Origin and Domestication of Foxtail Millet

Abstract

Setaria is a genus of panicoid grasses that utilizes the highly efficient C4 photosynthetic pathway and is related to other C4 grasses such as switchgrass, pearl millet, maize, and sorghum. The Setaria system comprises two species (considered subspecies by some authors), namely, wild green foxtail (S. viridis), one of the most widespread weeds on the planet, and its domesticated cousin, foxtail millet (S. italica), a drought-hardy and nutritious cereal important in China, India, and Africa. Together, the two species make up a remarkable system for investigating different aspects of plant biology, ranging from the processes of ecological differentiation, domestication, morphological and developmental change, genetic regulation, C4 photosynthesis, breeding, and genome evolution. The aim of this book is to introduce the Setaria system to a wider audience, explore current research in Setaria, and provide protocols and guidance for crossing, mutant production, creation of genetic resources, transformation, and genetic analysis. The wide latitudinal and ecological range of green foxtail and foxtail millet has led to population divergence and local adaptation to a variety of conditions. The model S. viridis accession, A10.1, is consistent in its growth form under controlled conditions but very sensitive to growth environment, making it ideal for examining the molecular basis of abiotic stress. Accession A10.1 is a small variety of green foxtail that can be grown in growth chamber, greenhouse, and field trials and needs no special growth conditions. In particular, the small physical size and rapid life cycle of A10.1, coupled with a small diploid genome, lend itself to genetic analyses such as those that have commonly been performed in Arabidopsis. Further model accessions are being developed that combine attributes of foxtail millet, especially non-seed shattering, with other desirable traits. Multiple chapters in this volume speak to the utility of Setaria as a model system for C4 grass biology, including C4 photosynthesis, cell wall regulation, root and shoot regulation, root-microbe interactions, herbicide tolerance, and drought stress. These studies are enabled by genetic and genomic resources, including multiple genome sequences for foxtail millet and green foxtail, multiple sequenced diversity lines for population genetics, and genome-wide association studies, a renewed interest in creating mapping populations and mutant collections, and efficient vi transformation techniques, including the promise of a spike dip protocol analogous to the floral dip protocol for transformation that revolutionized Arabidopsis genetic research. High-throughput sequencing (HTS) has been an important component in the development of several of these resources, including genome by sequencing and whole-genome sequencing of diversity lines, and rapid identification of candidate loci underlying quantitative trait loci (QTL) and mutant phenotypes. Coupled with HTS, new gene editing techniques are allowing rapid and efficient reverse genetic approaches, including testing candidate loci generated from GWAS, QTL mapping, and fine mapping in mutant populations. An important aspect of the Setaria system is that it is part of a larger set of genetic model species in the grasses that allow inferences about gene and genome evolution that is simply not possible in any other family. These models include rice, maize, sorghum, and Brachypodium (B. distachyon and B. stacei), a pooid C3 model grass much like Setaria in its small size and ease of use. In addition, there is the tetraploid genome of switchgrass (Panicum virgatum), the diploid genome of its close relative Panicum hallii, and multiple draft genomes in progress in other species. These genomes span the vast majority of grass diversity and allow unparalleled opportunities for investigating genome evolution and the genetic basis of morphological and physiological evolution. In addition, grass synteny allows basic research in these model systems to be quickly and efficiently translated into agronomically important crops such as maize, rice, and wheat. Setaria is unique among these grass model systems because it encompasses both an important domesticated cereal and an emerging model system. The drought hardiness of Setaria makes it an attractive crop in parts of China, India, and Africa and an alternative to other cereals such as pearl millet and sorghum. The use of Setaria as a model system to understand drought stress will rapidly be translated into both better Setaria varieties and the possibility of better drought-hardy cereals in general. The small size and ease of genetic analysis in Setaria also make it the model of choice for understanding the genetics and physiology of C4 photosynthesis, and Setaria is a key tool in the grand challenge of converting C3 photosynthetic grasses like rice into highly efficient C4 cereals to feed an ever-growing human population. Finally, a note on nomenclature. Setaria is used in this volume to denote the Setaria system (both Setaria italica and its wild progenitor Setaria viridis), written without italicization and starting with a capital letter. The two species names are written in italics as is normal for Latin binomials. In addition, S. viridis has been referred to in past literature as both green millet and green foxtail, but we advocate the use of green foxtail as its correct common name, as it is not a millet cereal grain.