Genome-wide identification and comprehensive analysis of microRNAs and phased small interfering RNAs in watermelon

Background: MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs involved in the post-transcriptional gene regulation and play a critical role in plant growth, development and stress responses. Watermelon (Citrullus lanatus L.) is one of the important agricultural crops worldwide. However, the watermelon miRNAs and phasiRNAs and their functions are not well explored. Results: Here we carried out computational and experimental analysis of miRNAs and phased small interfering RNAs (phasiRNAs) in watermelon by analyzing 14 small RNA profiles from roots, leaves, androecium, petals, and fruits, and one published small RNA profile of mixed tissues. To identify the targets of miRNAs and phasiRNAs, we generated a degradome profile for watermelon leaf which is analyzed using the SeqTar algorithm. We identified 97 conserved pre-miRNAs, of which 58 have not been reported previously and 348 conserved mature miRNAs without precursors. We also found 9 novel pre-miRNAs encoding 18 mature miRNAs. One hundred and one 21 nucleotide (nt) PHAS loci, and two hundred and forty one 24 nt PHAS loci were also identified. We identified 127 conserved targets of the conserved miRNAs and TAS3-derived tasiRNAs by analyzing a degradome profile of watermelon leaf. Conclusions: The presented results provide a comprehensive view of small regulatory RNAs and their targets in watermelon.