UC Davis

The cell wall is an integral and dynamic structure surrounding plant cells, involved in plant growth and development. During cytokinesis, cell plate formation takes place, via fusion of cytokinetic vesicles, that after membrane transformation and deposition of polysaccharides transitions to a new cross wall. Callose, a β(1,3) glucan polysaccharide, is transiently deposited at the cell plate during this fundamental process. Callose deposition is thought to play a vital role in stabilizing the cell plate and contributing to a spreading force during cell plate expansion.

Cytokinesis-specific callose synthases are recalcitrant to study by genetic means, because null mutations in the corresponding genes cause lethality. Additionally, limitations in live cell imaging of polysaccharides add to the difficulties in elucidating the biological role and regulation of callose biosynthesis at the cell plate.

To overcome these challenges, I have taken a multidisciplinary approach. To better understand the transition from a vesicular network to a fenestrated sheet and finally a mature cell plate, we implemented a modeling approach, adopting the Helfrich energy that examines the elastic properties of lipid bilayers. Our model predicts the requirement of a spreading/stabilizing force for cell plate expansion and maturation. The transient accumulation of callose, coinciding with the predicted cell plate stages requiring this spreading force, are consistent with the proposed model. Furthermore, we used a specific cytokinetic callose inhibitor, Endosidin7 (ES7), to dissect the role of callose. ES7 does not inhibit wound or stress-induced callose deposition; however, it specifically causes failure in cell plate expansion and maturation, indicating that callose is a contributor to the stabilizing or spreading force as predicted by our model.

We further used advanced light microscopic techniques to explore cell plate dynamics. I studied cell plate development across four dimensions using a cytokinesis-specific GTPase YFP-RABA2a as a vesicle marker. With the aid of a robust cell plate volume analysis pipeline, I identified three easily trackable cell plate developmental phases that can be interrogated to study cell plate development. Inhibition of callose through ES7 suppressed phase transitions, establishing a critical role and timing of polysaccharide deposition in cell plate expansion and maturation.

Using this suite of interdisciplinary techniques, we contributed to breaking apart the molecular black box surrounding callose deposition during cytokinesis.