Keywords 14Vegetative phase change, shoot apical meristem, miR156, SPL, AMP1, translational 15 repression 16 17 18 Summary statement 19We show that loss of the carboxypeptidase, AMP1, does not interfere with the function 20 of miR156 or miR159, suggesting that AMP1 is not universally required for miRNA-21 mediated translational repression in Arabidopsis. 22 Abstract 24In Arabidopsis, loss of the carboxypeptidase, ALTERED MERISTEM PROGRAM1 25 (AMP1), produces an increase in the rate of leaf initiation, an enlarged shoot apical 26 meristem and an increase in the number of juvenile leaves. This phenotype is also 27 observed in plants with reduced levels of miR156-targeted SQUAMOSA PROMOTER 28 BINDING PROTEIN-LIKE (SPL) transcription factors, suggesting that AMP1 may 29 promote SPL activity. However, we found that the amp1 phenotype is only partially 30 corrected by elevated SPL gene expression, and that amp1 has no significant effect on 31 SPL transcript levels, or on the level or the activity of miR156. Although evidence from 32 a previous study suggests that AMP1 promotes miRNA-mediated translational 33 repression, amp1 did not prevent the translational repression of the miR156 target, 34 SPL9, or the miR159 target, MYB33. These results suggest that AMP1 regulates 35 vegetative phase change downstream of, or in parallel to, the miR156/SPL pathway and 36 that it is not universally required for miRNA-mediated translational repression. 37 38 39 40 41 42 43We then examined the expression of these genes in successive rosette leaf 132 primordia (LP) of plants grown in SD. Because amp1 initiates leaves more rapidly than 133 WT, LP were grouped according to the time of harvest rather than position on the shoot. 134Both the level and rate of decline of miR156 were almost identical in WT and amp1 (Fig. 135