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ABSTRACT Plants develop throughout their lives: seeds become seedlings that mature and form fruits and seeds. Although the underlying mechanisms that drive these developmental phase transitions have been well elucidated for shoots, the extent to which they affect the root is less clear. However, root anatomy does change as some plants mature; meristems enlarge and radial thickening occurs. Here, in Arabidopsis thaliana, we show that overexpressing miR156A, a gene that promotes the juvenile phase, increased the density of the root system, even in grafted plants in which only the rootstock had the overexpression genotype. In the root, overexpression of miR156A resulted in lower levels of PLETHORA 2, a protein that affects formation of the meristem and elongation zone. Crossing in an extra copy of PLETHORA 2 partially rescued the effects of miR156A overexpression on traits affecting root architecture, including meristem length and the rate of lateral root emergence. Consistent with this, PLETHORA 2 also inhibited the root-tip expression of another miR156 gene, miR156C. We conclude that the system driving phase change in the shoot affects developmental progression in the root, and that PLETHORA 2 participates in this network.