To construct small RNA libraries, total RNA was extracted from leaves using Trizol reagent (Invitrogen, USA) according to the manufacturer's protocol as modified by LC Sciences (Houston, USA). The small RNA fraction (10-40 nt) was isolated by polyacrylamide gel electrophoresis and ligated with proprietary adaptors (Illumina). Short RNAs were then reverse-transcribed into cDNA by RT-PCR. Small RNA libraries were constructed for each maize line and sequenced using a Genome Analyzer GA-IIX (Illumina) following the manufacturer's instructions. Small RNA sequencing data was processed with the in-house program ACGT101-miR (LC Sciences, USA). Program parameters were set as previously described 36 (link) with modifications to adapt the program for plant miRNA prediction.