MicroRNAs (miRNAs) are a class of 20-to 24-nt noncoding RNAs that function as important regulators of plant development and stress responses.Most 21-nt miRNAs direct the cleavage of complementary target mRNAs or inhibit their translation,while some 22-nt miRNAs not only cleave target mRNAs but also trigger the biogenesis of secondary small interfering RNAs (siRNAs) to further repress target mRNAs.