Plant plasmodesmata bridges form through ER-dependent incomplete cytokinesis

Diverging from conventional cell division models, plant cells undergo incomplete division to generate plasmodesmata communication bridges between daughter cells. While fundamental for plant multicellularity, the molecular events leading to bridge stabilization, as opposed to severing, remain unknown. Using electron tomography, we mapped the transition from cell plate fenestrae to plasmodesmata. We show that the ER connects daughter cells across fenestrae, and as the cell plate matures, fenestrae contract, causing the PM to mold around constricted ER tubes. The ER’s presence prevents fenestrae fusion, forming plasmodesmata, while its absence results in closure. The ER-PM tethers MCTP3, 4, and 6 further stabilize nascent plasmodesmata during fenestrae contraction. Genetic deletion inArabidopsisreduces plasmodesmata formation. Our findings reveal how plants undergo incomplete division to promote intercellular communication.