Dynamic Regulation of Novel and Conserved miRNAs Across Various Tissues of Diverse Cucurbit Species

MicroRNA genes (miRNAs) encoding small non-coding RNAs are abundant in plant genomes and play a key role in regulating several biological mechanisms. Five conserved miRNAs, miR156, miR168-1, miR168-2, miR164, and miR166 were selected for analysis from the 21 known plant miRNA families that were recovered from deep sequencing data of small RNA libraries of pumpkin and squash. A total of six novel miRNAs that were not reported before were found to have precursors with reliable fold-back structures and hence considered novel and were designated as cuc_nov_miRNAs. A set of five conserved, six novel miRNAs, and five uncharacterized small RNAs from the deep sequencing data were profiled for their dynamic regulation using qPCR. The miRNAs were evaluated for differential regulation across the tissues among four diverse cucurbit species, including pumpkin and squash (Cucurbita moschata Duch. Ex Poir. and Cucurbita pepo L.), bitter melon (Momordica charantia L.), and Luffa (Loofah) (Luffa acutangula Roxb.). Expression analysis revealed differential regulation of various miRNAs in leaf, stem, and fruit tissues. Importantly, differences in the expression levels were also found in the leaves and fruits of closely related C. moschata and C. pepo. Comparative miRNA profiling and expression analysis in four cucurbits led to identification of conserved miRNAs in cucurbits. Predicted targets for two of the conserved miRNAs suggested miRNAs are involved in regulating similar biological mechanisms in various species of cucurbits.