| The soft-rot disease caused by Erwinia carotovora subsp. carotovora (Ecc) is one of the three major diseases of Chinese cabbage. It causes severe reduction in the production of several species of plants. The understanding of the resistance mechanism is of great importance to construct plants resistant to Ecc infection. In this work, expression sequence tags (ESTs) were analyzed in the process of Ecc pathogenesis to understand the resistance mechanism at global transcriptional level.A suppression subtractive hybridization ( SSH) cDNA library was first constructed from Chinese cabbage leaves challenged with Ecc at the early stages of soft-rot disease. The Chinese cabbage cultivar Dong-Nong 905 showing medium resistance to bacterial soft-rot was used in this study. In total, 1216 cDNA clones were randomly selected and sequenced and 1107 EST sequences longer than 100 bp with good quality were then obtained. The sequence similarity analysis was done using our local BLAST software by aligning the sequences with those of Non-redundant nucleotide sequence database (NR) of GenBank, and 1058 of the ESTs' were identified encoding putative proteins. Out of the 1058 ESTs, 793 coding for functionally known proteins, 256 for the hypothetical, putative and unknown proteins, and 49 had not match with the sequences in GenBank. The best matched protein sequences with those encoded by the 793 ESTs were from 24 plant species, most of which from Arabidopsis thaliana and Brassica species, and only 0.76% were of published sequences from Chinese cabbage.After EST analysis, we supposed that calcium-mediated signal transduction system and MAPK-mediated signal transduction system play roles in Chinese cabbage's resistance to Envinia carotovora subsp.carotovora. Ethylene, jasminate and salicylic acid have been shown to play important roles in plant's disease resistance. Among the ESTs we obtained, many ESTs encode enzymes involved in the biosynthesis of ethylene, jasminate and salicylic acid, suggesting they take part in the signal transduction of cabbage's defense reaction towards Ecc, which is consistent with the results obtained in Arabidopsis thaliana. However, the interactions among various signal transduction pathways are not clear yet.In Chinese cabbage's responses to infection of Ecc four pathogenesis-related proteins, PR2, 3, 4 and 5, except for PR-1, exist in the sequenced ESTs. Among these, ESTs of endo-chitinase and class I -1, 3 glucanase have relative high copy number, 10 and 12 copies,respectively, in the 1058 ESTs. Therefore, these two enzymes play important roles in cabbage's resistance to Ecc. In addition, in the ESTs obtained, there are many ESTs for abiotic stress-induced proteins, such as drought induced protein, low-temperature induced protein, cold induced protein, heat shock protein, and dehydration induced protein. Among the 265 ESTs encoding putative proteins, 46.8% were homologous to those induced under biotic and non-biotic stresses. The result suggests that a possible common mechanism exist during the stress reaction of plants towards different environments.In the infection process of the soft rot bacteria, the Chinese cabbage accumulated many transcripts encoding the enzymes involved in the biosynthesis of secondary metabolites. Five different ESTs of the genes for lignin biosynthesis have been cloned and sequenced from the SSH cDNA library, and they all exist in more than three copies. This result suggested that the lignin play an important role in the defenses against Ecc. Cytochrome P450 has been reported to be involved in various secondary metabolisms and defense responses, and EST analysis indicated that it was abundantly expressed in the responses of Chinese cabbage to Ecc infection. Other genes related to disease resistance and defenses were also identified, such as those for ascorbate peroxidase and monodehydroascorbate reductase in vitamin C biosynthesis. Furthermore, catalase and glutathione S-transferase were also shown to be involved in the defensive responses.Some cDNA fragmen...
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