Narrative
A sorghum [Sorghum bicolor (L.) Moench] carbon-partitioning nested association mapping (CP_NAM) multiparent population was developed at Clemson University, SC, using 11 diverse founder accessions, each crossed with a recurrent female parent ‘Grassl’. During the development of the CP_NAM population, the panicles in each generation (F1 to F6) were bagged to ensure selfing and to avoid outcrossing. The parental lines of the CP_NAM were judiciously selected from the Sorghum Bioenergy Panel (BAP) based on carbon-partitioning regimes that make the CP_NAM an ideal genetic resource for dissecting a wide range of traits related to carbon accumulation and bioenergy. Grassl (PI154844) was selected as the recurrent parent due to its ability to accumulate significant biomass and fermentable carbohydrates per unit time and area. The CP_NAM includes all five botanical races (bicolor, caudatum, durra, guinea, and kafir) and the four major sorghum types (cellulosic, forage, grain and sweet). All the 12 parental accessions (PI154844, PI22913, PI586454, PI297130, PI506069, PI508366, PI510757, PI329311, PI563295, PI229841, PI297155 and PI655972) were obtained through the Agricultural Research Service-Germplasm Resources Information Network (ARS GRIN) (http://www.ars-grin.gov). A set of 11 recombinant inbred line (RIL) families were maintained that consists of 2484 (F6) individuals. Each RIL family contained a minimum of 193 individuals (CP_NAM01) and maximum of 287 individuals (CP_NAM06). The parental lines of the CP_NAM were phenotyped for various traits segregating for anthesis date, stalk weight, leaf weight, panicle weight, juice weight, brix, wet weight, dry weight, total weight, plant height, acid detergent fiber, neutral detergent fiber, non-fibrous carbohydrates, lignin, and water-soluble carbohydrates. Recently, all 11 RIL families of the CP_NAM were used to generate a high-quality single nucleotide polymorphic (SNP) genetic resource using genotype by sequencing (GBS) as well as phenotypic resources. The combination of genotypic information along with phenotypic data were used for the genetic characterization of the CP_NAM and to explore the range of phenotypes controlled by carbon-partitioning dynamics. The complete set of the CP_NAM was used to identify quantitative trait loci (QTLs) associated with pericarp color which segregates for red, brown, yellow and white. Currently, a partial set of 110 selected diverse RILs of each CP_NAM were used for QTL mapping and Genome Wide Association Study (GWAS) for various biomass and its compositional traits that includes anthesis date, wet weight, dry weight, total weight, plant height, acid detergent fiber, neutral detergent fiber, non-fibrous carbohydrates, lignin, and water-soluble carbohydrates. This population is a unique resource for researchers to study the contrasting carbon-partitioning traits in sorghum to better understand the genetic variation underlying carbon fixation, translocation, and utilization.