Response of sorghum lines carrying recently identified brown midrib (bmr) mutations to stalk rot pathogens and water deficit

Sorghum (Sorghum bicolor) is drought-tolerant and has diverse germplasm for food, feed, forage and bioenergy. However, stalk diseases reduce quality and yield of biomass and grain, especially under drought. Previously, brown midrib (bmr) mutations in monolignol biosynthesis were shown to reduce lignin content and alter composition but were not more susceptible to stalk diseases than wild-type lines. Recently characterized bmr mutations were shown to affect flavonoid biosynthesis (chalcone isomerase; bmr30-1) or 1-carbon metabolism (folylpolyglutamate synthase; bmr19-1107, bmr19-1168 and bmr19-1937). Two other mutations, bmr29-1 and bmr31-1, have not yet been characterized. The six mutations were incorporated into elite genetic backgrounds (RTx430, BTx623 and BWheatland) to develop near-isogenic lines containing each mutation. Using peduncle inoculations with Fusarium thapsinum and F. proliferatum (Fusarium stalk rot) and Macrophomina phaseolina (charcoal rot) under well-watered conditions, most bmr lines were at least as resistant as the corresponding wild type, except for RTx430 bmr19-1937 that had significantly longer mean lesion lengths when inoculated with M. phaseolina. Based on significantly reduced lesion lengths following inoculations with F. proliferatum and F. thapsinum, respectively, bmr29-1 and bmr31-1 lines were screened using basal stalk inoculations under well-watered and water-deficit conditions. The bmr lines were at least as resistant as the corresponding wild-type lines. Wild-type BTx623 was highly susceptible to M. phaseolina under water deficit, but near-isogenic bmr29-1 and bmr31-1 lines had significantly shorter mean lesion lengths. Incorporation of these mutations can increase resistance to stalk pathogens in cultivar and hybrid development for feed, bioenergy and production of biomass-based green chemicals.