Disruption of actin filaments delays accumulation of cell plate membranes after chromosome separation

Phragmoplasts, which comprise microtubules, actin filaments, and membrane vesicles, are responsible for cell plate formation and expansion during plant cytokinesis. Our previous research using the actin polymerization inhibitor latrunculin B (LatB) to investigate the role of actin filaments suggested the existence of two types of microtubules: 1) initial microtubules sensitive to LatB but unassociated with NACK1 kinesin and 2) later LatB-insensitive, NACK1-associated microtubules. The organization of initial phragmoplast microtubules might have been disrupted by the LatB treatment; this hypothesis remained unverified, however, as the exact timing of cell plate membrane accumulation could not be determined. In the present study, we further investigated the timing of cell plate formation during LatB treatment. We monitored chromosome separation during anaphase as well as accumulation of FM4-64-stained cell plate membranes in dividing transgenic tobacco BY-2 cells expressing RFP-tagged histone H2B. We observed that LatB treatment prolonged the time between the slowdown of daughter chromosome migration and the accumulation of cell plate membranes. This result suggests that disruption of actin filaments resulted in delayed cell plate formation possibly by perturbation of initial phragmoplast microtubules or cell plate assembly.