Discovery And Fine Mapping Of Southern Rust Resistance Gene In Maize

Maize?Zea mays L.?is a major crop,and its safe production affects national food security in China.Southern corn rust?SCR?,an airborne disease caused by Puccinia polysora Underw.,can severely reduce the yield and quality of maize.As a result of susceptible varieties deploying and climate change,SCR has been widely found in tropical and subtropical and Huang-huan-hai corn growing areas.The most economical and effective method for controlling SCR is the use of resistant varieties.Screening resistant resources,mining and mapping resistance genes or quantitative trait loci?QTLs?are the basis for resistance breeding.In our study,some maize germplasm was screened for resistance to SCR.Then,using recombinant inbred lines?RIL?derived from a cross between susceptible inbred line Ye478 and resistant line Qi319 in combination with their high-density genetic map,we located quantitative trait loci?QTLs?against SCR,and the chromosome segment substitution lines?CSSL?constructed with Qi319 as the donor parent and Ye478 as the recurrent parent were used to further map and verify the QTL resistance to SCR.The main results were obtained as follow:1.Identification and evaluation of maize germplasm resources:A total of 337 introduced maize germplasm resources were screened for resistance to SCR via artificial inoculation in the field.Fifty-seven resources were reisitant?R?to SCR.There were 105 moderately resistant?MR?,95susceptible?S?,and 77 highly susceptible?HS?accessions,respectively.The results showed that there existed relatively abundant resistant resources among these introduced accessions,which will provide basic materials for SCR-resistant breeding.2.QTL mapping analysis for SCR resistance using a RIL population with high-density genetic linkage map construction:A RIL population was bred using Qi319 and Ye478 as parents.Genome resequencing was performed among a RIL population containing 300 lines,which and their parents.A total of 88,268 SNPs were developed,of which 4,183 polymorphic SNPs between parents were finally selected to construct the linkage map.A high-density map covering 10 chromosomes was constructed for QTL mapping in RIL population.We located five quantitative trait loci?QTLs?against SCR on chromosomes 3,5,6,9,and 10,respectively.Each QTL could explain 2.84%to 24.15%of the total phenotypic variation.qSCR6.01,detected on chromosome 6,with the highest effect value,was a stably major resistant QTL.The physical distance of qSCR6.01 was 75.30?78.25 Mb based on the B73RefGen?v3 sequence.3.Selection of suitable CSSLs and molecular markers:According to the rich genetic variation between Qi319 and Ye478,201 polymorphic makers between parents uniformly distributed on 10chromosomes,including 55 InDels and 146 SSRs,were selected from the universal primers in the database of MAIZE GDB and the entire genome sequenced information of the two parents.After generating CSSL population with elite inbred line Qi319 as the donor and Ye478 as the receptor,we performed marker-assisted selection based on 201 polymorphic markers,uniformly distributed on the chromosome to obtain 198 CSSLs?BC₅F₃?covering the complete maize chromosome.These CSSLs with the average background recovery rates was aboved 96%,which eliminated the influence of genetic background.4.Fine mapping of the major QTL qSCR6.01 using a CSSL population:Six CSSLs covering chromosome 6 were selected for mapping and verification.According to phenotypic values,CL183 was significantly different from the susceptible parent?P=0.0038?,which indicated CL183 carried qSCR6.01,and the replacement fragments were located between umc1133 and Y6q80.The hybrid progeny of CL183 and Ye478 were selected to narrow down the target segment.The target segment was finally located between InDel markers Y6q78 and Y6q79 at a physical distance ranging from 78.9 Mb to79.6 Mb.There were 12 genes in this region,among which GRMZM2G041822 was a gene related to the expression of Ser/Thr protein phosphatase family protein,and it was regarded as a candidate gene for resistance to SCR.qSCR6.01 was a newly identified major QTL resistance to SCR on chromosome 6 for the first time,which provided important gene resource and genetic information for breeding resistant varieties.