Robust division orientation of cambium stem cells requires cortical division zone components but not the preprophase band

The control of cell division plane orientation is fundamental for organizing developmental processes and shaping bodies of multicellular organisms. In plants, radial organ growth is mediated by the cambium, a stem cell niche deeply embedded in expanding organs and continuously producing xylem and phloem in a strictly bidirectional manner. Cambium stem cells (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 characterized microtubule organization during CSC divisions in Arabidopsis thaliana and found that division plane orientation is established independently from spindle orientation and the preprophase band (PPB). This conclusion is based on the remarkable variability of spindle orientation during metaphase and the robustness of CSC divisions in PPB-deficient mutants. 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.