The normal functioning of miRNAs is a prerequisite for plant development. The loss of function of the key genes involved in miRNA biogenesis would cause significant mutant phenotypes in plant growth and development (Jacobsen et al. 1999; Lu et al. 2000; Park et al. 2002; Vaucheret et al. 2004). For example, the loss of function of DCL1, an important gene directly involved in the processing of pri-miRNAs and pre-miRNAs, would impact the maturation of miRNAs causing multiple deficiencies in plant development, such as abnormal leaf shape, delayed flowering and early embryo arrest (Reinhart et al. 2002; Dugas and Bartel 2004; Liu et al. 2005; Nodine and Bartel 2010). The mutants of other miRNA biogenesis-related genes including hyl1, hen1, and hst, all showed developmental deficiencies (Han et al. 2004; Park et al. 2005). These data demonstrated that miRNAs are largely involved in regulating plant development and play vital roles.

In recent years, the impacts of miRNAs on plant development have been extensively studied, especially in the model specie Arabidopsis thaliana. For example, Palatnik et al. (2003) found that miR319/JAW could target some members of TCP family, affecting leaf shape formation. Overexpression of miR319 led to down regulation of several TCP targets, resulting in uneven leaf shape and curvature, cotyledon epinasty, a modest delay in flowering and crinkled fruits phenotypes (Palatnik et al. 2003). Similarly, our study in creeping bentgrass (Agrostis stolonifera) also demonstrated that over expression of a rice miR319 gene causes pleiotropic phenotypes in transgenic plants including increased leaf expansion and stem diameter, which are associated with down regulation of at least four putative target TCP genes (Zhou et al. 2013). MiR165/166 targets some members of Class III HD — Zip and KANADI families, of which PHABULOSA (PHB), PHAVOLUTA (PHV), REVOLUTA (REV), KAN1, KAN2, and KAN3 play important roles in regulating leaf and flower development and vascular polarity (Chen 2005). miR172 regulats floral organ identity, reproductive development through regulating its targets APETALA2 (AP2) and AP2-like genes, such as TOE1, TOE2 and TOE3 (Chen 2004, Zhu et al. 2009).