Strategies for Improving Switchgrass Vegetative Growth and Biomass Yield

Increasing biomass yield is one of the main goals in switchgrass breeding programs (Schubert 2006; Sticklen 2006; Bouton 2007). In general, delaying flowering can extend and promote vegetative growth thereby increasing biomass yield. Biomass accumulation in plants can stop when transitioning from vegetative to reproductive growth. Therefore, controlling flowering time could be an effective strategy to change the biomass yield in switchgrass genetic improvement. For most plant species, flowering is one of the complex traits regulated by many factors, including genetic and environmental components (temperature, photoperiod and other factors) (Distelfeld et al. 2009; Jung and Muller 2009). Several miRNAs have been implicated in this process. As mentioned above, miR156 has been used to modify flowering time and biomass production in switchgrass (Chuck et al. 2011; Fu et al. 2012). MiR156s delay vegetative growth phase by repressing the expression of their targeted SPL genes, which are positive regulators of another miRNA, miR172 (Xie et al. 2006; Wang et al. 2009; Wu et al.

2009) . In Arabidopsis, miR172 acts downstream of miR156 through SPL9 and represses one of its target SMZ, which represses the transcription of FLOWERING LOCUS T (FT) (Mathieu et al. 2009). This pathway conserves in many plant species, so it is possible to manipulate miR172 and its target genes to regulate flowering time and biomass yield in switchgrass. It also has been reported that overexpression of miR319 leads to down-regulation of several TCP targets and results in delayed flowering in Arabidopsis (Palatnik et al. 2003) and rice (Yang et al. 2013). This suggests that miR319 might be another candidate useful for adjusting flowering time and biomass production in switchgrass.