Molecular Analysis And Location Of Related Resistance QTL Of Rice Blast Fungus In The Southern US

The rice blast resistance gene Pi-ta has been effectively deployed in the Southern US rice germplasm for over two decades. Pi-ta is effective in preventing infection of blast fungus strains carry the corresponding avirulence gene AVR-Pital in a gene-for-gene fashion. In the present study, we examined genetic makeup of the field isolates of rice blast fungus from rice cultivars with or without Pi-ta to predict the functionality of the Pi-ta gene in the southern US. A total of169field isolates from1996and2009collections were analyzed using an international differential system, pathogenicity assay, rep-PCR and DNA sequencing of the avirulence gene AVR-Pital. These isolates were determined to belong to the races IA1, IB1, IB17, IC1, and IC17of M. oryzae, and were classified as15distinct groups within a major rep-PCR pattern in each group. Most of the fungal isolates of the races IA1, IB1, IB17, and IC17were unable to infect the Pi-ta-containing rice cultivar Katy suggesting that Pi-ta was effective. One AVR-Pital allele was consistently amplified by AVR-Pital-specific primers from78avirulent fungal isolates as expected; however, two to three AVR-Pital alleles were amplified in each of other12avirulent fungal isolates verifying again the effectiveness of the Pi-ta gene. Through sequencing and comparing the AVR-Pital gene that amplified, we found10different AVR-Pital alleles, in which6of them were same with those found before, and4of them were new.We separated all the blast isolates into two types according if there was Pi-ta gene and did the Chi square independent statistics test to the relation between the advent of virulent isolate and rice with or without Pi-ta gene. The test demonstrated that there was a significant difference of the occurrence of the virulent isolates between isolates from Pi-ta and non-Pi-ta cultivars, and Pi-ta gene was related to the occurrence of the virulent isolates. However, most isolates from the race IC1were virulent to Katy suggesting that some virulent isolates overcoming Pi-ta resistance have been adapted to the Southern US environment.10blast isolates that belong to IA1, IB1, IB17and IC1respectively were used to examine the blast resistance QTL. Finally,6blast resistant Quantitative trait loci (QTLs):qBLR8, qBLR10-1, qBLR10-2, qBLR10-3, qBLR12-1and qBLR12-2were identified on the chromosome8,10and12respectively with the recombinant inbred lines population that inherited from the hybridization between Lemont (partial resistant) and Jasmine85(resistant), which included total227lines. qBLR10-3was the new QTL that identified in this study, and the other5QTLs have been reported in the previous research. qBLR8、qBLR12-1and qBLR12-2were from Jasmine85, but qBLR10-1, qBLR10-2and qBLR10-3were from Lemont。The phenotype variance contribution of QTL was from5.37%～39.18%. qBLR12-2was the most widely resistance to the blast isolates in this study, especially to the new isolates belong to IA1and the dominant races IB land IB17that was just identified recently, but for another one, IC1, there was no resistance. However, qBLR10-1, qBLR10-2and qBLR10-3show part resistance to IC1. The blast resistance QTLs that identified from commercial rice Lemont and Jasmine85in this study can provide reference to rice breeding for blast resistance.