A novel cell-to-cell trafficking assay indicates that the KNOX homeodomain is necessary and sufficient for intercellular protein and mRNA trafficking.

Cell-to-cell trafficking of regulatory proteins is a novel mechanism for communication during cell fate specification in plants. Although several developmental proteins traffic cell-to-cell, no signals that are both necessary and sufficient for this function in developmental proteins have been described. We developed a novel trafficking assay using trichome rescue in Arabidopsis. Fusion to KNOTTED1 (KN1) conferred gain-of-trafficking function to the cell-autonomous GLABROUS1 (GL1) protein. We show that the KNOX homeodomain (HD) is necessary and sufficient for intercellular trafficking, identifying a novel function for the HD as the minimal sequence required for trafficking of KN1 and its associated mRNA.