Early-Generation Evaluation of Marker-Assisted Transfer of bmr6 and bmr12 Alleles into Sweet and High-biomass Sorghum

Abstract

The effects of transferring brown midrib (bmr) alleles in sweet and high biomass sorghums (SS/HBM) are not fully understood, and available works produced mixed results depending upon the genetic background of the recurrent parent used. Early-generation testing of the effects of the two mutations in the background of the recurrent parents can help predict the final products and make appropriate early go/no-go breeding decisions, yet, this information is missing. The present study was therefore carried out to reduce these gaps. Marker-assisted transfer of the bmr6 and bmr12 was performed using four (SSV84, ICSB474, ICSV18003, ICSV100324) SS and one (ICSV15024) HBM wild type (WT) lines as recurrent parents, and one bmr6 (N609) and two bmr12 (ICSV101039 and N600) lines as donor parents. Crossing and backcrossing techniques were used to transfer the bmr alleles to develop BC2F2 and F3 populations. Kompetitive allele-specific PCR (KASP) marker genotyping assays were performed and implemented for the indirect selection of the bmr transfer in F1 and BC_F1 populations. While the bmr6 markers were polymorphic, the bmr12 markers were monomorphic. The successful bmr6 transfer was therefore confirmed by markers in F1 and BC_F1, while for bmr12 the transfer was confirmed after segregation assessment in F2 and BC_F2 populations. The transfer of bmr12 improved the cellulose content relative to WT genotypes, and bmr12 populations were shorter-statured relative to bmr6 individuals, meaning that bmr12 gene can be considered in sorghum breeding to boost 2G bioethanol bioconversion and to control lodging, and hence favouring mechanical harvesting and limiting yield losses. The observed earliness in the bmr lines is an attractive trait conferring adaptation in the drylands, particularly in the world’s semi-arid tropics characterized by terminal drought stress. Both bmr alleles did not show any negative trade-offs relative to the must-have sorghum plant characteristics, implying that they can safely be deployed in sorghum breeding for bioenergy production purposes. The observed superior bmr lines with farmer-preferred phenotypic acceptance are candidates not only for advancement in breeding pipelines but also for subsequent pyramiding bmr6 and bmr12 in the same genetic background, further improving the bmr-conferred benefits. The successfully used molecular markers will be uploaded in the public domain and can help breed sorghum products other than SS/HBM such as high-quality forage sorghums.