To control gene expression by directly responding to hormone concentrations,both animal and plant cells have exploited comparable mechanisms to sense small-molecule hormones in nucleus.Whether nuclear entry of these hormones is actively transported or passively diffused,as conventionally postulated,through the nuclear pore complex,remains enigmatic.Here,we identified and characterized a jasmonate transporter in Arabidopsis thaliana,AtJAT1/AtABCG16,which exhibits an unexpected dual localization at the nuclear envelope and plasma membrane.We show that AtJAT1/AtABCG16 controls the cytoplasmic and nuclear partition of jasmonate phytohormones by mediating both cellular efflux of jasmonic acid (JA) and nuclear influx of jasmonoyl-isoleucine (JA-Ile),and is essential for maintaining a critical nuclear JA-Ile concentration to activate JA signaling.These results illustrate that transporter-mediated nuclear entry of small hormone molecules is a new mechanism to regulate nuclear hormone signaling.Our findings provide an avenue to develop pharmaceutical agents targeting the nuclear entry of small molecules.