Function of AGO1 in the miRNA regulatory pathway
miRNAs are loaded onto AGO1 , which acts as an RNA slicer in plants (Baumberger and Baulcombe, 2005). miR-168 directs the cleavage of AGO1 mRNA, indicating that miR-168 regulates the activity of its own miRNA pathway (Vaucheret et al. 2004).AGO1 -null alleles reduce the expression levels of some miRNAs, such as miR-171 , and increase the levels of the corresponding target mRNAs (Vaucheret et al. 2004). In 35S:rSlAGO1 , eight miRNAs were upregulated, and their targets were downregulated. Moreover, in 35S:SlmiR-168a , miR-8006 and miR-8007 were downregulated, and their targets were upregulated. Thus, these miRNAs may be associated with AGO1 protein, and impairment of AGO1 expression could decrease the expression levels of these miRNAs, resulting in upregulation of their targets to affect the plant response to the environment. Additionally, in 35S:SlmiR-168a , 71 miRNAs were upregulated, and their targets were downregulated. These miRNAs included many widely known molecules, such as miR-167 , miR-156 ,miR-396 , miR166 , miR-319b , and miR-172 . These miRNAs may be involved in various hormone signalling pathways, including auxin, ethylene, and gibberellin signalling. InAGO1- null plants, miR-156 /-157 and miR-167were found to accumulate to levels similar to or higher than those in WT plants (Vaucheret et al. 2004). Furthermore, Lynn et al. (1999) reported that PINHEAD/ZWILLE is 75% similar to and has overlapping functions with AGO1. miRNAs regulatory network is not only relying on AGO but also other proteins. AGO proteins can bind to single-stranded RNAs that are at least 5 nt in length and to double-stranded RNA, enabling AGO protein to directly associate with miRNAs before and after they recognise their mRNA targets (Yan et al. 2003). Importantly, miRNAs are regulated by AGO in time and space fine-tuned and complex regulatory networks. Thus,miR-168 may function with AGO1 to control the mRNA levels of miRNA targets through a complex pathway.