| Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most serious diseases of rice. Xa23, a new BB resistance gene was identified from O.rufipogon. A pair of rice near iso-genic line, CBB23 (carrying Xa23) and JG30 (without BB resistance gene) was used in this study. In order to study the molecular mechanism of BB resistance and clone new genes related to BB, mRNA differential display (DD) and Resistance Gene Analog (RGA) cloning methods was used to analyze the difference of gene expression between a JG30 and CBB23.Five differentially expressed cDNA fragments were obtained by DD. Reverse Northern Blotting analysis indicated that the expression of CU10 is higher in CBB23 than in JG30 .It was also noticed that the expression of CU10 become stronger in CBB23 following 1d to 10d after inoculation with Xoo, while was no change in JG30. Sequencing and blast searching indicated that clone CU10 has 99% homology with 4 ESTs from rice leaves 6, 24 h after inoculation with Pyricularia grisea. The results suggested that CU10 is a fragment of rice genome. The expression of CU10 was strongly increased by bacterium inoculation in CBB23. It is predicted that the product of CU10 may be one of the common signal transduction components in rice disease resistance. CU10 was localized on chromosome 2 .The other 4 cDNA fragments had no significant similarity in the GeneBank, so they may be new gene fragments.A 472bp fragment was amplified by STK conserved primers based on RGA cloning methods. Sequence analysis indicated that it shares 100% homology with CU10. It was extended to 1030bp based on two 100% homologue ESTs and designated as RTK Multiple sequence alignment showed that the up stream sequences of RTK are the same as the two ESTs obtained from the 6h,24h after inoculation with Pyricularia grisea, but quite different from the unaffected control. There are no differences between the down stream seqsuences of the ESTs obtained 6h,24h after inoculation with Pyricularia grisea and the unaffected control. Full length of RTK can be amplified from the resistance line CBB23 by primers designed base on RTK, but could not amplified from the susceptible line JG30 by the same primers. This results suggested that the extension of RTK is right and RTK has some relationship with disease resistance. RTK has a perfect ORF that code proteins with signal-peptide and transmembrane region. It is predicted that the product of RTK may be a transmembrane protein with phosphorylation site.Two sorts of RGAs were obtained based on NBS conserved domains. NB1 and NB2 shared 30% homology in DNA sequence and were all localized on chromosome 11. It is known that several other disease resistance genes are localized on chromosome 11 too. Deduced amino acid sequences of NB1 and NB2 have the typical conserved domains of NBS. Blast search showed that NBland NB2 had high homology with cloned NBS-LRR resistance gene analogs (RGAs). It is vident that NB1 and NB2 belong to the class of NBS-LRR resistance gene analogs. The relationship between NB1,NB2 and disease resistance remain unknown until the full sequence of NB1 and NB2 will be understood.LR1 and LR2, two kinds of RGAs, were obtained based on LRR conserved domains. Sequenceanalysis showed that LR1 and LR2 had no significant LRR conserved domain. They may have no more effects for resistance to diseaseRT-PCR analysis was carried out to identify the expressions manners of 4 candidate Xa23 genes that were obtained from map-based cloning. Results show that all 4 candidate genes were expressed in mRNA level.
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