected and sequenced, and 188 high quality ESTs were obtained. And 161 Unigenes, including 17 Contigs and 144 Singlets, were obtained after clustering.2. Similarity analysis based on BLASTx and BLASTn softwares in GenBank was performed. 171 (96.6 %) of the ESTs could find encoding putative proteins. 6 (3.5 %) of the ESTs were found with no significant similarity and they might represent new genes or high variant non-coding regions of cDNAs. Function of 112 ESTs (63.3 %) were known, 65 (36.7 %) were unknown. The best matched protein sequences with those encoded by the 170 ESTs were from many plant species, such as cucumber, Arabidopsis thaliana, maize, Ricinus communis, watermelon and rice.3. DnaJ, WD-repeat protein, serine/threonine protein kinase, ubiquitin-conjugating enzyme, ankyrin repeat-containing protein, CBF4 transcription factor, NAC domain protein and HD-ZIP protein genes were supposed to function as signal transduction and transcription regulating genes in cucumber after inoculated with Fusarium oxysporum. Peroxydase, 2-Cys peroxiredoxin, cytochrome P450, enzyme of the cupin superfamily, Clp protease, cysteine protease and Hsp70 might be involved in the main resistant course of cucumber. Secondary metabolism relating genes also appeared, such as cinnamate-4-hydroxylase, cinnamoyl-CoA reductase, zeaxanthin epoxidase and cellulose synthase genes. Some abiotic stress induced genes, including dehydration-responsive protein and metallothionein-like protein coding genes, also occurred several times, which indicated that there was crosstalk between biotic and abiotic resistance. Photosynthesis and energy metabolism relating genes were also included in acquired ESTs, indicating primary metabolites, and they might function in cucumber resistance to Fusarium wilt.4. All of the acquired ESTs were analyzed with Gene Ontology in cellular component, molecular function and biological process. Results showed that sort and number of cucumber resistance relating genes induced by Fusarium oxysporum were significantly different. Genes relating to metabolic process and cellular process existed high expression, binding and catalytic molecules increased. And these led biological regulation and response stimulus enhanced; cell, cell part and organelle relating genes also expressed extensively. And these differencially expressed genes might be closely related to cucumber resistance to Fusarium wilt.
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