Further Fine Mapping Of The QTL Qblsr5a That Confers Resistance To Bacterial Leaf Streak In Rice And Analysis Of Its Candidate Genes

Bacterial leaf streak(BLS),caused by pathogen Xanthomonas campestris pv.Oryzicola,is one of the most destructive diseases in rice.Studies have shown that BLS resistance in rice is quantitatively inherited,controlled by multiple quantitative trait loci(QTLs)of small effect;no major resistance genes against BLS have been found.Identifying the quantitative resistance loci(QRL)conferring the resistance to BLS in rice is very important for the understanding of the mechanism of quantitative resistance and for the breeding of resistant rice varieties.So far,13 QTLs conferring the resistance to BLS have been mapped in rice.Among these QTLs,qBlsr5a has a relatively larger effect on the resistance.Subsequent studies confirmed the existence of qBlsr5a and fine mapped the target QTL in a small region on chromosome 5.To confirm the previous result and further fine map qBlsr5a,in this study,a large secondary F2 population with 9498 plants was constructed from a cross between the susceptible variety H359 and its chromosome segment substitution line(CSSL)H359-BLSR5A,which had the genetic background of the susceptible parent H359 but carried a segment harboring qBlsr5a from the resistant parent Acc8558.Fifty-nine recombinants were identified among the F2 plants using flanking molecular markers and allowed to self-pollinate.In the F3 lines from the F2 recombinants,homozygous recombinant plants were identified according to the flanking markers so that a set of overlapping sub-CSSLs were developed.By genotyping the sub-CSSLs with molecular markers covering the qBlsr5a region and phenotyping the sub-CSSLs with artificial inoculation,the interval of qBlsr5a was reduced to 78.3 kb,and the previous mapping error was corrected.By searching relevant rice genome databases,there are 10 annotated putative genes in the 78.3 kb region where qBlsr5a was located(LOC_Os05g01690,LOC_Os05g01700,LOC_Os05g01710,LOC_Os05g01730,LOC_Os05g01750,LOC_Os05g01760,LOC_Os05g01770,LOC_Os05g01780,LOC_Os05g01790 and LOC_Os05g01810).Among them,LOC_Os05g01770 is annotated for transposon protein,two genes(LOC_Os05g01690 and LOC_Os05g01790)encode unknown proteins,other seven genes encode proteins with known functions.B informatics analysis initially determined 3 of the annotated genes(LOC_Os05g01700,LOC_Os05g01710 and LOC_Os05g01810)as the most possible for the candidate gene of QTL qBlsr5a.Then,with the susceptive parent H359 and the resistant parent H359-BLSR5A as materials,we analyzed the expression dynamics of these nine annotated genes(excluded the transposon protein LOC_Os05g01770)after inoculation with BLS pathogen using qRT-PCR.Results indicated that:all genes showed expression in the two varieties,eight of them did not show respond to the infection of BLS pathogen,and LOC_Os05g01700 show downregulated respond to the infection of BLS pathogen and exhibited differential expression between H359 and H359-BLSR5A.Bioinformatics analysis showed that LOC_Os05g01700 has the conserved domains of ATP-binding cassette(ABC)protein family,it has two nucleotide-binding folds and two transmembrane domains(TMD).It is knowed that ABC transporters are involved in transporting many kinds of substrates in living organisms and closely related to some important biological processes,involved in plant diseases resistance.Binformatics and expression dynamics analysis of the annotated genes demonstrated that:LOC_Os05g01700 should be the candidate gene of qBlsr5a.In conclusion,the present study demonstrating an efficient approach for fine mapping minor QTLs,and the result will facilitate the map-based cloning of the target QTL.