During its development, a plant shoot progresses from a juvenile to an adult phase of vegetative growth and from a reproductively incompetent to a reproductively competent state. In Arabidopsis, Loss-of-function mutations in SQUINT (SQN) reduced the number of juvenile Leaves and had subtle effects on inflorescence morphology but had no effect on flowering time or on reproductive competence. SQN encodes the Arabidopsis homolog of cyclophilin 40 (CyP40), a protein found in association with the Hsp90 chaperone complex in yeast, mammals, and plants. Thus, in Arabidopsis, CyP40 is specifically required for the vegetative but not the reproductive maturation of the shoot.