Stress Resistance

Along with large-scale, intensive production of switchgrass, agronomic trait improvement, such as disease and insect resistance, will become more and more important (Gressel 2008). Native switchgrass has extensive genetic diversity with fair resistance to the majority of potential pathogens (Bouton 2007). However, without knowledge of the genetic basis of disease resistance in switchgrass and the structure of pathogen populations, current and future switchgrass breeding programs that target high biomass yield and improved feedstock quality are likely to reduce the genetic diversity of disease resistance (Tanksley and McCouch 1997). Airborne foliar fungal pathogens like rust have a great potential to cause nationwide epidemics on switchgrass, resulting in significant biomass yield losses (Gustafson et al. 2003). Foliar diseases, in addition to reducing yields, can reduce the availability of saccharifiable cellulose due to increased lignification of host cell walls (Moerschbacher 1989; Parrish and Fike 2009; Shen et al. 2009). Among all potential switchgrass diseases that could negatively impact the commercial production of switchgrass, rust caused by the fungus Puccinia emaculata Schwein is the most destructive and widespread disease problem (Zeiders 1984; Gravert and Munkvold 2002; Gustafson et al. 2003; Krupinsky et al. 2004; Parrish and Fike 2005; Carris et al. 2008; Zale et al. 2008; Crouch et al. 2009; Hirsch et al. 2010; Tomaso-Peterson and Balbalian 2010).

Additionally, switchgrass seedheads can be heavily infected by smut and bunt caused by Tilletia maclagani (Berk.) G. P. Clinton and T. pulcherrima Syd. & P. Syd., respectively. While the impact of bunt infection on switchgrass production is not clear beyond plant inspection issues (Carris et al. 2008), smut has been shown to severely reduce seed and biomass yields in Iowa (Gravert et al. 2000; Thomsen et al. 2008) and has heavily infected Nebraska switchgrass accessions in Oklahoma (S. Marek, personal communications). In general, these seedborne diseases should be remediated by treating seeds with fungicides (Taylor and Harman 1990). Switchgrass is also affected by numerous fungal leaf spot diseases (Roane and Roane 1997; Gravert and Munkvold 2002; Farr and Rossman 2010), including anthracnose caused by Colletotrichumgraminicola (Ces.) G. W. Wilson and C. navitas J. A. Crouch, B. B. Clarke & B. I. Hillman (Crouch et al. 2009; Li et al. 2009), Helminthosporium leaf spot, caused by Bipolaris sorokiana (Sacc.) Shoemaker and B. oryzae (Breda de Haan) Shoemaker (Zeiders 1984, Artigiano and Bedendo 1995; Krupinsky et al. 2004; Tomaso-Peterson and Balbalian 2010), and to a minor extent tar spot, caused by Phyllachora graminis (Pers.) Fuckel, as well as undocumented diseases caused by Pyrenophora sp. and Phaeosphaeria sp. (Farr and Rossman 2010; and S. Marek, unpublished observations), could potentially impact biomass yields. In addition to these fungal diseases, at least two viral diseases, Panicum mosaic and barley yellow dwarf, affect switchgrass, with the former disease sometimes causing the death of tillers and plants (Sill and Pickett 1957; Garrett et al. 2004). Host resistance is the most effective, economical, and environmentally friendly way to control plant disease. Screening germplasm to identify resistance resources to various switchgrass diseases and developing durable and broad spectrum disease resistance will be one of the key breeding objectives in the future. Other than traditional breeding selection, genetic engineering may also have great contributions for disease control in switchgrass (Punja 2001; Stuiver and Custers 2001; Venter 2007; Collinge et al. 2008).

In addition to biotic stress, abiotic stress tolerance, such as tolerance to salinity and drought, will also be very useful. Towards that direction, Ceres has introduced a salinity-tolerance gene into switchgrass, which allows the plants to grow in sea water. The company implied that the unprecedented salt tolerance level could help in growing switchgrass (and other crops) on the 15 million acres of salt-affected soils in the U. S., as well as growing switchgrass in over a billion acres of abandoned cropland all over the world.

Concluding Remarks

Switchgrass is an important biomass/biofuel crop which would contribute substantially to our renewable energy in the future. Although molecular and genetic engineering studies just started several years ago, exciting results on quality improvement of switchgrass as a biofuel feedstock have been obtained. In addition, value-added engineering has emerged, which could be the first step towards improving the economics of biofuel production from lignocellulosic materials. With substantially improved transformation technology, many genes, which have been shown useful in model plant species, or emerge from molecular and genomic studies, could be introduced into switchgrass for its improvement via biotechnology.

As in other outcrossing transgenic plants, transgene escape, mainly through pollen grains, will be a concern. Kausch et al. (2009) has a detailed discussion to address the issue and potential solutions. Interested readers are referred to that review article.

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