ALTERED MERISTEM PROGRAM1 regulates leaf identity independent of miR156-mediated translational repression

In Arabidopsis, loss of the carboxypeptidase, ALTERED MERISTEM PROGRAM1 (AMP1), produces an increase in the rate of leaf initiation, an enlarged shoot apical meristem and an increase in the number of juvenile leaves. This phenotype is also observed in plants with reduced levels of miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors, suggesting that AMP1 may promote SPL activity. However, we found that the amp1 phenotype is only partially corrected by elevated SPL gene expression, and that amp1 has no significant effect on SPL transcript levels, or on the level or the activity of miR156. Although evidence from a previous study suggests that AMP1 promotes miRNA-mediated translational repression, amp1 did not prevent the translational repression of the miR156 target, SPL9, or the miR159 target, MYB33. These results suggest that AMP1 regulates vegetative phase change downstream of, or in parallel to, the miR156/SPL pathway and that it is not universally required for miRNA-mediated translational repression. Summary statementWe show that loss of the carboxypeptidase, AMP1, does not interfere with the function of miR156 or miR159, suggesting that AMP1 is not universally required for miRNA-mediated translational repression in Arabidopsis.