Research The preprophase band is dispensable for robust cell division orientation in wood-forming cambium stem cells

Cambium stem cells (CSCs) are the source of wood formation and, thus, of a large part of terrestrial biomass. CSCs are unique in comparison to other plant stem cells as they consistently divide along their longest axis clearly overriding the commonly found ‘short axis’ rule. Despite the topological consistency of CSC divisions and the functional importance of proper division plane orientation for CSC derivatives, the regulatory mechanisms behind this consistency remain unknown. Here, we reveal for the first time the subcellular dynamics of the cell division machinery during CSC divisions and identify determinants of the fascinating regularity of their division orientation. We found that division plane orientation is established independently from spindle orientation and the preprophase band (PPB). Instead, the orientation of division planes depends on the cortical division zone (CDZ) and CDZ-related PHRAGMOPLAST ORIENTING KINESIN (POK) proteins. Our results highlight the importance of wood-forming CSCs and their subcellular characterization as an instructive example for the determination of cell division orientation in plants.