Update on the Foxtail Millet System Foxtail Millet: A Sequence-Driven Grass Model System

Foxtail millet (Setaria italica) is a small diploid C4 panicoid crop species, whose genome is being sequenced by the Joint Genome Institute (JGI) of the Department of Energy. The rationale for sequencing foxtail millet is that it is closely related to the bioenergy grasses switchgrass (Panicum virgatum), napiergrass (Pennisetum purpureum), and pearl millet (Pennisetum glaucum), yet is a more tractable experimental model because of its small diploid genome (1C genome size = 490 Mb) and inbreeding nature. This compares to the larger genomes of the outbreeding species pearl millet (diploid, 1C = 2,352 Mb), napiergrass (tetraploid, 1C = 2,254 Mb), and switchgrass (tetraploid, 1C =1,372– 1,666 Mb, octaploid 1C = 2,352–3,136 Mb; Bennett et al., 2000). Switchgrass is largely self-incompatible, which significantly complicates forward or reverse genetics, trait mapping, genetic engineering, and genome sequencing. Foxtail millet is self-compatible and has a highly conserved genome structure relative to the ancestral grass lineage (Devos et al., 1998). Some genetic resources such as genetic maps (Devos et al., 1998; Wang et al., 1998) and a small collection of ESTs (Zhang et al., 2007) are already available for foxtail millet, but most of the tool development and genetic research in foxtail millet will be sequence driven. Foxtail millet will be useful as an experimental crop to investigate many aspects of plant architecture, genome evolution, and physiology in the bioenergy grasses. Foxtail millet, pearl millet, napiergrass, maize (Zea mays), and sorghum (Sorghum bicolor) differ from switchgrass in the decarboxylation enzyme used during C4 photosynthesis (NADP-malic versus NADmalic; Watson and Dallwitz, 1992), but are otherwise similar in C4 photosynthesis, making foxtail millet of value in understanding this important trait. Last, but not least, the draft foxtail millet genome will provide an assembly guide for any future switchgrass sequencing project, as ongoing work in switchgrass indicates that there is good colinearity between switchgrass and foxtail millet (C. Tobias, personal communication). ORIGIN AND RELATIONSHIPS OF FOXTAIL MILLET