Genetic mapping of greenbug resistance loci in sorghum [Sorghum bicolor (L.) Moench] & expression analysis of candidate genes in response to greenbug infestation

Abstract

Scope and Method of Study: Greenbug is a damaging pest of sorghum in the Great Plains of the United States. This study was taken with the objective to identify the genomic regions contributing resistance to greenbug biotype I using sorghum accession, PI 607900 as the resistance source. To accomplish this objective I undertook two projects involving three independent studies. These three studies involved linkage-based mapping and gene expression studies. The first study was conducted in an F 2 mapping population consisting of 371 individuals developed from a cross of BTx623 (susceptible to greenbug) by PI 607900 (resistant to greenbug) to identify QTLs contributing greenbug resistance in sorghum. The second study was performed to identify and confirm QTLs for greenbug resistance in an intercross population developed from a previously used F 2 mapping population. The third study was carried out to examine the differential expression of candidate genes induced by greenbug using real-time PCR experiments in two contrasting parental lines.

Findings and Conclusions: I found that two major loci for greenbug resistance were on sorghum chromosome 9 from two independent QTL mapping studies. The second QTL mapping project with an intercross population revealed potential candidate genes in a narrowed confidence interval compared to first project's QTL analysis with the F 2 population. These loci were mapped to a region on chromosome 9 flanked by markers Starssbnm 93-Starssbnm 102. The QTL mapping studies identified two novel loci for greenbug resistance using 48 newly developed nuclear and genic SSRs. The identified greenbug resistance loci were linked to a receptor-like kinase Xa21 -binding protein 3, a gene known to increase Xa21 -mediated resistance in rice. Relative quantification of gene expression in the two parental lines indicated that receptor-like kinase Xa21 -binding protein 3 and map kinase phosphatase were differentially expressed upon greenbug infestation. The markers/QTLs identified in the study will have applications in MAS and mapbased cloning experiments for the improvement of greenbug resistance in sorghum.