Abstract: 【Objective】 In order to understand the mechanism of Pi-ta resistance and provide a basis for the development of more durable resistant varieties, the relationship between the genetic diversity in 3'-UTR and the resistance function of the Pi-ta gene were studied. 【Method】 A total of 137 Yunnan rice landraces were collected. DNA from seedling at the three- to four-leaf stage was extracted and the DNA coding sequences of Pi-ta gene 3'-UTR were amplified with designed primers in Yunnan rice landraces which possess high genetic diversity. The sequence from functionally key nucleoside 6 640 to the stop codon 6 675 was also amplified. Through bidirectional sequencing, DNA sequences in 3'-UTR region of all genetic varieties in Yunnan rice landraces were obtained and submitted to the GenBank. The genetic diversity of Pi-ta 3'-UTR region in Yunnan rice landraces was analyzed based on detecting polymorphic sites. The distribution of different haplotypes was analyzed in combination with the key resistance locus 6640 G in coding region. The network diagram of the relationship between different haplotypes was constructed based on maximum parsimony. 【Result】 The Pi-ta gene 3'-UTR of Yunnan rice landraces shows a high degree of genetic diversity. A total of 12 SNPs were found in the 3'-UTR with a length of 1.1 kb. The 137 rice landraces could be divided into 7 haplotypes by these SNPs. No recombination signal was found among haplotypes. The length of Pi-ta 3'-UTR coding region is 1 120 bp, 5 times more than the average length of plant gene 3'-UTR (200 bp). The content of G+C is relatively low, 40.43%, and there is no insertion or deletion polymorphism in the 3'-UTR coding region. There are many non-conservative potential polyA sites in the 3'-UTR sequence of Pi-ta, in addition, there is a very high frequency TTTT sequence in the 3'-UTR region of Pi-ta, which suggests that Pi-ta has a complex regulatory mechanism when terminate transcription. Analysis of the different transcripts of Pi-ta also showed that the 3'-UTR has complex and variable transcripts as contrasted to the coding sequence, indicating that the selective splicing of 3'-UTR may be related to resistance determination. The analysis of the haplotype distribution indicated that the 3'-UTR high polymorphism appeared only in the susceptible rice landraces, and all the resistant individuals share one haplotype. Interestingly, the only 3'-UTR resistant haplotype corresponds to the only resistant haplotype in the Pi-ta coding region, that is, the haplotype containing 6640 G is also the 3'-UTR resistant haplotype. This indicates that 3'-UTR is closely linked to the coding region, and is consistent and continuous in terms of function and selection pressure. The Pi-ta resistant haplotype range has been extended from the coding region to the 3'-UTR region, and when introducing Pi-ta into broad-spectrum-resistant varieties, it is necessary to ensure that the 3'-UTR region must have no other SNPs, all should be 3'-UTR resistant-haplotype-specific SNPs. 【Conclusion】 The 3'-UTR is closely linked to the encoding region in Pi-ta gene. In resistant rice landraces, the 3'-UTR suffered purifying selection, and maintained a single haplotype. Results of the study demonstrate that the 3'-UTR is indispensable for the resistance function of Pi-ta gene.

Key words: Oryza sativa, rice blast, resistance gene, Pi-ta, UTR, genetic polymorphism