Bridging classical and molecular genetics of sorghum plant stature and maturity

Most initial sorghum introductions into the USA were too tall for mechanical grain harvest and flowered late or not at all at temperate latitudes. A small number of spontaneous mutations for dwarf stature and early maturity arose in farmers' fields and were quickly disseminated. Several subsequent sorghum introductions were determined by allelism tests to carry recessive alleles at the same loci. These early experiments led to the conclusion that there are four major loci for dwarfing in sorghum (Dw1-Dw4) and four major loci for maturity (Ma1-Ma4). The relatively simple inheritance of these important agronomic traits was later exploited by the Sorghum Conversion Program, which introgressed QTL for dwarfing and early maturity into hundreds of exotic lines through a strategy of backcrossing with phenotypic selection. To date, molecular work has identified just two of these classical dwarfing/maturity loci in sorghum: Dw3 encodes an MDR-class auxin efflux carrier, and Ma3 encodes a phytochrome B. In this chapter, we provide an overview of current understanding of the genetic architecture of plant height and flowering time in sorghum, including recent progress in mapping additional loci beyond the classical Dw1-Dw4 and Ma1-Ma4. We also discuss prospects for identifying the remaining major loci for height and maturity in sorghum using both linkage-and association-based methods. Most of our understanding of sorghum phenotypic variation currently comes from studies in dwarf grain sorghum genetic backgrounds in which variation for other developmental traits may be masked. Growing interest in C4 grasses as bioenergy crops is likely to necessitate study of a broader range of sorghum genetic backgrounds.