Gene flow matters in switchgrass (Panicum virgatumL.), a potential widespread biofuel feedstock

Abstract

Abstract. There currently exists a large push for the use, improvement, and expansion via landscape modification of dedicated biofuel crops (feedstocks) in the United States and in many parts of the world. Ecological concerns have been voiced because many biofuel feedstocks exhibit characteristics associated with invasiveness, and due to potential negative consequences of agronomic genes in native wild populations. Seed purity concerns for biofuel feedstock cultivars whose seeds would be harvested in agronomic fields also exist from the agribusiness sector. The common thread underlying these concerns, which have regulatory implications, is gene flow; thus detailed knowledge of gene flow in biofuel crop plants is important in the formulation of environmental risk management plans. Here, we synthesize the current state of knowledge of gene flow in an exemplary biofuel crop, switchgrass (Panicum virgatum L.), which is native to eastern North America and is currently experiencing conventional and technological advances in biomass yields and ethanol production. Surprisingly little is known regarding aspects of switchgrass pollen flow and seed dispersal, and whether native populations of conspecific or congeneric relatives will readily cross with current agronomic switchgrass cultivars. We pose that filling these important gaps will be required to confront the sustainability challenges of widespread planting of biofuel feedstocks.

Genetic diversity and structure of natural and agronomic switchgrass (Panicum virgatum L.) populations

“…This information is required to determine if it is possible for agronomic switchgrass populations to share genes with wild populations. Since ploidy levels in switchgrass can influence compatibility (Martinez-Reyna and Vogel 2002;Stottlemyer 2012;Kwit and Stewart 2012) and hence introgression, gene flow could occur between the natural and the agronomic populations, allowing agronomic genes to escape cultivation and introgress into the natural populations. However, a number of prerequisites such as overlapping flowering phenologies, close proximity of agronomic fields and demographic swamping from crop plants are to be fulfilled for introgression to occur (Stottlemyer 2012; Kwit and Stewart 2012).…”

“…As a biofuel crop, switchgrass exhibits certain traits associated with invasiveness [e.g., rapid early-season growth, effective belowground partitioning of nutrients in the dormant season, high water-use efficiency (Raghu et al 2006)], even though there is no documented evidence of its weediness or invasiveness. Still, there may be regulatory and ecological concerns about risks of transgene or agronomic gene introgression into native populations (Kausch et al 2010;Kwit and Stewart 2012). Furthermore, since the geographic origins of agronomic switchgrass cultivars may not coincide with current or future planting locations, such introgression may be possible and discernable, especially where ploidy levels of agronomic and natural populations are equivalent (Martinez-Reyna and Vogel 2002).…”

Pollen-mediated gene flow from transgenic to non-transgenic switchgrass (Panicum virgatum L.) in the field

“…Elsewhere, it is typically not a large component of natural areas [10, 11] and is often found in much lower densities than those grown in agronomic settings. Traditionally switchgrass has been grown as a forage crop [12, 13], and because it is highly adaptable, it has served a variety of purposes including conservation, wildlife habitat, prairie restoration, erosion control, and in ornamental gardens [12, 14, 15]. Owing to its high biomass yield and suitability for marginal land, switchgrass has been the target of extensive development as a dedicated lignocellulosic biofuel feedstock [16].…”

“…Owing to its high biomass yield and suitability for marginal land, switchgrass has been the target of extensive development as a dedicated lignocellulosic biofuel feedstock [16]. With the current drive to meet U.S. government-mandated benchmarks for energy production (16 billion gallons (1 gallon = 3.79 L) cellulosic biofuel by 2022; [17]), large-scale agronomic plantings of dedicated biofuel crops, including switchgrass, are projected to increase significantly through 2022 [14, 18, 19]. Although switchgrass research and development of conventional cultivars has significantly expanded over the last 25 years [20], extensive research efforts have been undertaken to produce transgenic switchgrass cultivars with increased biomass, enhanced saccharification efficiency, and modified lignin biosynthesis for improved biofuel properties [21–30].…”

“…Although genetic engineering provides an opportunity to increase biomass yields and efficiencies in switchgrass production, transgenes from large-scale switchgrass plantings could escape to (1) non-transgenic switchgrass that might be used for livestock feed or other future uses, and (2) wild relatives in the landscape matrix [14, 31, 34]. Switchgrass is also being engineered for abiotic and biotic resistance traits (e.g.…”