Stresses have been the major limiting factors in plant growth and reproduction. Although switchgrass is one of the toughest plant species which can thrive on marginal lands and tolerate adverse circumstances, increasing stress tolerance will further improve plant growth and is one of the important goals in switchgrass breeding programs. A number of extensive studies have revealed that the expression levels of certain miRNAs are regulated in plants exposed to various stresses and suggested that miRNAs are an integral part of plant stress regulatory networks (Jones — Rhoades and Bartel 2004; Sunkar and Zhu 2004; Sunkar et al. 2007, 2012; Zhao et al. 2007; Shukla et al. 2008; Wang et al. 2011; Khraiwesh et al. 2012; Zhou et al. 2012), suggesting that regulating the expressions of miRNAs could be one of the effective strategies to genetically improve plant stress tolerance in switchgrass.

The roles miRNAs play in plant stress responses have recently been studied in switchgrass. Sun et al. (2012) investigated how drought and salinity alter the expression of miRNAs. Using real-time RT-PCR, they analyzed 12 conserved miRNAs in switchgrass, which have been implicated in salt and/or drought stress in other plant species, and found that both salt and drought stresses could impact the expression pattern of many miRNAs. Under high drought stress, the expression levels of miR156 and miR162 changed significantly suggesting that miRNAs may contribute to plant adaptation to stress and are potential candidates for improving switchgrass (Sun et al. 2012). We have explored the potential of manipulating miRNAs in transgenics for improving plant resistance to environmental stress. Transgenic creeping bentgrass (Agrostis stolonifera L.) plants overexpressing a rice miR319 gene, Osa-miR319a, were generated and found to exhibit enhanced drought and salt tolerance. The enhanced abiotic stress tolerance in transgenic plants was related to significant down-regulation of miR319 target genes, and associated with increased leaf wax content and water retention, but reduced sodium uptake (Zhou et al. 2013). Similar strategy can also be applied in other crop species, including switchgrass to genetically engineer plants for enhanced resistance to environmental stress.