Identification of microRNAs and their targets in switchgrass, a model biofuel plant species

In recent years, several plant species such as switchgrass, Miscanthus and Brachypodium have been recognized as potential model plant species for cellulosic bioenergy production. Of these, switchgrass has attracted much attention in the United States and worldwide because it can grow well on marginal lands and tolerate frequent drought spells. However, little is known about the basic biology of the traits that control these important characteristics in switchgrass. Genome-encoded approximately 21-24nt microRNAs (miRNAs) have emerged as critical regulators of gene expression important for normal growth and development and adaptation to abiotic stress, including nutrient-deprived conditions. To understand miRNA-guided post-transcriptional gene regulatory networks in this plant species, we sought to identify miRNAs in switchgrass. Using computational and experimental approaches, we identified approximately 20 conserved miRNA families. Temporal expression analysis indicated that some miRNAs have distinct tissue-specific expression, although most are ubiquitously expressed. Unlike in Arabidopsis and other plants, miR395 and miR399 were detected in plants grown on optimal levels of sulfate or phosphate in switchgrass, and were only slightly altered when exposed to sulfate or phosphate deficit conditions. Thirty-seven genes were predicted as targets for miRNAs, and 4 target mRNAs (Squamosa promoter binding-like factor, apetala 2-like, NAC domain containing transcription factor and HD-Zip homologs) were validated by 5'-RACE assays. These findings provide a snapshot of the miRNA component and possible targets in switchgrass.