Setaria viridis: A Model for C4 Photosynthesis  

Abstract

C 4 photosynthesis drives productivity in several major food crops and bioenergy grasses, including maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), Miscanthus x giganteus, and switchgrass (Panicum virgatum). Gains in productivity associated with C 4 photosynthesis include improved water and nitrogen use efficiencies. Thus, engineering C 4 traits into C 3 crops is an attractive target for crop improvement. However, the lack of a small, rapid cycling genetic model system to study C 4 photosynthesis has limited progress in dissecting the regulatory networks underlying the C 4 syndrome. Setaria viridis is a member of the Panicoideae clade and is a close relative of several major feed, fuel, and bioenergy grasses. It is a true diploid with a relatively small genome of ;510 Mb. Its short stature, simple growth requirements, and rapid life cycle will greatly facilitate genetic studies of the C 4 grasses. Importantly, S. viridis uses an NADP-malic enzyme subtype C 4 photosynthetic system to fix carbon and therefore is a potentially powerful model system for dissecting C 4 photosynthesis. Here, we summarize some of the recent advances that promise greatly to accelerate the use of S. viridis as a genetic system. These include our recent successful efforts at regenerating plants from seed callus, establishing a transient transformation system, and developing stable transformation. Why Study C 4 ?C 4 photosynthesis is the primary mode of carbon capture for some of the world's most important food, feed, and fuel crops, including maize (Zea mays), sorghum (Sorghum bicolor), sugarcane (Saccharum officinarum), millets (e.g. Panicum miliaceum, Pennisetum glaucum, and Setaria italica), Miscanthus x giganteus, and switchgrass (Panicum virgatum). In contrast with C 3 plants, C 4 plants first fix CO 2 into a C 4 acid before delivering the CO 2 to the Calvin cycle (Hatch and Slack, 1966;Hatch, 1971). For example, in maize and sorghum leaves, CO 2 entering mesophyll (M) cells is first fixed into oxaloacetate, which is then reduced to malate in the M chloroplasts. The malate then diffuses into the inner bundle sheath (BS) cells and is transported into the BS chloroplast. There, malate is decarboxylated by NADP-malic enzyme, releasing CO 2 close to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). This carbon shuttle greatly lowers rates of photorespiration as Rubisco is both isolated from the site of O 2 evolution (oxygen evolving complex of photosystem II) and also maintained in a CO 2 -rich environment. Indeed, in mature maize or sorghum leaves, rates of photorespiration are at the limits of detection under conditions where C 3 plants lose up to 30% of their photosynthetic capacity due to photorespiration (Zhu et al., 2008).

Grasses suppress shoot-borne roots to conserve water during drought

“…These studies used the emerging grass model species Setaria viridis, which is a C 4 -grass model for other agronomically important panicoid grasses, such as maize and sorghum (15,16). Our results define the primary developmental mechanism through which WD affects root system architecture and reveals the physiological relevance of such a response.…”

Genetic diversity and origin of North American green foxtail [Setaria viridis (L.) Beauv.] accessions

“…org/cvalues/), diploid nature (2n = 18), and short life cycle, S. viridis and its domesticated form, S. italica, have become important models to study the genetics of the biofuel crop switchgrass and for C 4 photosynthesis (Brutnell et al 2010;Doust et al 2009;Li and Brutnell 2011). The genome of S. italica has been sequenced and assembled into 9 pseudomolecules, corresponding to the 9 chromosomes, and covers *80 % of the genome and more than 95 % of the gene space (Bennetzen et al 2012;Zhang et al 2012).…”

Initiation of Setaria as a model plant

“…With a high quality reference genome sequence [4] and a high-density haplotype map of genome variation [5] and other genomic data, the accumulated genomic information for foxtail millet and green foxtail has reached the level where this species pair can now truly be considered a novel model system. Several publications, foxtail millet: a sequencedriven grass model system by Doust et al [6], Setaria viridis: a model for C 4 photosynthesis by Brutnell et al [7], and two review papers of using foxtail millet and green foxtail as model [8,9] have helped foster enthusiasm and forward momentum that is leading to the development of Setaria as a model system. A note on terminology: although Setaria is the name of a genus, Setaria is used here to refer to either the domesticated species, foxtail millet, or the wild ancestor, green foxtail, following the common convention (i.e.…”