For RNA extraction,10-day-old plants grown horizontally on the round plates were used. Seedlings were immediatly frozen in liquid nitrogen. NA Young seedlings were grown horizontally on round plates containing ½ MS medium with 1% sucrose and 0.9% agar at 21˚C under a 16h/8h day/night condition. NA NA RNA was extracted with the miRNeasy Mini Kit (Qiagen). RNA integrity and quantity were measured using the Agilent 2100/4200 system.  The RNAs were first selected via enrichment of their poly(A) tail and then further fragmented into 300~350 bp fragments to build the libraries.