Background: In Arabidopsis, AGO1 and AGO2 associate with small RNAs that exhibit a Uridine and an Adenosine at their 5' end, respectively. Because most plant miRNAs have a 5'U, AGO1 plays many essential roles in miRNA-mediated regulation of development and stress responses. In contrast, AGO2 has only been implicated in antibacterial defense in association with miR393*, which has a 5'A. AGO2 also participates in antiviral defense in association with viral siRNAs.
Principal findings: This study reveals that miR408, which has a 5'A, regulates its target Plantacyanin through either AGO1 or AGO2. Indeed, neither ago1 nor ago2 single mutations abolish miR408-mediated regulation of Plantacyanin. Only an ago1 ago2 double mutant appears compromised in miR408-mediated regulation of Plantacyanin, suggesting that AGO1 and AGO2 have redundant roles in this regulation. Moreover, the nature of the 5' nucleotide of miR408 does not appear essential for its regulatory role because both a wildtype 5'A-MIR408 and a mutant 5'U-MIR408 gene complement a mir408 mutant.
Conclusions/significance: These results suggest that miR408 associates with both AGO1 and AGO2 based on criteria that differ from the 5' end rule, reminiscent of miR390-AGO7 and miR165/166-AGO10 associations, which are not based on the nature of the 5' nucleotide.